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	<!-- ****************************************************** -->
	<!-- Header  -->
	<!-- ****************************************************** -->
	<book id="Writing-an-ALSA-Driver">
	  <bookinfo>
	    <title>Writing an ALSA Driver</title>
	    <author>
	      <firstname>Takashi</firstname>
	      <surname>Iwai</surname>
	      <affiliation>
	        <address>
	          <email>tiwai@suse.de</email>
	        </address>
	      </affiliation>
	     </author>
	
	     <date>Oct 15, 2007</date>
	     <edition>0.3.7</edition>
	
	    <abstract>
	      <para>
	        This document describes how to write an ALSA (Advanced Linux
	        Sound Architecture) driver.
	      </para>
	    </abstract>
	
	    <legalnotice>
	    <para>
	    Copyright (c) 2002-2005  Takashi Iwai <email>tiwai@suse.de</email>
	    </para>
	
	    <para>
	    This document is free; you can redistribute it and/or modify it
	    under the terms of the GNU General Public License as published by
	    the Free Software Foundation; either version 2 of the License, or
	    (at your option) any later version. 
	    </para>
	
	    <para>
	    This document is distributed in the hope that it will be useful,
	    but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the
	    implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A
	    PARTICULAR PURPOSE</emphasis>. See the GNU General Public License
	    for more details.
	    </para>
	
	    <para>
	    You should have received a copy of the GNU General Public
	    License along with this program; if not, write to the Free
	    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
	    MA 02111-1307 USA
	    </para>
	    </legalnotice>
	
	  </bookinfo>
	
	<!-- ****************************************************** -->
	<!-- Preface  -->
	<!-- ****************************************************** -->
	  <preface id="preface">
	    <title>Preface</title>
	    <para>
	      This document describes how to write an
	      <ulink url="http://www.alsa-project.org/"><citetitle>
	      ALSA (Advanced Linux Sound Architecture)</citetitle></ulink>
	      driver. The document focuses mainly on PCI soundcards.
	      In the case of other device types, the API might
	      be different, too. However, at least the ALSA kernel API is
	      consistent, and therefore it would be still a bit help for
	      writing them.
	    </para>
	
	    <para>
	    This document targets people who already have enough
	    C language skills and have basic linux kernel programming
	    knowledge.  This document doesn't explain the general
	    topic of linux kernel coding and doesn't cover low-level
	    driver implementation details. It only describes
	    the standard way to write a PCI sound driver on ALSA.
	    </para>
	
	    <para>
	      If you are already familiar with the older ALSA ver.0.5.x API, you
	    can check the drivers such as <filename>sound/pci/es1938.c</filename> or
	    <filename>sound/pci/maestro3.c</filename> which have also almost the same
	    code-base in the ALSA 0.5.x tree, so you can compare the differences.
	    </para>
	
	    <para>
	      This document is still a draft version. Any feedback and
	    corrections, please!!
	    </para>
	  </preface>
	
	
	<!-- ****************************************************** -->
	<!-- File Tree Structure  -->
	<!-- ****************************************************** -->
	  <chapter id="file-tree">
	    <title>File Tree Structure</title>
	
	    <section id="file-tree-general">
	      <title>General</title>
	      <para>
	        The ALSA drivers are provided in two ways.
	      </para>
	
	      <para>
	        One is the trees provided as a tarball or via cvs from the
	      ALSA's ftp site, and another is the 2.6 (or later) Linux kernel
	      tree. To synchronize both, the ALSA driver tree is split into
	      two different trees: alsa-kernel and alsa-driver. The former
	      contains purely the source code for the Linux 2.6 (or later)
	      tree. This tree is designed only for compilation on 2.6 or
	      later environment. The latter, alsa-driver, contains many subtle
	      files for compiling ALSA drivers outside of the Linux kernel tree,
	      wrapper functions for older 2.2 and 2.4 kernels, to adapt the latest kernel API,
	      and additional drivers which are still in development or in
	      tests.  The drivers in alsa-driver tree will be moved to
	      alsa-kernel (and eventually to the 2.6 kernel tree) when they are
	      finished and confirmed to work fine.
	      </para>
	
	      <para>
	        The file tree structure of ALSA driver is depicted below. Both
	        alsa-kernel and alsa-driver have almost the same file
	        structure, except for <quote>core</quote> directory. It's
	        named as <quote>acore</quote> in alsa-driver tree. 
	
	        <example>
	          <title>ALSA File Tree Structure</title>
	          <literallayout>
	        sound
	                /core
	                        /oss
	                        /seq
	                                /oss
	                                /instr
	                /ioctl32
	                /include
	                /drivers
	                        /mpu401
	                        /opl3
	                /i2c
	                        /l3
	                /synth
	                        /emux
	                /pci
	                        /(cards)
	                /isa
	                        /(cards)
	                /arm
	                /ppc
	                /sparc
	                /usb
	                /pcmcia /(cards)
	                /oss
	          </literallayout>
	        </example>
	      </para>
	    </section>
	
	    <section id="file-tree-core-directory">
	      <title>core directory</title>
	      <para>
	        This directory contains the middle layer which is the heart
	      of ALSA drivers. In this directory, the native ALSA modules are
	      stored. The sub-directories contain different modules and are
	      dependent upon the kernel config. 
	      </para>
	
	      <section id="file-tree-core-directory-oss">
	        <title>core/oss</title>
	
	        <para>
	          The codes for PCM and mixer OSS emulation modules are stored
	        in this directory. The rawmidi OSS emulation is included in
	        the ALSA rawmidi code since it's quite small. The sequencer
	        code is stored in <filename>core/seq/oss</filename> directory (see
	        <link linkend="file-tree-core-directory-seq-oss"><citetitle>
	        below</citetitle></link>).
	        </para>
	      </section>
	
	      <section id="file-tree-core-directory-ioctl32">
	        <title>core/ioctl32</title>
	
	        <para>
	          This directory contains the 32bit-ioctl wrappers for 64bit
	        architectures such like x86-64, ppc64 and sparc64. For 32bit
	        and alpha architectures, these are not compiled. 
	        </para>
	      </section>
	
	      <section id="file-tree-core-directory-seq">
	        <title>core/seq</title>
	        <para>
	          This directory and its sub-directories are for the ALSA
	        sequencer. This directory contains the sequencer core and
	        primary sequencer modules such like snd-seq-midi,
	        snd-seq-virmidi, etc. They are compiled only when
	        <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel
	        config. 
	        </para>
	      </section>
	
	      <section id="file-tree-core-directory-seq-oss">
	        <title>core/seq/oss</title>
	        <para>
	          This contains the OSS sequencer emulation codes.
	        </para>
	      </section>
	
	      <section id="file-tree-core-directory-deq-instr">
	        <title>core/seq/instr</title>
	        <para>
	          This directory contains the modules for the sequencer
	        instrument layer. 
	        </para>
	      </section>
	    </section>
	
	    <section id="file-tree-include-directory">
	      <title>include directory</title>
	      <para>
	        This is the place for the public header files of ALSA drivers,
	      which are to be exported to user-space, or included by
	      several files at different directories. Basically, the private
	      header files should not be placed in this directory, but you may
	      still find files there, due to historical reasons :) 
	      </para>
	    </section>
	
	    <section id="file-tree-drivers-directory">
	      <title>drivers directory</title>
	      <para>
	        This directory contains code shared among different drivers
	      on different architectures.  They are hence supposed not to be
	      architecture-specific.
	      For example, the dummy pcm driver and the serial MIDI
	      driver are found in this directory. In the sub-directories,
	      there is code for components which are independent from
	      bus and cpu architectures. 
	      </para>
	
	      <section id="file-tree-drivers-directory-mpu401">
	        <title>drivers/mpu401</title>
	        <para>
	          The MPU401 and MPU401-UART modules are stored here.
	        </para>
	      </section>
	
	      <section id="file-tree-drivers-directory-opl3">
	        <title>drivers/opl3 and opl4</title>
	        <para>
	          The OPL3 and OPL4 FM-synth stuff is found here.
	        </para>
	      </section>
	    </section>
	
	    <section id="file-tree-i2c-directory">
	      <title>i2c directory</title>
	      <para>
	        This contains the ALSA i2c components.
	      </para>
	
	      <para>
	        Although there is a standard i2c layer on Linux, ALSA has its
	      own i2c code for some cards, because the soundcard needs only a
	      simple operation and the standard i2c API is too complicated for
	      such a purpose. 
	      </para>
	
	      <section id="file-tree-i2c-directory-l3">
	        <title>i2c/l3</title>
	        <para>
	          This is a sub-directory for ARM L3 i2c.
	        </para>
	      </section>
	    </section>
	
	    <section id="file-tree-synth-directory">
	        <title>synth directory</title>
	        <para>
	          This contains the synth middle-level modules.
	        </para>
	
	        <para>
	          So far, there is only Emu8000/Emu10k1 synth driver under
	        the <filename>synth/emux</filename> sub-directory. 
	        </para>
	    </section>
	
	    <section id="file-tree-pci-directory">
	      <title>pci directory</title>
	      <para>
	        This directory and its sub-directories hold the top-level card modules
	      for PCI soundcards and the code specific to the PCI BUS.
	      </para>
	
	      <para>
	        The drivers compiled from a single file are stored directly
	      in the pci directory, while the drivers with several source files are
	      stored on their own sub-directory (e.g. emu10k1, ice1712). 
	      </para>
	    </section>
	
	    <section id="file-tree-isa-directory">
	      <title>isa directory</title>
	      <para>
	        This directory and its sub-directories hold the top-level card modules
	      for ISA soundcards. 
	      </para>
	    </section>
	
	    <section id="file-tree-arm-ppc-sparc-directories">
	      <title>arm, ppc, and sparc directories</title>
	      <para>
	        They are used for top-level card modules which are
	      specific to one of these architectures. 
	      </para>
	    </section>
	
	    <section id="file-tree-usb-directory">
	      <title>usb directory</title>
	      <para>
	        This directory contains the USB-audio driver. In the latest version, the
	      USB MIDI driver is integrated in the usb-audio driver. 
	      </para>
	    </section>
	
	    <section id="file-tree-pcmcia-directory">
	      <title>pcmcia directory</title>
	      <para>
	        The PCMCIA, especially PCCard drivers will go here. CardBus
	      drivers will be in the pci directory, because their API is identical
	      to that of standard PCI cards. 
	      </para>
	    </section>
	
	    <section id="file-tree-oss-directory">
	      <title>oss directory</title>
	      <para>
	        The OSS/Lite source files are stored here in Linux 2.6 (or
	      later) tree. In the ALSA driver tarball, this directory is empty,
	      of course :) 
	      </para>
	    </section>
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- Basic Flow for PCI Drivers  -->
	<!-- ****************************************************** -->
	  <chapter id="basic-flow">
	    <title>Basic Flow for PCI Drivers</title>
	
	    <section id="basic-flow-outline">
	      <title>Outline</title>
	      <para>
	        The minimum flow for PCI soundcards is as follows:
	
	        <itemizedlist>
	          <listitem><para>define the PCI ID table (see the section
	          <link linkend="pci-resource-entries"><citetitle>PCI Entries
	          </citetitle></link>).</para></listitem> 
	          <listitem><para>create <function>probe()</function> callback.</para></listitem>
	          <listitem><para>create <function>remove()</function> callback.</para></listitem>
	          <listitem><para>create a <structname>pci_driver</structname> structure
		  containing the three pointers above.</para></listitem>
	          <listitem><para>create an <function>init()</function> function just calling
		  the <function>pci_register_driver()</function> to register the pci_driver table
		  defined above.</para></listitem>
	          <listitem><para>create an <function>exit()</function> function to call
		  the <function>pci_unregister_driver()</function> function.</para></listitem>
	        </itemizedlist>
	      </para>
	    </section>
	
	    <section id="basic-flow-example">
	      <title>Full Code Example</title>
	      <para>
	        The code example is shown below. Some parts are kept
	      unimplemented at this moment but will be filled in the
	      next sections. The numbers in the comment lines of the
	      <function>snd_mychip_probe()</function> function
	      refer to details explained in the following section. 
	
	        <example>
	          <title>Basic Flow for PCI Drivers - Example</title>
	          <programlisting>
	<![CDATA[
	  #include <linux/init.h>
	  #include <linux/pci.h>
	  #include <linux/slab.h>
	  #include <sound/core.h>
	  #include <sound/initval.h>
	
	  /* module parameters (see "Module Parameters") */
	  /* SNDRV_CARDS: maximum number of cards supported by this module */
	  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
	  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
	  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
	
	  /* definition of the chip-specific record */
	  struct mychip {
	          struct snd_card *card;
	          /* the rest of the implementation will be in section
	           * "PCI Resource Management"
	           */
	  };
	
	  /* chip-specific destructor
	   * (see "PCI Resource Management")
	   */
	  static int snd_mychip_free(struct mychip *chip)
	  {
	          .... /* will be implemented later... */
	  }
	
	  /* component-destructor
	   * (see "Management of Cards and Components")
	   */
	  static int snd_mychip_dev_free(struct snd_device *device)
	  {
	          return snd_mychip_free(device->device_data);
	  }
	
	  /* chip-specific constructor
	   * (see "Management of Cards and Components")
	   */
	  static int __devinit snd_mychip_create(struct snd_card *card,
	                                         struct pci_dev *pci,
	                                         struct mychip **rchip)
	  {
	          struct mychip *chip;
	          int err;
	          static struct snd_device_ops ops = {
	                 .dev_free = snd_mychip_dev_free,
	          };
	
	          *rchip = NULL;
	
	          /* check PCI availability here
	           * (see "PCI Resource Management")
	           */
	          ....
	
	          /* allocate a chip-specific data with zero filled */
	          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
	          if (chip == NULL)
	                  return -ENOMEM;
	
	          chip->card = card;
	
	          /* rest of initialization here; will be implemented
	           * later, see "PCI Resource Management"
	           */
	          ....
	
	          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
	          if (err < 0) {
	                  snd_mychip_free(chip);
	                  return err;
	          }
	
	          snd_card_set_dev(card, &pci->dev);
	
	          *rchip = chip;
	          return 0;
	  }
	
	  /* constructor -- see "Constructor" sub-section */
	  static int __devinit snd_mychip_probe(struct pci_dev *pci,
	                               const struct pci_device_id *pci_id)
	  {
	          static int dev;
	          struct snd_card *card;
	          struct mychip *chip;
	          int err;
	
	          /* (1) */
	          if (dev >= SNDRV_CARDS)
	                  return -ENODEV;
	          if (!enable[dev]) {
	                  dev++;
	                  return -ENOENT;
	          }
	
	          /* (2) */
	          err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
	          if (err < 0)
	                  return err;
	
	          /* (3) */
	          err = snd_mychip_create(card, pci, &chip);
	          if (err < 0) {
	                  snd_card_free(card);
	                  return err;
	          }
	
	          /* (4) */
	          strcpy(card->driver, "My Chip");
	          strcpy(card->shortname, "My Own Chip 123");
	          sprintf(card->longname, "%s at 0x%lx irq %i",
	                  card->shortname, chip->ioport, chip->irq);
	
	          /* (5) */
	          .... /* implemented later */
	
	          /* (6) */
	          err = snd_card_register(card);
	          if (err < 0) {
	                  snd_card_free(card);
	                  return err;
	          }
	
	          /* (7) */
	          pci_set_drvdata(pci, card);
	          dev++;
	          return 0;
	  }
	
	  /* destructor -- see the "Destructor" sub-section */
	  static void __devexit snd_mychip_remove(struct pci_dev *pci)
	  {
	          snd_card_free(pci_get_drvdata(pci));
	          pci_set_drvdata(pci, NULL);
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	    </section>
	
	    <section id="basic-flow-constructor">
	      <title>Constructor</title>
	      <para>
	        The real constructor of PCI drivers is the <function>probe</function> callback.
	      The <function>probe</function> callback and other component-constructors which are called
	      from the <function>probe</function> callback should be defined with
	      the <parameter>__devinit</parameter> prefix. You 
	      cannot use the <parameter>__init</parameter> prefix for them,
	      because any PCI device could be a hotplug device. 
	      </para>
	
	      <para>
	        In the <function>probe</function> callback, the following scheme is often used.
	      </para>
	
	      <section id="basic-flow-constructor-device-index">
	        <title>1) Check and increment the device index.</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int dev;
	  ....
	  if (dev >= SNDRV_CARDS)
	          return -ENODEV;
	  if (!enable[dev]) {
	          dev++;
	          return -ENOENT;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	
	        where enable[dev] is the module option.
	        </para>
	
	        <para>
	          Each time the <function>probe</function> callback is called, check the
	        availability of the device. If not available, simply increment
	        the device index and returns. dev will be incremented also
	        later (<link
	        linkend="basic-flow-constructor-set-pci"><citetitle>step
	        7</citetitle></link>). 
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-create-card">
	        <title>2) Create a card instance</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  struct snd_card *card;
	  int err;
	  ....
	  err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          The details will be explained in the section
	          <link linkend="card-management-card-instance"><citetitle>
	          Management of Cards and Components</citetitle></link>.
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-create-main">
	        <title>3) Create a main component</title>
	        <para>
	          In this part, the PCI resources are allocated.
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  struct mychip *chip;
	  ....
	  err = snd_mychip_create(card, pci, &chip);
	  if (err < 0) {
	          snd_card_free(card);
	          return err;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	
	          The details will be explained in the section <link
	        linkend="pci-resource"><citetitle>PCI Resource
	        Management</citetitle></link>.
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-main-component">
	        <title>4) Set the driver ID and name strings.</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  strcpy(card->driver, "My Chip");
	  strcpy(card->shortname, "My Own Chip 123");
	  sprintf(card->longname, "%s at 0x%lx irq %i",
	          card->shortname, chip->ioport, chip->irq);
	]]>
	            </programlisting>
	          </informalexample>
	
	          The driver field holds the minimal ID string of the
	        chip. This is used by alsa-lib's configurator, so keep it
	        simple but unique. 
	          Even the same driver can have different driver IDs to
	        distinguish the functionality of each chip type. 
	        </para>
	
	        <para>
	          The shortname field is a string shown as more verbose
	        name. The longname field contains the information
	        shown in <filename>/proc/asound/cards</filename>. 
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-create-other">
	        <title>5) Create other components, such as mixer, MIDI, etc.</title>
	        <para>
	          Here you define the basic components such as
	          <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>,
	          mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>),
	          MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>),
	          and other interfaces.
	          Also, if you want a <link linkend="proc-interface"><citetitle>proc
	        file</citetitle></link>, define it here, too.
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-register-card">
	        <title>6) Register the card instance.</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  err = snd_card_register(card);
	  if (err < 0) {
	          snd_card_free(card);
	          return err;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          Will be explained in the section <link
	        linkend="card-management-registration"><citetitle>Management
	        of Cards and Components</citetitle></link>, too. 
	        </para>
	      </section>
	
	      <section id="basic-flow-constructor-set-pci">
	        <title>7) Set the PCI driver data and return zero.</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	        pci_set_drvdata(pci, card);
	        dev++;
	        return 0;
	]]>
	            </programlisting>
	          </informalexample>
	
	          In the above, the card record is stored. This pointer is
	        used in the remove callback and power-management
	        callbacks, too. 
	        </para>
	      </section>
	    </section>
	
	    <section id="basic-flow-destructor">
	      <title>Destructor</title>
	      <para>
	        The destructor, remove callback, simply releases the card
	      instance. Then the ALSA middle layer will release all the
	      attached components automatically. 
	      </para>
	
	      <para>
	        It would be typically like the following:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void __devexit snd_mychip_remove(struct pci_dev *pci)
	  {
	          snd_card_free(pci_get_drvdata(pci));
	          pci_set_drvdata(pci, NULL);
	  }
	]]>
	          </programlisting>
	        </informalexample>
	
	        The above code assumes that the card pointer is set to the PCI
		driver data.
	      </para>
	    </section>
	
	    <section id="basic-flow-header-files">
	      <title>Header Files</title>
	      <para>
	        For the above example, at least the following include files
	      are necessary. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  #include <linux/init.h>
	  #include <linux/pci.h>
	  #include <linux/slab.h>
	  #include <sound/core.h>
	  #include <sound/initval.h>
	]]>
	          </programlisting>
	        </informalexample>
	
		where the last one is necessary only when module options are
	      defined in the source file.  If the code is split into several
	      files, the files without module options don't need them.
	      </para>
	
	      <para>
	        In addition to these headers, you'll need
	      <filename>&lt;linux/interrupt.h&gt;</filename> for interrupt
	      handling, and <filename>&lt;asm/io.h&gt;</filename> for I/O
	      access. If you use the <function>mdelay()</function> or
	      <function>udelay()</function> functions, you'll need to include
	      <filename>&lt;linux/delay.h&gt;</filename> too. 
	      </para>
	
	      <para>
	      The ALSA interfaces like the PCM and control APIs are defined in other
	      <filename>&lt;sound/xxx.h&gt;</filename> header files.
	      They have to be included after
	      <filename>&lt;sound/core.h&gt;</filename>.
	      </para>
	
	    </section>
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- Management of Cards and Components  -->
	<!-- ****************************************************** -->
	  <chapter id="card-management">
	    <title>Management of Cards and Components</title>
	
	    <section id="card-management-card-instance">
	      <title>Card Instance</title>
	      <para>
	      For each soundcard, a <quote>card</quote> record must be allocated.
	      </para>
	
	      <para>
	      A card record is the headquarters of the soundcard.  It manages
	      the whole list of devices (components) on the soundcard, such as
	      PCM, mixers, MIDI, synthesizer, and so on.  Also, the card
	      record holds the ID and the name strings of the card, manages
	      the root of proc files, and controls the power-management states
	      and hotplug disconnections.  The component list on the card
	      record is used to manage the correct release of resources at
	      destruction. 
	      </para>
	
	      <para>
	        As mentioned above, to create a card instance, call
	      <function>snd_card_create()</function>.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_card *card;
	  int err;
	  err = snd_card_create(index, id, module, extra_size, &card);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The function takes five arguments, the card-index number, the
	        id string, the module pointer (usually
	        <constant>THIS_MODULE</constant>),
	        the size of extra-data space, and the pointer to return the
	        card instance.  The extra_size argument is used to
	        allocate card-&gt;private_data for the
	        chip-specific data.  Note that these data
	        are allocated by <function>snd_card_create()</function>.
	      </para>
	    </section>
	
	    <section id="card-management-component">
	      <title>Components</title>
	      <para>
	        After the card is created, you can attach the components
	      (devices) to the card instance. In an ALSA driver, a component is
	      represented as a struct <structname>snd_device</structname> object.
	      A component can be a PCM instance, a control interface, a raw
	      MIDI interface, etc.  Each such instance has one component
	      entry.
	      </para>
	
	      <para>
	        A component can be created via
	        <function>snd_device_new()</function> function. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        This takes the card pointer, the device-level
	      (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the
	      callback pointers (<parameter>&amp;ops</parameter>). The
	      device-level defines the type of components and the order of
	      registration and de-registration.  For most components, the
	      device-level is already defined.  For a user-defined component,
	      you can use <constant>SNDRV_DEV_LOWLEVEL</constant>.
	      </para>
	
	      <para>
	      This function itself doesn't allocate the data space. The data
	      must be allocated manually beforehand, and its pointer is passed
	      as the argument. This pointer is used as the
	      (<parameter>chip</parameter> identifier in the above example)
	      for the instance. 
	      </para>
	
	      <para>
	        Each pre-defined ALSA component such as ac97 and pcm calls
	      <function>snd_device_new()</function> inside its
	      constructor. The destructor for each component is defined in the
	      callback pointers.  Hence, you don't need to take care of
	      calling a destructor for such a component.
	      </para>
	
	      <para>
	        If you wish to create your own component, you need to
	      set the destructor function to the dev_free callback in
	      the <parameter>ops</parameter>, so that it can be released
	      automatically via <function>snd_card_free()</function>.
	      The next example will show an implementation of chip-specific
	      data.
	      </para>
	    </section>
	
	    <section id="card-management-chip-specific">
	      <title>Chip-Specific Data</title>
	      <para>
	      Chip-specific information, e.g. the I/O port address, its
	      resource pointer, or the irq number, is stored in the
	      chip-specific record.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct mychip {
	          ....
	  };
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        In general, there are two ways of allocating the chip record.
	      </para>
	
	      <section id="card-management-chip-specific-snd-card-new">
	        <title>1. Allocating via <function>snd_card_create()</function>.</title>
	        <para>
	          As mentioned above, you can pass the extra-data-length
		  to the 4th argument of <function>snd_card_create()</function>, i.e.
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  err = snd_card_create(index[dev], id[dev], THIS_MODULE,
	                        sizeof(struct mychip), &card);
	]]>
	            </programlisting>
	          </informalexample>
	
	          struct <structname>mychip</structname> is the type of the chip record.
	        </para>
	
	        <para>
	          In return, the allocated record can be accessed as
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  struct mychip *chip = card->private_data;
	]]>
	            </programlisting>
	          </informalexample>
	
	          With this method, you don't have to allocate twice.
	          The record is released together with the card instance.
	        </para>
	      </section>
	
	      <section id="card-management-chip-specific-allocate-extra">
	        <title>2. Allocating an extra device.</title>
	
	        <para>
	          After allocating a card instance via
	          <function>snd_card_create()</function> (with
	          <constant>0</constant> on the 4th arg), call
	          <function>kzalloc()</function>. 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  struct snd_card *card;
	  struct mychip *chip;
	  err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
	  .....
	  chip = kzalloc(sizeof(*chip), GFP_KERNEL);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          The chip record should have the field to hold the card
	          pointer at least, 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  struct mychip {
	          struct snd_card *card;
	          ....
	  };
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          Then, set the card pointer in the returned chip instance.
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  chip->card = card;
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          Next, initialize the fields, and register this chip
	          record as a low-level device with a specified
	          <parameter>ops</parameter>, 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static struct snd_device_ops ops = {
	          .dev_free =        snd_mychip_dev_free,
	  };
	  ....
	  snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
	]]>
	            </programlisting>
	          </informalexample>
	
	          <function>snd_mychip_dev_free()</function> is the
	        device-destructor function, which will call the real
	        destructor. 
	        </para>
	
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_mychip_dev_free(struct snd_device *device)
	  {
	          return snd_mychip_free(device->device_data);
	  }
	]]>
	            </programlisting>
	          </informalexample>
	
	          where <function>snd_mychip_free()</function> is the real destructor.
	        </para>
	      </section>
	    </section>
	
	    <section id="card-management-registration">
	      <title>Registration and Release</title>
	      <para>
	        After all components are assigned, register the card instance
	      by calling <function>snd_card_register()</function>. Access
	      to the device files is enabled at this point. That is, before
	      <function>snd_card_register()</function> is called, the
	      components are safely inaccessible from external side. If this
	      call fails, exit the probe function after releasing the card via
	      <function>snd_card_free()</function>. 
	      </para>
	
	      <para>
	        For releasing the card instance, you can call simply
	      <function>snd_card_free()</function>. As mentioned earlier, all
	      components are released automatically by this call. 
	      </para>
	
	      <para>
	        As further notes, the destructors (both
	      <function>snd_mychip_dev_free</function> and
	      <function>snd_mychip_free</function>) cannot be defined with
	      the <parameter>__devexit</parameter> prefix, because they may be
	      called from the constructor, too, at the false path. 
	      </para>
	
	      <para>
	      For a device which allows hotplugging, you can use
	      <function>snd_card_free_when_closed</function>.  This one will
	      postpone the destruction until all devices are closed.
	      </para>
	
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- PCI Resource Management  -->
	<!-- ****************************************************** -->
	  <chapter id="pci-resource">
	    <title>PCI Resource Management</title>
	
	    <section id="pci-resource-example">
	      <title>Full Code Example</title>
	      <para>
	        In this section, we'll complete the chip-specific constructor,
	      destructor and PCI entries. Example code is shown first,
	      below. 
	
	        <example>
	          <title>PCI Resource Management Example</title>
	          <programlisting>
	<![CDATA[
	  struct mychip {
	          struct snd_card *card;
	          struct pci_dev *pci;
	
	          unsigned long port;
	          int irq;
	  };
	
	  static int snd_mychip_free(struct mychip *chip)
	  {
	          /* disable hardware here if any */
	          .... /* (not implemented in this document) */
	
	          /* release the irq */
	          if (chip->irq >= 0)
	                  free_irq(chip->irq, chip);
	          /* release the I/O ports & memory */
	          pci_release_regions(chip->pci);
	          /* disable the PCI entry */
	          pci_disable_device(chip->pci);
	          /* release the data */
	          kfree(chip);
	          return 0;
	  }
	
	  /* chip-specific constructor */
	  static int __devinit snd_mychip_create(struct snd_card *card,
	                                         struct pci_dev *pci,
	                                         struct mychip **rchip)
	  {
	          struct mychip *chip;
	          int err;
	          static struct snd_device_ops ops = {
	                 .dev_free = snd_mychip_dev_free,
	          };
	
	          *rchip = NULL;
	
	          /* initialize the PCI entry */
	          err = pci_enable_device(pci);
	          if (err < 0)
	                  return err;
	          /* check PCI availability (28bit DMA) */
	          if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
	              pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
	                  printk(KERN_ERR "error to set 28bit mask DMA\n");
	                  pci_disable_device(pci);
	                  return -ENXIO;
	          }
	
	          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
	          if (chip == NULL) {
	                  pci_disable_device(pci);
	                  return -ENOMEM;
	          }
	
	          /* initialize the stuff */
	          chip->card = card;
	          chip->pci = pci;
	          chip->irq = -1;
	
	          /* (1) PCI resource allocation */
	          err = pci_request_regions(pci, "My Chip");
	          if (err < 0) {
	                  kfree(chip);
	                  pci_disable_device(pci);
	                  return err;
	          }
	          chip->port = pci_resource_start(pci, 0);
	          if (request_irq(pci->irq, snd_mychip_interrupt,
	                          IRQF_SHARED, KBUILD_MODNAME, chip)) {
	                  printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
	                  snd_mychip_free(chip);
	                  return -EBUSY;
	          }
	          chip->irq = pci->irq;
	
	          /* (2) initialization of the chip hardware */
	          .... /*   (not implemented in this document) */
	
	          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
	          if (err < 0) {
	                  snd_mychip_free(chip);
	                  return err;
	          }
	
	          snd_card_set_dev(card, &pci->dev);
	
	          *rchip = chip;
	          return 0;
	  }        
	
	  /* PCI IDs */
	  static struct pci_device_id snd_mychip_ids[] = {
	          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
	            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
	          ....
	          { 0, }
	  };
	  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
	
	  /* pci_driver definition */
	  static struct pci_driver driver = {
	          .name = KBUILD_MODNAME,
	          .id_table = snd_mychip_ids,
	          .probe = snd_mychip_probe,
	          .remove = __devexit_p(snd_mychip_remove),
	  };
	
	  /* module initialization */
	  static int __init alsa_card_mychip_init(void)
	  {
	          return pci_register_driver(&driver);
	  }
	
	  /* module clean up */
	  static void __exit alsa_card_mychip_exit(void)
	  {
	          pci_unregister_driver(&driver);
	  }
	
	  module_init(alsa_card_mychip_init)
	  module_exit(alsa_card_mychip_exit)
	
	  EXPORT_NO_SYMBOLS; /* for old kernels only */
	]]>
	          </programlisting>
	        </example>
	      </para>
	    </section>
	
	    <section id="pci-resource-some-haftas">
	      <title>Some Hafta's</title>
	      <para>
	        The allocation of PCI resources is done in the
	      <function>probe()</function> function, and usually an extra
	      <function>xxx_create()</function> function is written for this
	      purpose.
	      </para>
	
	      <para>
	        In the case of PCI devices, you first have to call
	      the <function>pci_enable_device()</function> function before
	      allocating resources. Also, you need to set the proper PCI DMA
	      mask to limit the accessed I/O range. In some cases, you might
	      need to call <function>pci_set_master()</function> function,
	      too.
	      </para>
	
	      <para>
	        Suppose the 28bit mask, and the code to be added would be like:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  err = pci_enable_device(pci);
	  if (err < 0)
	          return err;
	  if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
	      pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
	          printk(KERN_ERR "error to set 28bit mask DMA\n");
	          pci_disable_device(pci);
	          return -ENXIO;
	  }
	  
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	    </section>
	
	    <section id="pci-resource-resource-allocation">
	      <title>Resource Allocation</title>
	      <para>
	        The allocation of I/O ports and irqs is done via standard kernel
	      functions. Unlike ALSA ver.0.5.x., there are no helpers for
	      that. And these resources must be released in the destructor
	      function (see below). Also, on ALSA 0.9.x, you don't need to
	      allocate (pseudo-)DMA for PCI like in ALSA 0.5.x.
	      </para>
	
	      <para>
	        Now assume that the PCI device has an I/O port with 8 bytes
	        and an interrupt. Then struct <structname>mychip</structname> will have the
	        following fields:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct mychip {
	          struct snd_card *card;
	
	          unsigned long port;
	          int irq;
	  };
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        For an I/O port (and also a memory region), you need to have
	      the resource pointer for the standard resource management. For
	      an irq, you have to keep only the irq number (integer). But you
	      need to initialize this number as -1 before actual allocation,
	      since irq 0 is valid. The port address and its resource pointer
	      can be initialized as null by
	      <function>kzalloc()</function> automatically, so you
	      don't have to take care of resetting them. 
	      </para>
	
	      <para>
	        The allocation of an I/O port is done like this:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  err = pci_request_regions(pci, "My Chip");
	  if (err < 0) { 
	          kfree(chip);
	          pci_disable_device(pci);
	          return err;
	  }
	  chip->port = pci_resource_start(pci, 0);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        <!-- obsolete -->
	        It will reserve the I/O port region of 8 bytes of the given
	      PCI device. The returned value, chip-&gt;res_port, is allocated
	      via <function>kmalloc()</function> by
	      <function>request_region()</function>. The pointer must be
	      released via <function>kfree()</function>, but there is a
	      problem with this. This issue will be explained later.
	      </para>
	
	      <para>
	        The allocation of an interrupt source is done like this:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  if (request_irq(pci->irq, snd_mychip_interrupt,
	                  IRQF_SHARED, KBUILD_MODNAME, chip)) {
	          printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
	          snd_mychip_free(chip);
	          return -EBUSY;
	  }
	  chip->irq = pci->irq;
	]]>
	          </programlisting>
	        </informalexample>
	
	        where <function>snd_mychip_interrupt()</function> is the
	      interrupt handler defined <link
	      linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>.
	      Note that chip-&gt;irq should be defined
	      only when <function>request_irq()</function> succeeded.
	      </para>
	
	      <para>
	      On the PCI bus, interrupts can be shared. Thus,
	      <constant>IRQF_SHARED</constant> is used as the interrupt flag of
	      <function>request_irq()</function>. 
	      </para>
	
	      <para>
	        The last argument of <function>request_irq()</function> is the
	      data pointer passed to the interrupt handler. Usually, the
	      chip-specific record is used for that, but you can use what you
	      like, too. 
	      </para>
	
	      <para>
	        I won't give details about the interrupt handler at this
	        point, but at least its appearance can be explained now. The
	        interrupt handler looks usually like the following: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
	  {
	          struct mychip *chip = dev_id;
	          ....
	          return IRQ_HANDLED;
	  }
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        Now let's write the corresponding destructor for the resources
	      above. The role of destructor is simple: disable the hardware
	      (if already activated) and release the resources. So far, we
	      have no hardware part, so the disabling code is not written here. 
	      </para>
	
	      <para>
	        To release the resources, the <quote>check-and-release</quote>
	        method is a safer way. For the interrupt, do like this: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  if (chip->irq >= 0)
	          free_irq(chip->irq, chip);
	]]>
	          </programlisting>
	        </informalexample>
	
	        Since the irq number can start from 0, you should initialize
	        chip-&gt;irq with a negative value (e.g. -1), so that you can
	        check the validity of the irq number as above.
	      </para>
	
	      <para>
	        When you requested I/O ports or memory regions via
		<function>pci_request_region()</function> or
		<function>pci_request_regions()</function> like in this example,
		release the resource(s) using the corresponding function,
		<function>pci_release_region()</function> or
		<function>pci_release_regions()</function>.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  pci_release_regions(chip->pci);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
		When you requested manually via <function>request_region()</function>
		or <function>request_mem_region</function>, you can release it via
		<function>release_resource()</function>.  Suppose that you keep
		the resource pointer returned from <function>request_region()</function>
		in chip-&gt;res_port, the release procedure looks like:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  release_and_free_resource(chip->res_port);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	      Don't forget to call <function>pci_disable_device()</function>
	      before the end.
	      </para>
	
	      <para>
	        And finally, release the chip-specific record.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  kfree(chip);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	      Again, remember that you cannot
	      use the <parameter>__devexit</parameter> prefix for this destructor. 
	      </para>
	
	      <para>
	      We didn't implement the hardware disabling part in the above.
	      If you need to do this, please note that the destructor may be
	      called even before the initialization of the chip is completed.
	      It would be better to have a flag to skip hardware disabling
	      if the hardware was not initialized yet.
	      </para>
	
	      <para>
	      When the chip-data is assigned to the card using
	      <function>snd_device_new()</function> with
	      <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is 
	      called at the last.  That is, it is assured that all other
	      components like PCMs and controls have already been released.
	      You don't have to stop PCMs, etc. explicitly, but just
	      call low-level hardware stopping.
	      </para>
	
	      <para>
	        The management of a memory-mapped region is almost as same as
	        the management of an I/O port. You'll need three fields like
	        the following: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct mychip {
	          ....
	          unsigned long iobase_phys;
	          void __iomem *iobase_virt;
	  };
	]]>
	          </programlisting>
	        </informalexample>
	
	        and the allocation would be like below:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  if ((err = pci_request_regions(pci, "My Chip")) < 0) {
	          kfree(chip);
	          return err;
	  }
	  chip->iobase_phys = pci_resource_start(pci, 0);
	  chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
	                                      pci_resource_len(pci, 0));
	]]>
	          </programlisting>
	        </informalexample>
	        
	        and the corresponding destructor would be:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int snd_mychip_free(struct mychip *chip)
	  {
	          ....
	          if (chip->iobase_virt)
	                  iounmap(chip->iobase_virt);
	          ....
	          pci_release_regions(chip->pci);
	          ....
	  }
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	    </section>
	
	    <section id="pci-resource-device-struct">
	      <title>Registration of Device Struct</title>
	      <para>
		At some point, typically after calling <function>snd_device_new()</function>,
		you need to register the struct <structname>device</structname> of the chip
		you're handling for udev and co.  ALSA provides a macro for compatibility with
		older kernels.  Simply call like the following:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_card_set_dev(card, &pci->dev);
	]]>
	          </programlisting>
	        </informalexample>
		so that it stores the PCI's device pointer to the card.  This will be
		referred by ALSA core functions later when the devices are registered.
	      </para>
	      <para>
		In the case of non-PCI, pass the proper device struct pointer of the BUS
		instead.  (In the case of legacy ISA without PnP, you don't have to do
		anything.)
	      </para>
	    </section>
	
	    <section id="pci-resource-entries">
	      <title>PCI Entries</title>
	      <para>
	        So far, so good. Let's finish the missing PCI
	      stuff. At first, we need a
	      <structname>pci_device_id</structname> table for this
	      chipset. It's a table of PCI vendor/device ID number, and some
	      masks. 
	      </para>
	
	      <para>
	        For example,
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static struct pci_device_id snd_mychip_ids[] = {
	          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
	            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
	          ....
	          { 0, }
	  };
	  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The first and second fields of
	      the <structname>pci_device_id</structname> structure are the vendor and
	      device IDs. If you have no reason to filter the matching
	      devices, you can leave the remaining fields as above. The last
	      field of the <structname>pci_device_id</structname> struct contains
	      private data for this entry. You can specify any value here, for
	      example, to define specific operations for supported device IDs.
	      Such an example is found in the intel8x0 driver. 
	      </para>
	
	      <para>
	        The last entry of this list is the terminator. You must
	      specify this all-zero entry. 
	      </para>
	
	      <para>
	        Then, prepare the <structname>pci_driver</structname> record:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static struct pci_driver driver = {
	          .name = KBUILD_MODNAME,
	          .id_table = snd_mychip_ids,
	          .probe = snd_mychip_probe,
	          .remove = __devexit_p(snd_mychip_remove),
	  };
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The <structfield>probe</structfield> and
	      <structfield>remove</structfield> functions have already
	      been defined in the previous sections.
	      The <structfield>remove</structfield> function should
	      be defined with the 
	      <function>__devexit_p()</function> macro, so that it's not
	      defined for built-in (and non-hot-pluggable) case. The
	      <structfield>name</structfield> 
	      field is the name string of this device. Note that you must not
	      use a slash <quote>/</quote> in this string. 
	      </para>
	
	      <para>
	        And at last, the module entries:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int __init alsa_card_mychip_init(void)
	  {
	          return pci_register_driver(&driver);
	  }
	
	  static void __exit alsa_card_mychip_exit(void)
	  {
	          pci_unregister_driver(&driver);
	  }
	
	  module_init(alsa_card_mychip_init)
	  module_exit(alsa_card_mychip_exit)
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        Note that these module entries are tagged with
	      <parameter>__init</parameter> and 
	      <parameter>__exit</parameter> prefixes, not
	      <parameter>__devinit</parameter> nor
	      <parameter>__devexit</parameter>.
	      </para>
	
	      <para>
	        Oh, one thing was forgotten. If you have no exported symbols,
	        you need to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  EXPORT_NO_SYMBOLS;
	]]>
	          </programlisting>
	        </informalexample>
	
	        That's all!
	      </para>
	    </section>
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- PCM Interface  -->
	<!-- ****************************************************** -->
	  <chapter id="pcm-interface">
	    <title>PCM Interface</title>
	
	    <section id="pcm-interface-general">
	      <title>General</title>
	      <para>
	        The PCM middle layer of ALSA is quite powerful and it is only
	      necessary for each driver to implement the low-level functions
	      to access its hardware.
	      </para>
	
	      <para>
	        For accessing to the PCM layer, you need to include
	      <filename>&lt;sound/pcm.h&gt;</filename> first. In addition,
	      <filename>&lt;sound/pcm_params.h&gt;</filename> might be needed
	      if you access to some functions related with hw_param. 
	      </para>
	
	      <para>
	        Each card device can have up to four pcm instances. A pcm
	      instance corresponds to a pcm device file. The limitation of
	      number of instances comes only from the available bit size of
	      the Linux's device numbers. Once when 64bit device number is
	      used, we'll have more pcm instances available. 
	      </para>
	
	      <para>
	        A pcm instance consists of pcm playback and capture streams,
	      and each pcm stream consists of one or more pcm substreams. Some
	      soundcards support multiple playback functions. For example,
	      emu10k1 has a PCM playback of 32 stereo substreams. In this case, at
	      each open, a free substream is (usually) automatically chosen
	      and opened. Meanwhile, when only one substream exists and it was
	      already opened, the successful open will either block
	      or error with <constant>EAGAIN</constant> according to the
	      file open mode. But you don't have to care about such details in your
	      driver. The PCM middle layer will take care of such work.
	      </para>
	    </section>
	
	    <section id="pcm-interface-example">
	      <title>Full Code Example</title>
	      <para>
	      The example code below does not include any hardware access
	      routines but shows only the skeleton, how to build up the PCM
	      interfaces.
	
	        <example>
	          <title>PCM Example Code</title>
	          <programlisting>
	<![CDATA[
	  #include <sound/pcm.h>
	  ....
	
	  /* hardware definition */
	  static struct snd_pcm_hardware snd_mychip_playback_hw = {
	          .info = (SNDRV_PCM_INFO_MMAP |
	                   SNDRV_PCM_INFO_INTERLEAVED |
	                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
	                   SNDRV_PCM_INFO_MMAP_VALID),
	          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
	          .rates =            SNDRV_PCM_RATE_8000_48000,
	          .rate_min =         8000,
	          .rate_max =         48000,
	          .channels_min =     2,
	          .channels_max =     2,
	          .buffer_bytes_max = 32768,
	          .period_bytes_min = 4096,
	          .period_bytes_max = 32768,
	          .periods_min =      1,
	          .periods_max =      1024,
	  };
	
	  /* hardware definition */
	  static struct snd_pcm_hardware snd_mychip_capture_hw = {
	          .info = (SNDRV_PCM_INFO_MMAP |
	                   SNDRV_PCM_INFO_INTERLEAVED |
	                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
	                   SNDRV_PCM_INFO_MMAP_VALID),
	          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
	          .rates =            SNDRV_PCM_RATE_8000_48000,
	          .rate_min =         8000,
	          .rate_max =         48000,
	          .channels_min =     2,
	          .channels_max =     2,
	          .buffer_bytes_max = 32768,
	          .period_bytes_min = 4096,
	          .period_bytes_max = 32768,
	          .periods_min =      1,
	          .periods_max =      1024,
	  };
	
	  /* open callback */
	  static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          struct snd_pcm_runtime *runtime = substream->runtime;
	
	          runtime->hw = snd_mychip_playback_hw;
	          /* more hardware-initialization will be done here */
	          ....
	          return 0;
	  }
	
	  /* close callback */
	  static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          /* the hardware-specific codes will be here */
	          ....
	          return 0;
	
	  }
	
	  /* open callback */
	  static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          struct snd_pcm_runtime *runtime = substream->runtime;
	
	          runtime->hw = snd_mychip_capture_hw;
	          /* more hardware-initialization will be done here */
	          ....
	          return 0;
	  }
	
	  /* close callback */
	  static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          /* the hardware-specific codes will be here */
	          ....
	          return 0;
	
	  }
	
	  /* hw_params callback */
	  static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
	                               struct snd_pcm_hw_params *hw_params)
	  {
	          return snd_pcm_lib_malloc_pages(substream,
	                                     params_buffer_bytes(hw_params));
	  }
	
	  /* hw_free callback */
	  static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
	  {
	          return snd_pcm_lib_free_pages(substream);
	  }
	
	  /* prepare callback */
	  static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          struct snd_pcm_runtime *runtime = substream->runtime;
	
	          /* set up the hardware with the current configuration
	           * for example...
	           */
	          mychip_set_sample_format(chip, runtime->format);
	          mychip_set_sample_rate(chip, runtime->rate);
	          mychip_set_channels(chip, runtime->channels);
	          mychip_set_dma_setup(chip, runtime->dma_addr,
	                               chip->buffer_size,
	                               chip->period_size);
	          return 0;
	  }
	
	  /* trigger callback */
	  static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
	                                    int cmd)
	  {
	          switch (cmd) {
	          case SNDRV_PCM_TRIGGER_START:
	                  /* do something to start the PCM engine */
	                  ....
	                  break;
	          case SNDRV_PCM_TRIGGER_STOP:
	                  /* do something to stop the PCM engine */
	                  ....
	                  break;
	          default:
	                  return -EINVAL;
	          }
	  }
	
	  /* pointer callback */
	  static snd_pcm_uframes_t
	  snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          unsigned int current_ptr;
	
	          /* get the current hardware pointer */
	          current_ptr = mychip_get_hw_pointer(chip);
	          return current_ptr;
	  }
	
	  /* operators */
	  static struct snd_pcm_ops snd_mychip_playback_ops = {
	          .open =        snd_mychip_playback_open,
	          .close =       snd_mychip_playback_close,
	          .ioctl =       snd_pcm_lib_ioctl,
	          .hw_params =   snd_mychip_pcm_hw_params,
	          .hw_free =     snd_mychip_pcm_hw_free,
	          .prepare =     snd_mychip_pcm_prepare,
	          .trigger =     snd_mychip_pcm_trigger,
	          .pointer =     snd_mychip_pcm_pointer,
	  };
	
	  /* operators */
	  static struct snd_pcm_ops snd_mychip_capture_ops = {
	          .open =        snd_mychip_capture_open,
	          .close =       snd_mychip_capture_close,
	          .ioctl =       snd_pcm_lib_ioctl,
	          .hw_params =   snd_mychip_pcm_hw_params,
	          .hw_free =     snd_mychip_pcm_hw_free,
	          .prepare =     snd_mychip_pcm_prepare,
	          .trigger =     snd_mychip_pcm_trigger,
	          .pointer =     snd_mychip_pcm_pointer,
	  };
	
	  /*
	   *  definitions of capture are omitted here...
	   */
	
	  /* create a pcm device */
	  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
	  {
	          struct snd_pcm *pcm;
	          int err;
	
	          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
	          if (err < 0) 
	                  return err;
	          pcm->private_data = chip;
	          strcpy(pcm->name, "My Chip");
	          chip->pcm = pcm;
	          /* set operators */
	          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
	                          &snd_mychip_playback_ops);
	          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
	                          &snd_mychip_capture_ops);
	          /* pre-allocation of buffers */
	          /* NOTE: this may fail */
	          snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
	                                                snd_dma_pci_data(chip->pci),
	                                                64*1024, 64*1024);
	          return 0;
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	    </section>
	
	    <section id="pcm-interface-constructor">
	      <title>Constructor</title>
	      <para>
	        A pcm instance is allocated by the <function>snd_pcm_new()</function>
	      function. It would be better to create a constructor for pcm,
	      namely, 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
	  {
	          struct snd_pcm *pcm;
	          int err;
	
	          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
	          if (err < 0) 
	                  return err;
	          pcm->private_data = chip;
	          strcpy(pcm->name, "My Chip");
	          chip->pcm = pcm;
		  ....
	          return 0;
	  }
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The <function>snd_pcm_new()</function> function takes four
	      arguments. The first argument is the card pointer to which this
	      pcm is assigned, and the second is the ID string. 
	      </para>
	
	      <para>
	        The third argument (<parameter>index</parameter>, 0 in the
	      above) is the index of this new pcm. It begins from zero. If
	      you create more than one pcm instances, specify the
	      different numbers in this argument. For example,
	      <parameter>index</parameter> = 1 for the second PCM device.  
	      </para>
	
	      <para>
	        The fourth and fifth arguments are the number of substreams
	      for playback and capture, respectively. Here 1 is used for
	      both arguments. When no playback or capture substreams are available,
	      pass 0 to the corresponding argument.
	      </para>
	
	      <para>
	        If a chip supports multiple playbacks or captures, you can
	      specify more numbers, but they must be handled properly in
	      open/close, etc. callbacks.  When you need to know which
	      substream you are referring to, then it can be obtained from
	      struct <structname>snd_pcm_substream</structname> data passed to each callback
	      as follows: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_pcm_substream *substream;
	  int index = substream->number;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        After the pcm is created, you need to set operators for each
	        pcm stream. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
	                  &snd_mychip_playback_ops);
	  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
	                  &snd_mychip_capture_ops);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The operators are defined typically like this:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static struct snd_pcm_ops snd_mychip_playback_ops = {
	          .open =        snd_mychip_pcm_open,
	          .close =       snd_mychip_pcm_close,
	          .ioctl =       snd_pcm_lib_ioctl,
	          .hw_params =   snd_mychip_pcm_hw_params,
	          .hw_free =     snd_mychip_pcm_hw_free,
	          .prepare =     snd_mychip_pcm_prepare,
	          .trigger =     snd_mychip_pcm_trigger,
	          .pointer =     snd_mychip_pcm_pointer,
	  };
	]]>
	          </programlisting>
	        </informalexample>
	
	        All the callbacks are described in the
	        <link linkend="pcm-interface-operators"><citetitle>
	        Operators</citetitle></link> subsection.
	      </para>
	
	      <para>
	        After setting the operators, you probably will want to
	        pre-allocate the buffer. For the pre-allocation, simply call
	        the following: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
	                                        snd_dma_pci_data(chip->pci),
	                                        64*1024, 64*1024);
	]]>
	          </programlisting>
	        </informalexample>
	
	        It will allocate a buffer up to 64kB as default.
	      Buffer management details will be described in the later section <link
	      linkend="buffer-and-memory"><citetitle>Buffer and Memory
	      Management</citetitle></link>. 
	      </para>
	
	      <para>
	        Additionally, you can set some extra information for this pcm
	        in pcm-&gt;info_flags.
	        The available values are defined as
	        <constant>SNDRV_PCM_INFO_XXX</constant> in
	        <filename>&lt;sound/asound.h&gt;</filename>, which is used for
	        the hardware definition (described later). When your soundchip
	        supports only half-duplex, specify like this: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	    </section>
	
	    <section id="pcm-interface-destructor">
	      <title>... And the Destructor?</title>
	      <para>
	        The destructor for a pcm instance is not always
	      necessary. Since the pcm device will be released by the middle
	      layer code automatically, you don't have to call the destructor
	      explicitly.
	      </para>
	
	      <para>
	        The destructor would be necessary if you created
	        special records internally and needed to release them. In such a
	        case, set the destructor function to
	        pcm-&gt;private_free: 
	
	        <example>
	          <title>PCM Instance with a Destructor</title>
	          <programlisting>
	<![CDATA[
	  static void mychip_pcm_free(struct snd_pcm *pcm)
	  {
	          struct mychip *chip = snd_pcm_chip(pcm);
	          /* free your own data */
	          kfree(chip->my_private_pcm_data);
	          /* do what you like else */
	          ....
	  }
	
	  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
	  {
	          struct snd_pcm *pcm;
	          ....
	          /* allocate your own data */
	          chip->my_private_pcm_data = kmalloc(...);
	          /* set the destructor */
	          pcm->private_data = chip;
	          pcm->private_free = mychip_pcm_free;
	          ....
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	    </section>
	
	    <section id="pcm-interface-runtime">
	      <title>Runtime Pointer - The Chest of PCM Information</title>
		<para>
		  When the PCM substream is opened, a PCM runtime instance is
		allocated and assigned to the substream. This pointer is
		accessible via <constant>substream-&gt;runtime</constant>.
		This runtime pointer holds most information you need
		to control the PCM: the copy of hw_params and sw_params configurations, the buffer
		pointers, mmap records, spinlocks, etc.
		</para>
	
		<para>
		The definition of runtime instance is found in
		<filename>&lt;sound/pcm.h&gt;</filename>.  Here are
	       the contents of this file:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	struct _snd_pcm_runtime {
		/* -- Status -- */
		struct snd_pcm_substream *trigger_master;
		snd_timestamp_t trigger_tstamp;	/* trigger timestamp */
		int overrange;
		snd_pcm_uframes_t avail_max;
		snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
		snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
	
		/* -- HW params -- */
		snd_pcm_access_t access;	/* access mode */
		snd_pcm_format_t format;	/* SNDRV_PCM_FORMAT_* */
		snd_pcm_subformat_t subformat;	/* subformat */
		unsigned int rate;		/* rate in Hz */
		unsigned int channels;		/* channels */
		snd_pcm_uframes_t period_size;	/* period size */
		unsigned int periods;		/* periods */
		snd_pcm_uframes_t buffer_size;	/* buffer size */
		unsigned int tick_time;		/* tick time */
		snd_pcm_uframes_t min_align;	/* Min alignment for the format */
		size_t byte_align;
		unsigned int frame_bits;
		unsigned int sample_bits;
		unsigned int info;
		unsigned int rate_num;
		unsigned int rate_den;
	
		/* -- SW params -- */
		struct timespec tstamp_mode;	/* mmap timestamp is updated */
	  	unsigned int period_step;
		unsigned int sleep_min;		/* min ticks to sleep */
		snd_pcm_uframes_t start_threshold;
		snd_pcm_uframes_t stop_threshold;
		snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
							noise is nearest than this */
		snd_pcm_uframes_t silence_size;	/* Silence filling size */
		snd_pcm_uframes_t boundary;	/* pointers wrap point */
	
		snd_pcm_uframes_t silenced_start;
		snd_pcm_uframes_t silenced_size;
	
		snd_pcm_sync_id_t sync;		/* hardware synchronization ID */
	
		/* -- mmap -- */
		volatile struct snd_pcm_mmap_status *status;
		volatile struct snd_pcm_mmap_control *control;
		atomic_t mmap_count;
	
		/* -- locking / scheduling -- */
		spinlock_t lock;
		wait_queue_head_t sleep;
		struct timer_list tick_timer;
		struct fasync_struct *fasync;
	
		/* -- private section -- */
		void *private_data;
		void (*private_free)(struct snd_pcm_runtime *runtime);
	
		/* -- hardware description -- */
		struct snd_pcm_hardware hw;
		struct snd_pcm_hw_constraints hw_constraints;
	
		/* -- interrupt callbacks -- */
		void (*transfer_ack_begin)(struct snd_pcm_substream *substream);
		void (*transfer_ack_end)(struct snd_pcm_substream *substream);
	
		/* -- timer -- */
		unsigned int timer_resolution;	/* timer resolution */
	
		/* -- DMA -- */           
		unsigned char *dma_area;	/* DMA area */
		dma_addr_t dma_addr;		/* physical bus address (not accessible from main CPU) */
		size_t dma_bytes;		/* size of DMA area */
	
		struct snd_dma_buffer *dma_buffer_p;	/* allocated buffer */
	
	#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
		/* -- OSS things -- */
		struct snd_pcm_oss_runtime oss;
	#endif
	};
	]]>
	            </programlisting>
	          </informalexample>
		</para>
	
		<para>
		  For the operators (callbacks) of each sound driver, most of
		these records are supposed to be read-only.  Only the PCM
		middle-layer changes / updates them.  The exceptions are
		the hardware description (hw), interrupt callbacks
		(transfer_ack_xxx), DMA buffer information, and the private
		data.  Besides, if you use the standard buffer allocation
		method via <function>snd_pcm_lib_malloc_pages()</function>,
		you don't need to set the DMA buffer information by yourself.
		</para>
	
		<para>
		In the sections below, important records are explained.
		</para>
	
		<section id="pcm-interface-runtime-hw">
		<title>Hardware Description</title>
		<para>
		  The hardware descriptor (struct <structname>snd_pcm_hardware</structname>)
		contains the definitions of the fundamental hardware
		configuration.  Above all, you'll need to define this in
		<link linkend="pcm-interface-operators-open-callback"><citetitle>
		the open callback</citetitle></link>.
		Note that the runtime instance holds the copy of the
		descriptor, not the pointer to the existing descriptor.  That
		is, in the open callback, you can modify the copied descriptor
		(<constant>runtime-&gt;hw</constant>) as you need.  For example, if the maximum
		number of channels is 1 only on some chip models, you can
		still use the same hardware descriptor and change the
		channels_max later:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	          struct snd_pcm_runtime *runtime = substream->runtime;
	          ...
	          runtime->hw = snd_mychip_playback_hw; /* common definition */
	          if (chip->model == VERY_OLD_ONE)
	                  runtime->hw.channels_max = 1;
	]]>
	            </programlisting>
	          </informalexample>
		</para>
	
		<para>
		  Typically, you'll have a hardware descriptor as below:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static struct snd_pcm_hardware snd_mychip_playback_hw = {
	          .info = (SNDRV_PCM_INFO_MMAP |
	                   SNDRV_PCM_INFO_INTERLEAVED |
	                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
	                   SNDRV_PCM_INFO_MMAP_VALID),
	          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
	          .rates =            SNDRV_PCM_RATE_8000_48000,
	          .rate_min =         8000,
	          .rate_max =         48000,
	          .channels_min =     2,
	          .channels_max =     2,
	          .buffer_bytes_max = 32768,
	          .period_bytes_min = 4096,
	          .period_bytes_max = 32768,
	          .periods_min =      1,
	          .periods_max =      1024,
	  };
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
		<itemizedlist>
		<listitem><para>
	          The <structfield>info</structfield> field contains the type and
	        capabilities of this pcm. The bit flags are defined in
	        <filename>&lt;sound/asound.h&gt;</filename> as
	        <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you
	        have to specify whether the mmap is supported and which
	        interleaved format is supported.
	        When the is supported, add the
	        <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the
	        hardware supports the interleaved or the non-interleaved
	        formats, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or
	        <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must
	        be set, respectively. If both are supported, you can set both,
	        too. 
	        </para>
	
	        <para>
	          In the above example, <constant>MMAP_VALID</constant> and
	        <constant>BLOCK_TRANSFER</constant> are specified for the OSS mmap
	        mode. Usually both are set. Of course,
	        <constant>MMAP_VALID</constant> is set only if the mmap is
	        really supported. 
	        </para>
	
	        <para>
	          The other possible flags are
	        <constant>SNDRV_PCM_INFO_PAUSE</constant> and
	        <constant>SNDRV_PCM_INFO_RESUME</constant>. The
	        <constant>PAUSE</constant> bit means that the pcm supports the
	        <quote>pause</quote> operation, while the
	        <constant>RESUME</constant> bit means that the pcm supports
	        the full <quote>suspend/resume</quote> operation.
		If the <constant>PAUSE</constant> flag is set,
		the <structfield>trigger</structfield> callback below
	        must handle the corresponding (pause push/release) commands.
		The suspend/resume trigger commands can be defined even without
		the <constant>RESUME</constant> flag.  See <link
		linkend="power-management"><citetitle>
		Power Management</citetitle></link> section for details.
	        </para>
	
		<para>
		  When the PCM substreams can be synchronized (typically,
		synchronized start/stop of a playback and a capture streams),
		you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>,
		too.  In this case, you'll need to check the linked-list of
		PCM substreams in the trigger callback.  This will be
		described in the later section.
		</para>
		</listitem>
	
		<listitem>
	        <para>
	          <structfield>formats</structfield> field contains the bit-flags
	        of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>).
	        If the hardware supports more than one format, give all or'ed
	        bits.  In the example above, the signed 16bit little-endian
	        format is specified.
	        </para>
		</listitem>
	
		<listitem>
	        <para>
	        <structfield>rates</structfield> field contains the bit-flags of
	        supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>).
	        When the chip supports continuous rates, pass
	        <constant>CONTINUOUS</constant> bit additionally.
	        The pre-defined rate bits are provided only for typical
		rates. If your chip supports unconventional rates, you need to add
	        the <constant>KNOT</constant> bit and set up the hardware
	        constraint manually (explained later).
	        </para>
		</listitem>
	
		<listitem>
		<para>
		<structfield>rate_min</structfield> and
		<structfield>rate_max</structfield> define the minimum and
		maximum sample rate.  This should correspond somehow to
		<structfield>rates</structfield> bits.
		</para>
		</listitem>
	
		<listitem>
		<para>
		<structfield>channel_min</structfield> and
		<structfield>channel_max</structfield> 
		define, as you might already expected, the minimum and maximum
		number of channels.
		</para>
		</listitem>
	
		<listitem>
		<para>
		<structfield>buffer_bytes_max</structfield> defines the
		maximum buffer size in bytes.  There is no
		<structfield>buffer_bytes_min</structfield> field, since
		it can be calculated from the minimum period size and the
		minimum number of periods.
		Meanwhile, <structfield>period_bytes_min</structfield> and
		define the minimum and maximum size of the period in bytes.
		<structfield>periods_max</structfield> and
		<structfield>periods_min</structfield> define the maximum and
		minimum number of periods in the buffer.
	        </para>
	
		<para>
		The <quote>period</quote> is a term that corresponds to
		a fragment in the OSS world. The period defines the size at
		which a PCM interrupt is generated. This size strongly
		depends on the hardware. 
		Generally, the smaller period size will give you more
		interrupts, that is, more controls. 
		In the case of capture, this size defines the input latency.
		On the other hand, the whole buffer size defines the
		output latency for the playback direction.
		</para>
		</listitem>
	
		<listitem>
		<para>
		There is also a field <structfield>fifo_size</structfield>.
		This specifies the size of the hardware FIFO, but currently it
		is neither used in the driver nor in the alsa-lib.  So, you
		can ignore this field.
		</para>
		</listitem>
		</itemizedlist>
		</para>
		</section>
	
		<section id="pcm-interface-runtime-config">
		<title>PCM Configurations</title>
		<para>
		Ok, let's go back again to the PCM runtime records.
		The most frequently referred records in the runtime instance are
		the PCM configurations.
		The PCM configurations are stored in the runtime instance
		after the application sends <type>hw_params</type> data via
		alsa-lib.  There are many fields copied from hw_params and
		sw_params structs.  For example,
		<structfield>format</structfield> holds the format type
		chosen by the application.  This field contains the enum value
		<constant>SNDRV_PCM_FORMAT_XXX</constant>.
		</para>
	
		<para>
		One thing to be noted is that the configured buffer and period
		sizes are stored in <quote>frames</quote> in the runtime.
	        In the ALSA world, 1 frame = channels * samples-size.
		For conversion between frames and bytes, you can use the
		<function>frames_to_bytes()</function> and
	          <function>bytes_to_frames()</function> helper functions. 
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  period_bytes = frames_to_bytes(runtime, runtime->period_size);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
		<para>
		Also, many software parameters (sw_params) are
		stored in frames, too.  Please check the type of the field.
		<type>snd_pcm_uframes_t</type> is for the frames as unsigned
		integer while <type>snd_pcm_sframes_t</type> is for the frames
		as signed integer.
		</para>
		</section>
	
		<section id="pcm-interface-runtime-dma">
		<title>DMA Buffer Information</title>
		<para>
		The DMA buffer is defined by the following four fields,
		<structfield>dma_area</structfield>,
		<structfield>dma_addr</structfield>,
		<structfield>dma_bytes</structfield> and
		<structfield>dma_private</structfield>.
		The <structfield>dma_area</structfield> holds the buffer
		pointer (the logical address).  You can call
		<function>memcpy</function> from/to 
		this pointer.  Meanwhile, <structfield>dma_addr</structfield>
		holds the physical address of the buffer.  This field is
		specified only when the buffer is a linear buffer.
		<structfield>dma_bytes</structfield> holds the size of buffer
		in bytes.  <structfield>dma_private</structfield> is used for
		the ALSA DMA allocator.
		</para>
	
		<para>
		If you use a standard ALSA function,
		<function>snd_pcm_lib_malloc_pages()</function>, for
		allocating the buffer, these fields are set by the ALSA middle
		layer, and you should <emphasis>not</emphasis> change them by
		yourself.  You can read them but not write them.
		On the other hand, if you want to allocate the buffer by
		yourself, you'll need to manage it in hw_params callback.
		At least, <structfield>dma_bytes</structfield> is mandatory.
		<structfield>dma_area</structfield> is necessary when the
		buffer is mmapped.  If your driver doesn't support mmap, this
		field is not necessary.  <structfield>dma_addr</structfield>
		is also optional.  You can use
		<structfield>dma_private</structfield> as you like, too.
		</para>
		</section>
	
		<section id="pcm-interface-runtime-status">
		<title>Running Status</title>
		<para>
		The running status can be referred via <constant>runtime-&gt;status</constant>.
		This is the pointer to the struct <structname>snd_pcm_mmap_status</structname>
		record.  For example, you can get the current DMA hardware
		pointer via <constant>runtime-&gt;status-&gt;hw_ptr</constant>.
		</para>
	
		<para>
		The DMA application pointer can be referred via
		<constant>runtime-&gt;control</constant>, which points to the
		struct <structname>snd_pcm_mmap_control</structname> record.
		However, accessing directly to this value is not recommended.
		</para>
		</section>
	
		<section id="pcm-interface-runtime-private">
		<title>Private Data</title> 
		<para>
		You can allocate a record for the substream and store it in
		<constant>runtime-&gt;private_data</constant>.  Usually, this
		is done in
		<link linkend="pcm-interface-operators-open-callback"><citetitle>
		the open callback</citetitle></link>.
		Don't mix this with <constant>pcm-&gt;private_data</constant>.
		The <constant>pcm-&gt;private_data</constant> usually points to the
		chip instance assigned statically at the creation of PCM, while the 
		<constant>runtime-&gt;private_data</constant> points to a dynamic
		data structure created at the PCM open callback.
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_open(struct snd_pcm_substream *substream)
	  {
	          struct my_pcm_data *data;
	          ....
	          data = kmalloc(sizeof(*data), GFP_KERNEL);
	          substream->runtime->private_data = data;
	          ....
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          The allocated object must be released in
		<link linkend="pcm-interface-operators-open-callback"><citetitle>
		the close callback</citetitle></link>.
	        </para>
		</section>
	
		<section id="pcm-interface-runtime-intr">
		<title>Interrupt Callbacks</title>
		<para>
		The field <structfield>transfer_ack_begin</structfield> and
		<structfield>transfer_ack_end</structfield> are called at
		the beginning and at the end of
		<function>snd_pcm_period_elapsed()</function>, respectively. 
		</para>
		</section>
	
	    </section>
	
	    <section id="pcm-interface-operators">
	      <title>Operators</title>
	      <para>
	        OK, now let me give details about each pcm callback
	      (<parameter>ops</parameter>). In general, every callback must
	      return 0 if successful, or a negative error number
	      such as <constant>-EINVAL</constant>. To choose an appropriate
	      error number, it is advised to check what value other parts of
	      the kernel return when the same kind of request fails.
	      </para>
	
	      <para>
	        The callback function takes at least the argument with
	        <structname>snd_pcm_substream</structname> pointer. To retrieve
	        the chip record from the given substream instance, you can use the
	        following macro. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  int xxx() {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          ....
	  }
	]]>
	          </programlisting>
	        </informalexample>
	
		The macro reads <constant>substream-&gt;private_data</constant>,
		which is a copy of <constant>pcm-&gt;private_data</constant>.
		You can override the former if you need to assign different data
		records per PCM substream.  For example, the cmi8330 driver assigns
		different private_data for playback and capture directions,
		because it uses two different codecs (SB- and AD-compatible) for
		different directions.
	      </para>
	
	      <section id="pcm-interface-operators-open-callback">
	        <title>open callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_open(struct snd_pcm_substream *substream);
	]]>
	            </programlisting>
	          </informalexample>
	
	          This is called when a pcm substream is opened.
	        </para>
	
	        <para>
	          At least, here you have to initialize the runtime-&gt;hw
	          record. Typically, this is done by like this: 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_open(struct snd_pcm_substream *substream)
	  {
	          struct mychip *chip = snd_pcm_substream_chip(substream);
	          struct snd_pcm_runtime *runtime = substream->runtime;
	
	          runtime->hw = snd_mychip_playback_hw;
	          return 0;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	
	          where <parameter>snd_mychip_playback_hw</parameter> is the
	          pre-defined hardware description.
		</para>
	
		<para>
		You can allocate a private data in this callback, as described
		in <link linkend="pcm-interface-runtime-private"><citetitle>
		Private Data</citetitle></link> section.
		</para>
	
		<para>
		If the hardware configuration needs more constraints, set the
		hardware constraints here, too.
		See <link linkend="pcm-interface-constraints"><citetitle>
		Constraints</citetitle></link> for more details.
		</para>
	      </section>
	
	      <section id="pcm-interface-operators-close-callback">
	        <title>close callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_close(struct snd_pcm_substream *substream);
	]]>
	            </programlisting>
	          </informalexample>
	
	          Obviously, this is called when a pcm substream is closed.
	        </para>
	
	        <para>
	          Any private instance for a pcm substream allocated in the
	          open callback will be released here. 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_close(struct snd_pcm_substream *substream)
	  {
	          ....
	          kfree(substream->runtime->private_data);
	          ....
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-ioctl-callback">
	        <title>ioctl callback</title>
	        <para>
	          This is used for any special call to pcm ioctls. But
	        usually you can pass a generic ioctl callback, 
	        <function>snd_pcm_lib_ioctl</function>.
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-hw-params-callback">
	        <title>hw_params callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
	                               struct snd_pcm_hw_params *hw_params);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          This is called when the hardware parameter
	        (<structfield>hw_params</structfield>) is set
	        up by the application, 
	        that is, once when the buffer size, the period size, the
	        format, etc. are defined for the pcm substream. 
	        </para>
	
	        <para>
	          Many hardware setups should be done in this callback,
	        including the allocation of buffers. 
	        </para>
	
	        <para>
	          Parameters to be initialized are retrieved by
	          <function>params_xxx()</function> macros. To allocate
	          buffer, you can call a helper function, 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
	]]>
	            </programlisting>
	          </informalexample>
	
	          <function>snd_pcm_lib_malloc_pages()</function> is available
		  only when the DMA buffers have been pre-allocated.
		  See the section <link
		  linkend="buffer-and-memory-buffer-types"><citetitle>
		  Buffer Types</citetitle></link> for more details.
	        </para>
	
	        <para>
	          Note that this and <structfield>prepare</structfield> callbacks
	        may be called multiple times per initialization.
	        For example, the OSS emulation may
	        call these callbacks at each change via its ioctl. 
	        </para>
	
	        <para>
	          Thus, you need to be careful not to allocate the same buffers
	        many times, which will lead to memory leaks!  Calling the
	        helper function above many times is OK. It will release the
	        previous buffer automatically when it was already allocated. 
	        </para>
	
	        <para>
	          Another note is that this callback is non-atomic
	        (schedulable). This is important, because the
	        <structfield>trigger</structfield> callback 
	        is atomic (non-schedulable). That is, mutexes or any
	        schedule-related functions are not available in
	        <structfield>trigger</structfield> callback.
		Please see the subsection
		<link linkend="pcm-interface-atomicity"><citetitle>
		Atomicity</citetitle></link> for details.
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-hw-free-callback">
	        <title>hw_free callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          This is called to release the resources allocated via
	          <structfield>hw_params</structfield>. For example, releasing the
	          buffer via 
	          <function>snd_pcm_lib_malloc_pages()</function> is done by
	          calling the following: 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  snd_pcm_lib_free_pages(substream);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          This function is always called before the close callback is called.
	          Also, the callback may be called multiple times, too.
	          Keep track whether the resource was already released. 
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-prepare-callback">
	       <title>prepare callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_prepare(struct snd_pcm_substream *substream);
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          This callback is called when the pcm is
	        <quote>prepared</quote>. You can set the format type, sample
	        rate, etc. here. The difference from
	        <structfield>hw_params</structfield> is that the 
	        <structfield>prepare</structfield> callback will be called each
	        time 
	        <function>snd_pcm_prepare()</function> is called, i.e. when
	        recovering after underruns, etc. 
	        </para>
	
	        <para>
		Note that this callback is now non-atomic.
		You can use schedule-related functions safely in this callback.
	        </para>
	
	        <para>
	          In this and the following callbacks, you can refer to the
	        values via the runtime record,
	        substream-&gt;runtime.
	        For example, to get the current
	        rate, format or channels, access to
	        runtime-&gt;rate,
	        runtime-&gt;format or
	        runtime-&gt;channels, respectively. 
	        The physical address of the allocated buffer is set to
		runtime-&gt;dma_area.  The buffer and period sizes are
		in runtime-&gt;buffer_size and runtime-&gt;period_size,
		respectively.
	        </para>
	
	        <para>
	          Be careful that this callback will be called many times at
	        each setup, too. 
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-trigger-callback">
	        <title>trigger callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
	]]>
	            </programlisting>
	          </informalexample>
	
	          This is called when the pcm is started, stopped or paused.
	        </para>
	
	        <para>
	          Which action is specified in the second argument,
	          <constant>SNDRV_PCM_TRIGGER_XXX</constant> in
	          <filename>&lt;sound/pcm.h&gt;</filename>. At least,
	          the <constant>START</constant> and <constant>STOP</constant>
	          commands must be defined in this callback. 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  switch (cmd) {
	  case SNDRV_PCM_TRIGGER_START:
	          /* do something to start the PCM engine */
	          break;
	  case SNDRV_PCM_TRIGGER_STOP:
	          /* do something to stop the PCM engine */
	          break;
	  default:
	          return -EINVAL;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	          When the pcm supports the pause operation (given in the info
	        field of the hardware table), the <constant>PAUSE_PUSE</constant>
	        and <constant>PAUSE_RELEASE</constant> commands must be
	        handled here, too. The former is the command to pause the pcm,
	        and the latter to restart the pcm again. 
	        </para>
	
	        <para>
	          When the pcm supports the suspend/resume operation,
		regardless of full or partial suspend/resume support,
	        the <constant>SUSPEND</constant> and <constant>RESUME</constant>
	        commands must be handled, too.
	        These commands are issued when the power-management status is
	        changed.  Obviously, the <constant>SUSPEND</constant> and
	        <constant>RESUME</constant> commands
	        suspend and resume the pcm substream, and usually, they
	        are identical to the <constant>STOP</constant> and
	        <constant>START</constant> commands, respectively.
		  See the <link linkend="power-management"><citetitle>
		Power Management</citetitle></link> section for details.
	        </para>
	
	        <para>
	          As mentioned, this callback is atomic.  You cannot call
		  functions which may sleep.
		  The trigger callback should be as minimal as possible,
		  just really triggering the DMA.  The other stuff should be
		  initialized hw_params and prepare callbacks properly
		  beforehand.
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-pointer-callback">
	        <title>pointer callback</title>
	        <para>
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
	]]>
	            </programlisting>
	          </informalexample>
	
	          This callback is called when the PCM middle layer inquires
	        the current hardware position on the buffer. The position must
	        be returned in frames,
	        ranging from 0 to buffer_size - 1.
	        </para>
	
	        <para>
	          This is called usually from the buffer-update routine in the
	        pcm middle layer, which is invoked when
	        <function>snd_pcm_period_elapsed()</function> is called in the
	        interrupt routine. Then the pcm middle layer updates the
	        position and calculates the available space, and wakes up the
	        sleeping poll threads, etc. 
	        </para>
	
	        <para>
	          This callback is also atomic.
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-copy-silence">
	        <title>copy and silence callbacks</title>
	        <para>
	          These callbacks are not mandatory, and can be omitted in
	        most cases. These callbacks are used when the hardware buffer
	        cannot be in the normal memory space. Some chips have their
	        own buffer on the hardware which is not mappable. In such a
	        case, you have to transfer the data manually from the memory
	        buffer to the hardware buffer. Or, if the buffer is
	        non-contiguous on both physical and virtual memory spaces,
	        these callbacks must be defined, too. 
	        </para>
	
	        <para>
	          If these two callbacks are defined, copy and set-silence
	        operations are done by them. The detailed will be described in
	        the later section <link
	        linkend="buffer-and-memory"><citetitle>Buffer and Memory
	        Management</citetitle></link>. 
	        </para>
	      </section>
	
	      <section id="pcm-interface-operators-ack">
	        <title>ack callback</title>
	        <para>
	          This callback is also not mandatory. This callback is called
	        when the appl_ptr is updated in read or write operations.
	        Some drivers like emu10k1-fx and cs46xx need to track the
		current appl_ptr for the internal buffer, and this callback
		is useful only for such a purpose.
		</para>
		<para>
		  This callback is atomic.
		</para>
	      </section>
	
	      <section id="pcm-interface-operators-page-callback">
	        <title>page callback</title>
	
	        <para>
	          This callback is optional too. This callback is used
	        mainly for non-contiguous buffers. The mmap calls this
	        callback to get the page address. Some examples will be
	        explained in the later section <link
	        linkend="buffer-and-memory"><citetitle>Buffer and Memory
	        Management</citetitle></link>, too. 
	        </para>
	      </section>
	    </section>
	
	    <section id="pcm-interface-interrupt-handler">
	      <title>Interrupt Handler</title>
	      <para>
	        The rest of pcm stuff is the PCM interrupt handler. The
	      role of PCM interrupt handler in the sound driver is to update
	      the buffer position and to tell the PCM middle layer when the
	      buffer position goes across the prescribed period size. To
	      inform this, call the <function>snd_pcm_period_elapsed()</function>
	      function. 
	      </para>
	
	      <para>
	        There are several types of sound chips to generate the interrupts.
	      </para>
	
	      <section id="pcm-interface-interrupt-handler-boundary">
	        <title>Interrupts at the period (fragment) boundary</title>
	        <para>
	          This is the most frequently found type:  the hardware
	        generates an interrupt at each period boundary.
		In this case, you can call
	        <function>snd_pcm_period_elapsed()</function> at each 
	        interrupt. 
	        </para>
	
	        <para>
	          <function>snd_pcm_period_elapsed()</function> takes the
	        substream pointer as its argument. Thus, you need to keep the
	        substream pointer accessible from the chip instance. For
	        example, define substream field in the chip record to hold the
	        current running substream pointer, and set the pointer value
	        at open callback (and reset at close callback). 
	        </para>
	
	        <para>
	          If you acquire a spinlock in the interrupt handler, and the
	        lock is used in other pcm callbacks, too, then you have to
	        release the lock before calling
	        <function>snd_pcm_period_elapsed()</function>, because
	        <function>snd_pcm_period_elapsed()</function> calls other pcm
	        callbacks inside. 
	        </para>
	
	        <para>
	          Typical code would be like:
	
	          <example>
		    <title>Interrupt Handler Case #1</title>
	            <programlisting>
	<![CDATA[
	  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
	  {
	          struct mychip *chip = dev_id;
	          spin_lock(&chip->lock);
	          ....
	          if (pcm_irq_invoked(chip)) {
	                  /* call updater, unlock before it */
	                  spin_unlock(&chip->lock);
	                  snd_pcm_period_elapsed(chip->substream);
	                  spin_lock(&chip->lock);
	                  /* acknowledge the interrupt if necessary */
	          }
	          ....
	          spin_unlock(&chip->lock);
	          return IRQ_HANDLED;
	  }
	]]>
	            </programlisting>
	          </example>
	        </para>
	      </section>
	
	      <section id="pcm-interface-interrupt-handler-timer">
	        <title>High frequency timer interrupts</title>
	        <para>
		This happense when the hardware doesn't generate interrupts
	        at the period boundary but issues timer interrupts at a fixed
	        timer rate (e.g. es1968 or ymfpci drivers). 
	        In this case, you need to check the current hardware
	        position and accumulate the processed sample length at each
	        interrupt.  When the accumulated size exceeds the period
	        size, call 
	        <function>snd_pcm_period_elapsed()</function> and reset the
	        accumulator. 
	        </para>
	
	        <para>
	          Typical code would be like the following.
	
	          <example>
		    <title>Interrupt Handler Case #2</title>
	            <programlisting>
	<![CDATA[
	  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
	  {
	          struct mychip *chip = dev_id;
	          spin_lock(&chip->lock);
	          ....
	          if (pcm_irq_invoked(chip)) {
	                  unsigned int last_ptr, size;
	                  /* get the current hardware pointer (in frames) */
	                  last_ptr = get_hw_ptr(chip);
	                  /* calculate the processed frames since the
	                   * last update
	                   */
	                  if (last_ptr < chip->last_ptr)
	                          size = runtime->buffer_size + last_ptr 
	                                   - chip->last_ptr; 
	                  else
	                          size = last_ptr - chip->last_ptr;
	                  /* remember the last updated point */
	                  chip->last_ptr = last_ptr;
	                  /* accumulate the size */
	                  chip->size += size;
	                  /* over the period boundary? */
	                  if (chip->size >= runtime->period_size) {
	                          /* reset the accumulator */
	                          chip->size %= runtime->period_size;
	                          /* call updater */
	                          spin_unlock(&chip->lock);
	                          snd_pcm_period_elapsed(substream);
	                          spin_lock(&chip->lock);
	                  }
	                  /* acknowledge the interrupt if necessary */
	          }
	          ....
	          spin_unlock(&chip->lock);
	          return IRQ_HANDLED;
	  }
	]]>
	            </programlisting>
	          </example>
	        </para>
	      </section>
	
	      <section id="pcm-interface-interrupt-handler-both">
	        <title>On calling <function>snd_pcm_period_elapsed()</function></title>
	        <para>
	          In both cases, even if more than one period are elapsed, you
	        don't have to call
	        <function>snd_pcm_period_elapsed()</function> many times. Call
	        only once. And the pcm layer will check the current hardware
	        pointer and update to the latest status. 
	        </para>
	      </section>
	    </section>
	
	    <section id="pcm-interface-atomicity">
	      <title>Atomicity</title>
	      <para>
	      One of the most important (and thus difficult to debug) problems
	      in kernel programming are race conditions.
	      In the Linux kernel, they are usually avoided via spin-locks, mutexes
	      or semaphores.  In general, if a race condition can happen
	      in an interrupt handler, it has to be managed atomically, and you
	      have to use a spinlock to protect the critical session. If the
	      critical section is not in interrupt handler code and
	      if taking a relatively long time to execute is acceptable, you
	      should use mutexes or semaphores instead.
	      </para>
	
	      <para>
	      As already seen, some pcm callbacks are atomic and some are
	      not.  For example, the <parameter>hw_params</parameter> callback is
	      non-atomic, while <parameter>trigger</parameter> callback is
	      atomic.  This means, the latter is called already in a spinlock
	      held by the PCM middle layer. Please take this atomicity into
	      account when you choose a locking scheme in the callbacks.
	      </para>
	
	      <para>
	      In the atomic callbacks, you cannot use functions which may call
	      <function>schedule</function> or go to
	      <function>sleep</function>.  Semaphores and mutexes can sleep,
	      and hence they cannot be used inside the atomic callbacks
	      (e.g. <parameter>trigger</parameter> callback).
	      To implement some delay in such a callback, please use
	      <function>udelay()</function> or <function>mdelay()</function>.
	      </para>
	
	      <para>
	      All three atomic callbacks (trigger, pointer, and ack) are
	      called with local interrupts disabled.
	      </para>
	
	    </section>
	    <section id="pcm-interface-constraints">
	      <title>Constraints</title>
	      <para>
	        If your chip supports unconventional sample rates, or only the
	      limited samples, you need to set a constraint for the
	      condition. 
	      </para>
	
	      <para>
	        For example, in order to restrict the sample rates in the some
	        supported values, use
		<function>snd_pcm_hw_constraint_list()</function>.
		You need to call this function in the open callback.
	
	        <example>
		  <title>Example of Hardware Constraints</title>
	          <programlisting>
	<![CDATA[
	  static unsigned int rates[] =
	          {4000, 10000, 22050, 44100};
	  static struct snd_pcm_hw_constraint_list constraints_rates = {
	          .count = ARRAY_SIZE(rates),
	          .list = rates,
	          .mask = 0,
	  };
	
	  static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
	  {
	          int err;
	          ....
	          err = snd_pcm_hw_constraint_list(substream->runtime, 0,
	                                           SNDRV_PCM_HW_PARAM_RATE,
	                                           &constraints_rates);
	          if (err < 0)
	                  return err;
	          ....
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	
	      <para>
	        There are many different constraints.
	        Look at <filename>sound/pcm.h</filename> for a complete list.
	        You can even define your own constraint rules.
	        For example, let's suppose my_chip can manage a substream of 1 channel
	        if and only if the format is S16_LE, otherwise it supports any format
	        specified in the <structname>snd_pcm_hardware</structname> structure (or in any
	        other constraint_list). You can build a rule like this:
	
	        <example>
		  <title>Example of Hardware Constraints for Channels</title>
		  <programlisting>
	<![CDATA[
	  static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
	                                        struct snd_pcm_hw_rule *rule)
	  {
	          struct snd_interval *c = hw_param_interval(params,
	                SNDRV_PCM_HW_PARAM_CHANNELS);
	          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
	          struct snd_mask fmt;
	
	          snd_mask_any(&fmt);    /* Init the struct */
	          if (c->min < 2) {
	                  fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
	                  return snd_mask_refine(f, &fmt);
	          }
	          return 0;
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	 
	      <para>
	        Then you need to call this function to add your rule:
	
	       <informalexample>
		 <programlisting>
	<![CDATA[
	  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
	                      hw_rule_channels_by_format, 0, SNDRV_PCM_HW_PARAM_FORMAT,
	                      -1);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The rule function is called when an application sets the number of
	        channels. But an application can set the format before the number of
	        channels. Thus you also need to define the inverse rule:
	
	       <example>
		 <title>Example of Hardware Constraints for Channels</title>
		 <programlisting>
	<![CDATA[
	  static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
	                                        struct snd_pcm_hw_rule *rule)
	  {
	          struct snd_interval *c = hw_param_interval(params,
	                        SNDRV_PCM_HW_PARAM_CHANNELS);
	          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
	          struct snd_interval ch;
	
	          snd_interval_any(&ch);
	          if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
	                  ch.min = ch.max = 1;
	                  ch.integer = 1;
	                  return snd_interval_refine(c, &ch);
	          }
	          return 0;
	  }
	]]>
	          </programlisting>
	        </example>
	      </para>
	
	      <para>
	      ...and in the open callback:
	       <informalexample>
		 <programlisting>
	<![CDATA[
	  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
	                      hw_rule_format_by_channels, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
	                      -1);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        I won't give more details here, rather I
	        would like to say, <quote>Luke, use the source.</quote>
	      </para>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- Control Interface  -->
	<!-- ****************************************************** -->
	  <chapter id="control-interface">
	    <title>Control Interface</title>
	
	    <section id="control-interface-general">
	      <title>General</title>
	      <para>
	        The control interface is used widely for many switches,
	      sliders, etc. which are accessed from user-space. Its most
	      important use is the mixer interface. In other words, since ALSA
	      0.9.x, all the mixer stuff is implemented on the control kernel API.
	      </para>
	
	      <para>
	        ALSA has a well-defined AC97 control module. If your chip
	      supports only the AC97 and nothing else, you can skip this
	      section. 
	      </para>
	
	      <para>
	        The control API is defined in
	      <filename>&lt;sound/control.h&gt;</filename>.
	      Include this file if you want to add your own controls.
	      </para>
	    </section>
	
	    <section id="control-interface-definition">
	      <title>Definition of Controls</title>
	      <para>
	        To create a new control, you need to define the
		following three
	      callbacks: <structfield>info</structfield>,
	      <structfield>get</structfield> and
	      <structfield>put</structfield>. Then, define a
	      struct <structname>snd_kcontrol_new</structname> record, such as: 
	
	        <example>
		  <title>Definition of a Control</title>
	          <programlisting>
	<![CDATA[
	  static struct snd_kcontrol_new my_control __devinitdata = {
	          .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
	          .name = "PCM Playback Switch",
	          .index = 0,
	          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
	          .private_value = 0xffff,
	          .info = my_control_info,
	          .get = my_control_get,
	          .put = my_control_put
	  };
	]]>
	          </programlisting>
	        </example>
	      </para>
	
	      <para>
	        Most likely the control is created via
	      <function>snd_ctl_new1()</function>, and in such a case, you can
	      add the <parameter>__devinitdata</parameter> prefix to the
	      definition as above. 
	      </para>
	
	      <para>
	        The <structfield>iface</structfield> field specifies the control
	      type, <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>, which
	      is usually <constant>MIXER</constant>.
	      Use <constant>CARD</constant> for global controls that are not
	      logically part of the mixer.
	      If the control is closely associated with some specific device on
	      the sound card, use <constant>HWDEP</constant>,
	      <constant>PCM</constant>, <constant>RAWMIDI</constant>,
	      <constant>TIMER</constant>, or <constant>SEQUENCER</constant>, and
	      specify the device number with the
	      <structfield>device</structfield> and
	      <structfield>subdevice</structfield> fields.
	      </para>
	
	      <para>
	        The <structfield>name</structfield> is the name identifier
	      string. Since ALSA 0.9.x, the control name is very important,
	      because its role is classified from its name. There are
	      pre-defined standard control names. The details are described in
	      the <link linkend="control-interface-control-names"><citetitle>
	      Control Names</citetitle></link> subsection.
	      </para>
	
	      <para>
	        The <structfield>index</structfield> field holds the index number
	      of this control. If there are several different controls with
	      the same name, they can be distinguished by the index
	      number. This is the case when 
	      several codecs exist on the card. If the index is zero, you can
	      omit the definition above. 
	      </para>
	
	      <para>
	        The <structfield>access</structfield> field contains the access
	      type of this control. Give the combination of bit masks,
	      <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there.
	      The details will be explained in
	      the <link linkend="control-interface-access-flags"><citetitle>
	      Access Flags</citetitle></link> subsection.
	      </para>
	
	      <para>
	        The <structfield>private_value</structfield> field contains
	      an arbitrary long integer value for this record. When using
	      the generic <structfield>info</structfield>,
	      <structfield>get</structfield> and
	      <structfield>put</structfield> callbacks, you can pass a value 
	      through this field. If several small numbers are necessary, you can
	      combine them in bitwise. Or, it's possible to give a pointer
	      (casted to unsigned long) of some record to this field, too. 
	      </para>
	
	      <para>
	      The <structfield>tlv</structfield> field can be used to provide
	      metadata about the control; see the
	      <link linkend="control-interface-tlv">
	      <citetitle>Metadata</citetitle></link> subsection.
	      </para>
	
	      <para>
	        The other three are
		<link linkend="control-interface-callbacks"><citetitle>
		callback functions</citetitle></link>.
	      </para>
	    </section>
	
	    <section id="control-interface-control-names">
	      <title>Control Names</title>
	      <para>
	        There are some standards to define the control names. A
	      control is usually defined from the three parts as
	      <quote>SOURCE DIRECTION FUNCTION</quote>. 
	      </para>
	
	      <para>
	        The first, <constant>SOURCE</constant>, specifies the source
	      of the control, and is a string such as <quote>Master</quote>,
	      <quote>PCM</quote>, <quote>CD</quote> and
	      <quote>Line</quote>. There are many pre-defined sources. 
	      </para>
	
	      <para>
	        The second, <constant>DIRECTION</constant>, is one of the
	      following strings according to the direction of the control:
	      <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass
	      Playback</quote> and <quote>Bypass Capture</quote>. Or, it can
	      be omitted, meaning both playback and capture directions. 
	      </para>
	
	      <para>
	        The third, <constant>FUNCTION</constant>, is one of the
	      following strings according to the function of the control:
	      <quote>Switch</quote>, <quote>Volume</quote> and
	      <quote>Route</quote>. 
	      </para>
	
	      <para>
	        The example of control names are, thus, <quote>Master Capture
	      Switch</quote> or <quote>PCM Playback Volume</quote>. 
	      </para>
	
	      <para>
	        There are some exceptions:
	      </para>
	
	      <section id="control-interface-control-names-global">
	        <title>Global capture and playback</title>
	        <para>
	          <quote>Capture Source</quote>, <quote>Capture Switch</quote>
	        and <quote>Capture Volume</quote> are used for the global
	        capture (input) source, switch and volume. Similarly,
	        <quote>Playback Switch</quote> and <quote>Playback
	        Volume</quote> are used for the global output gain switch and
	        volume. 
	        </para>
	      </section>
	
	      <section id="control-interface-control-names-tone">
	        <title>Tone-controls</title>
	        <para>
	          tone-control switch and volumes are specified like
	        <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control -
	        Switch</quote>, <quote>Tone Control - Bass</quote>,
	        <quote>Tone Control - Center</quote>.  
	        </para>
	      </section>
	
	      <section id="control-interface-control-names-3d">
	        <title>3D controls</title>
	        <para>
	          3D-control switches and volumes are specified like <quote>3D
	        Control - XXX</quote>, e.g. <quote>3D Control -
	        Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D
	        Control - Space</quote>. 
	        </para>
	      </section>
	
	      <section id="control-interface-control-names-mic">
	        <title>Mic boost</title>
	        <para>
	          Mic-boost switch is set as <quote>Mic Boost</quote> or
	        <quote>Mic Boost (6dB)</quote>. 
	        </para>
	
	        <para>
	          More precise information can be found in
	        <filename>Documentation/sound/alsa/ControlNames.txt</filename>.
	        </para>
	      </section>
	    </section>
	
	    <section id="control-interface-access-flags">
	      <title>Access Flags</title>
	
	      <para>
	      The access flag is the bitmask which specifies the access type
	      of the given control.  The default access type is
	      <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>, 
	      which means both read and write are allowed to this control.
	      When the access flag is omitted (i.e. = 0), it is
	      considered as <constant>READWRITE</constant> access as default. 
	      </para>
	
	      <para>
	      When the control is read-only, pass
	      <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead.
	      In this case, you don't have to define
	      the <structfield>put</structfield> callback.
	      Similarly, when the control is write-only (although it's a rare
	      case), you can use the <constant>WRITE</constant> flag instead, and
	      you don't need the <structfield>get</structfield> callback.
	      </para>
	
	      <para>
	      If the control value changes frequently (e.g. the VU meter),
	      <constant>VOLATILE</constant> flag should be given.  This means
	      that the control may be changed without
	      <link linkend="control-interface-change-notification"><citetitle>
	      notification</citetitle></link>. Applications should poll such
	      a control constantly.
	      </para>
	
	      <para>
	      When the control is inactive, set
	      the <constant>INACTIVE</constant> flag, too.
	      There are <constant>LOCK</constant> and
	      <constant>OWNER</constant> flags to change the write
	      permissions.
	      </para>
	
	    </section>
	
	    <section id="control-interface-callbacks">
	      <title>Callbacks</title>
	
	      <section id="control-interface-callbacks-info">
	        <title>info callback</title>
	        <para>
	          The <structfield>info</structfield> callback is used to get
	        detailed information on this control. This must store the
	        values of the given struct <structname>snd_ctl_elem_info</structname>
	        object. For example, for a boolean control with a single
	        element: 
	
	          <example>
		    <title>Example of info callback</title>
	            <programlisting>
	<![CDATA[
	  static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
	                          struct snd_ctl_elem_info *uinfo)
	  {
	          uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
	          uinfo->count = 1;
	          uinfo->value.integer.min = 0;
	          uinfo->value.integer.max = 1;
	          return 0;
	  }
	]]>
	            </programlisting>
	          </example>
	        </para>
	
	        <para>
	          The <structfield>type</structfield> field specifies the type
	        of the control. There are <constant>BOOLEAN</constant>,
	        <constant>INTEGER</constant>, <constant>ENUMERATED</constant>,
	        <constant>BYTES</constant>, <constant>IEC958</constant> and
	        <constant>INTEGER64</constant>. The
	        <structfield>count</structfield> field specifies the 
	        number of elements in this control. For example, a stereo
	        volume would have count = 2. The
	        <structfield>value</structfield> field is a union, and 
	        the values stored are depending on the type. The boolean and
	        integer types are identical. 
	        </para>
	
	        <para>
	          The enumerated type is a bit different from others.  You'll
	          need to set the string for the currently given item index. 
	
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
	                          struct snd_ctl_elem_info *uinfo)
	  {
	          static char *texts[4] = {
	                  "First", "Second", "Third", "Fourth"
	          };
	          uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
	          uinfo->count = 1;
	          uinfo->value.enumerated.items = 4;
	          if (uinfo->value.enumerated.item > 3)
	                  uinfo->value.enumerated.item = 3;
	          strcpy(uinfo->value.enumerated.name,
	                 texts[uinfo->value.enumerated.item]);
	          return 0;
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
		  Some common info callbacks are available for your convenience:
		<function>snd_ctl_boolean_mono_info()</function> and
		<function>snd_ctl_boolean_stereo_info()</function>.
		Obviously, the former is an info callback for a mono channel
		boolean item, just like <function>snd_myctl_mono_info</function>
		above, and the latter is for a stereo channel boolean item.
		</para>
	
	      </section>
	
	      <section id="control-interface-callbacks-get">
	        <title>get callback</title>
	
	        <para>
	          This callback is used to read the current value of the
	        control and to return to user-space. 
	        </para>
	
	        <para>
	          For example,
	
	          <example>
		    <title>Example of get callback</title>
	            <programlisting>
	<![CDATA[
	  static int snd_myctl_get(struct snd_kcontrol *kcontrol,
	                           struct snd_ctl_elem_value *ucontrol)
	  {
	          struct mychip *chip = snd_kcontrol_chip(kcontrol);
	          ucontrol->value.integer.value[0] = get_some_value(chip);
	          return 0;
	  }
	]]>
	            </programlisting>
	          </example>
	        </para>
	
	        <para>
		The <structfield>value</structfield> field depends on 
	        the type of control as well as on the info callback.  For example,
		the sb driver uses this field to store the register offset,
	        the bit-shift and the bit-mask.  The
	        <structfield>private_value</structfield> field is set as follows:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  .private_value = reg | (shift << 16) | (mask << 24)
	]]>
	            </programlisting>
	          </informalexample>
		and is retrieved in callbacks like
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
	                                    struct snd_ctl_elem_value *ucontrol)
	  {
	          int reg = kcontrol->private_value & 0xff;
	          int shift = (kcontrol->private_value >> 16) & 0xff;
	          int mask = (kcontrol->private_value >> 24) & 0xff;
	          ....
	  }
	]]>
	            </programlisting>
	          </informalexample>
		</para>
	
		<para>
		In the <structfield>get</structfield> callback,
		you have to fill all the elements if the
	        control has more than one elements,
	        i.e. <structfield>count</structfield> &gt; 1.
		In the example above, we filled only one element
	        (<structfield>value.integer.value[0]</structfield>) since it's
	        assumed as <structfield>count</structfield> = 1.
	        </para>
	      </section>
	
	      <section id="control-interface-callbacks-put">
	        <title>put callback</title>
	
	        <para>
	          This callback is used to write a value from user-space.
	        </para>
	
	        <para>
	          For example,
	
	          <example>
		    <title>Example of put callback</title>
	            <programlisting>
	<![CDATA[
	  static int snd_myctl_put(struct snd_kcontrol *kcontrol,
	                           struct snd_ctl_elem_value *ucontrol)
	  {
	          struct mychip *chip = snd_kcontrol_chip(kcontrol);
	          int changed = 0;
	          if (chip->current_value !=
	               ucontrol->value.integer.value[0]) {
	                  change_current_value(chip,
	                              ucontrol->value.integer.value[0]);
	                  changed = 1;
	          }
	          return changed;
	  }
	]]>
	            </programlisting>
	          </example>
	
	          As seen above, you have to return 1 if the value is
	        changed. If the value is not changed, return 0 instead. 
		If any fatal error happens, return a negative error code as
	        usual.
	        </para>
	
	        <para>
		As in the <structfield>get</structfield> callback,
		when the control has more than one elements,
		all elements must be evaluated in this callback, too.
	        </para>
	      </section>
	
	      <section id="control-interface-callbacks-all">
	        <title>Callbacks are not atomic</title>
	        <para>
	          All these three callbacks are basically not atomic.
	        </para>
	      </section>
	    </section>
	
	    <section id="control-interface-constructor">
	      <title>Constructor</title>
	      <para>
	        When everything is ready, finally we can create a new
	      control. To create a control, there are two functions to be
	      called, <function>snd_ctl_new1()</function> and
	      <function>snd_ctl_add()</function>. 
	      </para>
	
	      <para>
	        In the simplest way, you can do like this:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
	  if (err < 0)
	          return err;
	]]>
	          </programlisting>
	        </informalexample>
	
	        where <parameter>my_control</parameter> is the
	      struct <structname>snd_kcontrol_new</structname> object defined above, and chip
	      is the object pointer to be passed to
	      kcontrol-&gt;private_data 
	      which can be referred to in callbacks. 
	      </para>
	
	      <para>
	        <function>snd_ctl_new1()</function> allocates a new
	      <structname>snd_kcontrol</structname> instance (that's why the definition
	      of <parameter>my_control</parameter> can be with
	      the <parameter>__devinitdata</parameter> 
	      prefix), and <function>snd_ctl_add</function> assigns the given
	      control component to the card. 
	      </para>
	    </section>
	
	    <section id="control-interface-change-notification">
	      <title>Change Notification</title>
	      <para>
	        If you need to change and update a control in the interrupt
	      routine, you can call <function>snd_ctl_notify()</function>. For
	      example, 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
	]]>
	          </programlisting>
	        </informalexample>
	
	        This function takes the card pointer, the event-mask, and the
	      control id pointer for the notification. The event-mask
	      specifies the types of notification, for example, in the above
	      example, the change of control values is notified.
	      The id pointer is the pointer of struct <structname>snd_ctl_elem_id</structname>
	      to be notified.
	      You can find some examples in <filename>es1938.c</filename> or
	      <filename>es1968.c</filename> for hardware volume interrupts. 
	      </para>
	    </section>
	
	    <section id="control-interface-tlv">
	      <title>Metadata</title>
	      <para>
	      To provide information about the dB values of a mixer control, use
	      on of the <constant>DECLARE_TLV_xxx</constant> macros from
	      <filename>&lt;sound/tlv.h&gt;</filename> to define a variable
	      containing this information, set the<structfield>tlv.p
	      </structfield> field to point to this variable, and include the
	      <constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the
	      <structfield>access</structfield> field; like this:
	      <informalexample>
	        <programlisting>
	<![CDATA[
	  static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
	
	  static struct snd_kcontrol_new my_control __devinitdata = {
	          ...
	          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
	                    SNDRV_CTL_ELEM_ACCESS_TLV_READ,
	          ...
	          .tlv.p = db_scale_my_control,
	  };
	]]>
	        </programlisting>
	      </informalexample>
	      </para>
	
	      <para>
	      The <function>DECLARE_TLV_DB_SCALE</function> macro defines
	      information about a mixer control where each step in the control's
	      value changes the dB value by a constant dB amount.
	      The first parameter is the name of the variable to be defined.
	      The second parameter is the minimum value, in units of 0.01 dB.
	      The third parameter is the step size, in units of 0.01 dB.
	      Set the fourth parameter to 1 if the minimum value actually mutes
	      the control.
	      </para>
	
	      <para>
	      The <function>DECLARE_TLV_DB_LINEAR</function> macro defines
	      information about a mixer control where the control's value affects
	      the output linearly.
	      The first parameter is the name of the variable to be defined.
	      The second parameter is the minimum value, in units of 0.01 dB.
	      The third parameter is the maximum value, in units of 0.01 dB.
	      If the minimum value mutes the control, set the second parameter to
	      <constant>TLV_DB_GAIN_MUTE</constant>.
	      </para>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- API for AC97 Codec  -->
	<!-- ****************************************************** -->
	  <chapter id="api-ac97">
	    <title>API for AC97 Codec</title>
	
	    <section>
	      <title>General</title>
	      <para>
	        The ALSA AC97 codec layer is a well-defined one, and you don't
	      have to write much code to control it. Only low-level control
	      routines are necessary. The AC97 codec API is defined in
	      <filename>&lt;sound/ac97_codec.h&gt;</filename>. 
	      </para>
	    </section>
	
	    <section id="api-ac97-example">
	      <title>Full Code Example</title>
	      <para>
	          <example>
		    <title>Example of AC97 Interface</title>
	            <programlisting>
	<![CDATA[
	  struct mychip {
	          ....
	          struct snd_ac97 *ac97;
	          ....
	  };
	
	  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
	                                             unsigned short reg)
	  {
	          struct mychip *chip = ac97->private_data;
	          ....
	          /* read a register value here from the codec */
	          return the_register_value;
	  }
	
	  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
	                                   unsigned short reg, unsigned short val)
	  {
	          struct mychip *chip = ac97->private_data;
	          ....
	          /* write the given register value to the codec */
	  }
	
	  static int snd_mychip_ac97(struct mychip *chip)
	  {
	          struct snd_ac97_bus *bus;
	          struct snd_ac97_template ac97;
	          int err;
	          static struct snd_ac97_bus_ops ops = {
	                  .write = snd_mychip_ac97_write,
	                  .read = snd_mychip_ac97_read,
	          };
	
	          err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
	          if (err < 0)
	                  return err;
	          memset(&ac97, 0, sizeof(ac97));
	          ac97.private_data = chip;
	          return snd_ac97_mixer(bus, &ac97, &chip->ac97);
	  }
	
	]]>
	          </programlisting>
	        </example>
	      </para>
	    </section>
	
	    <section id="api-ac97-constructor">
	      <title>Constructor</title>
	      <para>
	        To create an ac97 instance, first call <function>snd_ac97_bus</function>
	      with an <type>ac97_bus_ops_t</type> record with callback functions.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_ac97_bus *bus;
	  static struct snd_ac97_bus_ops ops = {
	        .write = snd_mychip_ac97_write,
	        .read = snd_mychip_ac97_read,
	  };
	
	  snd_ac97_bus(card, 0, &ops, NULL, &pbus);
	]]>
	          </programlisting>
	        </informalexample>
	
	      The bus record is shared among all belonging ac97 instances.
	      </para>
	
	      <para>
	      And then call <function>snd_ac97_mixer()</function> with an
	      struct <structname>snd_ac97_template</structname>
	      record together with the bus pointer created above.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_ac97_template ac97;
	  int err;
	
	  memset(&ac97, 0, sizeof(ac97));
	  ac97.private_data = chip;
	  snd_ac97_mixer(bus, &ac97, &chip->ac97);
	]]>
	          </programlisting>
	        </informalexample>
	
	        where chip-&gt;ac97 is a pointer to a newly created
	        <type>ac97_t</type> instance.
	        In this case, the chip pointer is set as the private data, so that
	        the read/write callback functions can refer to this chip instance.
	        This instance is not necessarily stored in the chip
		record.  If you need to change the register values from the
	        driver, or need the suspend/resume of ac97 codecs, keep this
	        pointer to pass to the corresponding functions.
	      </para>
	    </section>
	
	    <section id="api-ac97-callbacks">
	      <title>Callbacks</title>
	      <para>
	        The standard callbacks are <structfield>read</structfield> and
	      <structfield>write</structfield>. Obviously they 
	      correspond to the functions for read and write accesses to the
	      hardware low-level codes. 
	      </para>
	
	      <para>
	        The <structfield>read</structfield> callback returns the
	        register value specified in the argument. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
	                                             unsigned short reg)
	  {
	          struct mychip *chip = ac97->private_data;
	          ....
	          return the_register_value;
	  }
	]]>
	          </programlisting>
	        </informalexample>
	
	        Here, the chip can be cast from ac97-&gt;private_data.
	      </para>
	
	      <para>
	        Meanwhile, the <structfield>write</structfield> callback is
	        used to set the register value. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
	                       unsigned short reg, unsigned short val)
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	      These callbacks are non-atomic like the control API callbacks.
	      </para>
	
	      <para>
	        There are also other callbacks:
	      <structfield>reset</structfield>,
	      <structfield>wait</structfield> and
	      <structfield>init</structfield>. 
	      </para>
	
	      <para>
	        The <structfield>reset</structfield> callback is used to reset
	      the codec. If the chip requires a special kind of reset, you can
	      define this callback. 
	      </para>
	
	      <para>
	        The <structfield>wait</structfield> callback is used to
	      add some waiting time in the standard initialization of the codec. If the
	      chip requires the extra waiting time, define this callback. 
	      </para>
	
	      <para>
	        The <structfield>init</structfield> callback is used for
	      additional initialization of the codec.
	      </para>
	    </section>
	
	    <section id="api-ac97-updating-registers">
	      <title>Updating Registers in The Driver</title>
	      <para>
	        If you need to access to the codec from the driver, you can
	      call the following functions:
	      <function>snd_ac97_write()</function>,
	      <function>snd_ac97_read()</function>,
	      <function>snd_ac97_update()</function> and
	      <function>snd_ac97_update_bits()</function>. 
	      </para>
	
	      <para>
	        Both <function>snd_ac97_write()</function> and
	        <function>snd_ac97_update()</function> functions are used to
	        set a value to the given register
	        (<constant>AC97_XXX</constant>). The difference between them is
	        that <function>snd_ac97_update()</function> doesn't write a
	        value if the given value has been already set, while
	        <function>snd_ac97_write()</function> always rewrites the
	        value. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_ac97_write(ac97, AC97_MASTER, 0x8080);
	  snd_ac97_update(ac97, AC97_MASTER, 0x8080);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        <function>snd_ac97_read()</function> is used to read the value
	        of the given register. For example, 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  value = snd_ac97_read(ac97, AC97_MASTER);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        <function>snd_ac97_update_bits()</function> is used to update
	        some bits in the given register.  
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_ac97_update_bits(ac97, reg, mask, value);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        Also, there is a function to change the sample rate (of a
	        given register such as
	        <constant>AC97_PCM_FRONT_DAC_RATE</constant>) when VRA or
	        DRA is supported by the codec:
	        <function>snd_ac97_set_rate()</function>. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The following registers are available to set the rate:
	      <constant>AC97_PCM_MIC_ADC_RATE</constant>,
	      <constant>AC97_PCM_FRONT_DAC_RATE</constant>,
	      <constant>AC97_PCM_LR_ADC_RATE</constant>,
	      <constant>AC97_SPDIF</constant>. When
	      <constant>AC97_SPDIF</constant> is specified, the register is
	      not really changed but the corresponding IEC958 status bits will
	      be updated. 
	      </para>
	    </section>
	
	    <section id="api-ac97-clock-adjustment">
	      <title>Clock Adjustment</title>
	      <para>
	        In some chips, the clock of the codec isn't 48000 but using a
	      PCI clock (to save a quartz!). In this case, change the field
	      bus-&gt;clock to the corresponding
	      value. For example, intel8x0 
	      and es1968 drivers have their own function to read from the clock.
	      </para>
	    </section>
	
	    <section id="api-ac97-proc-files">
	      <title>Proc Files</title>
	      <para>
	        The ALSA AC97 interface will create a proc file such as
	      <filename>/proc/asound/card0/codec97#0/ac97#0-0</filename> and
	      <filename>ac97#0-0+regs</filename>. You can refer to these files to
	      see the current status and registers of the codec. 
	      </para>
	    </section>
	
	    <section id="api-ac97-multiple-codecs">
	      <title>Multiple Codecs</title>
	      <para>
	        When there are several codecs on the same card, you need to
	      call <function>snd_ac97_mixer()</function> multiple times with
	      ac97.num=1 or greater. The <structfield>num</structfield> field
	      specifies the codec number. 
	      </para>
	
	      <para>
	        If you set up multiple codecs, you either need to write
	      different callbacks for each codec or check
	      ac97-&gt;num in the callback routines. 
	      </para>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- MIDI (MPU401-UART) Interface  -->
	<!-- ****************************************************** -->
	  <chapter id="midi-interface">
	    <title>MIDI (MPU401-UART) Interface</title>
	
	    <section id="midi-interface-general">
	      <title>General</title>
	      <para>
	        Many soundcards have built-in MIDI (MPU401-UART)
	      interfaces. When the soundcard supports the standard MPU401-UART
	      interface, most likely you can use the ALSA MPU401-UART API. The
	      MPU401-UART API is defined in
	      <filename>&lt;sound/mpu401.h&gt;</filename>. 
	      </para>
	
	      <para>
	        Some soundchips have a similar but slightly different
	      implementation of mpu401 stuff. For example, emu10k1 has its own
	      mpu401 routines. 
	      </para>
	    </section>
	
	    <section id="midi-interface-constructor">
	      <title>Constructor</title>
	      <para>
	        To create a rawmidi object, call
	      <function>snd_mpu401_uart_new()</function>. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_rawmidi *rmidi;
	  snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
	                      irq, &rmidi);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The first argument is the card pointer, and the second is the
	      index of this component. You can create up to 8 rawmidi
	      devices. 
	      </para>
	
	      <para>
	        The third argument is the type of the hardware,
	      <constant>MPU401_HW_XXX</constant>. If it's not a special one,
	      you can use <constant>MPU401_HW_MPU401</constant>. 
	      </para>
	
	      <para>
	        The 4th argument is the I/O port address. Many
	      backward-compatible MPU401 have an I/O port such as 0x330. Or, it
	      might be a part of its own PCI I/O region. It depends on the
	      chip design. 
	      </para>
	
	      <para>
		The 5th argument is a bitflag for additional information.
	        When the I/O port address above is part of the PCI I/O
	      region, the MPU401 I/O port might have been already allocated
	      (reserved) by the driver itself. In such a case, pass a bit flag
	      <constant>MPU401_INFO_INTEGRATED</constant>,
	      and the mpu401-uart layer will allocate the I/O ports by itself. 
	      </para>
	
		<para>
		When the controller supports only the input or output MIDI stream,
		pass the <constant>MPU401_INFO_INPUT</constant> or
		<constant>MPU401_INFO_OUTPUT</constant> bitflag, respectively.
		Then the rawmidi instance is created as a single stream.
		</para>
	
		<para>
		<constant>MPU401_INFO_MMIO</constant> bitflag is used to change
		the access method to MMIO (via readb and writeb) instead of
		iob and outb. In this case, you have to pass the iomapped address
		to <function>snd_mpu401_uart_new()</function>.
		</para>
	
		<para>
		When <constant>MPU401_INFO_TX_IRQ</constant> is set, the output
		stream isn't checked in the default interrupt handler.  The driver
		needs to call <function>snd_mpu401_uart_interrupt_tx()</function>
		by itself to start processing the output stream in the irq handler.
		</para>
	
		<para>
		If the MPU-401 interface shares its interrupt with the other logical
		devices on the card, set <constant>MPU401_INFO_IRQ_HOOK</constant>
		(see <link linkend="midi-interface-interrupt-handler"><citetitle>
		below</citetitle></link>).
		</para>
	
	      <para>
	        Usually, the port address corresponds to the command port and
	        port + 1 corresponds to the data port. If not, you may change
	        the <structfield>cport</structfield> field of
	        struct <structname>snd_mpu401</structname> manually 
	        afterward. However, <structname>snd_mpu401</structname> pointer is not
	        returned explicitly by
	        <function>snd_mpu401_uart_new()</function>. You need to cast
	        rmidi-&gt;private_data to
	        <structname>snd_mpu401</structname> explicitly, 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_mpu401 *mpu;
	  mpu = rmidi->private_data;
	]]>
	          </programlisting>
	        </informalexample>
	
	        and reset the cport as you like:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  mpu->cport = my_own_control_port;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
		The 6th argument specifies the ISA irq number that will be
		allocated.  If no interrupt is to be allocated (because your
		code is already allocating a shared interrupt, or because the
		device does not use interrupts), pass -1 instead.
		For a MPU-401 device without an interrupt, a polling timer
		will be used instead.
	      </para>
	    </section>
	
	    <section id="midi-interface-interrupt-handler">
	      <title>Interrupt Handler</title>
	      <para>
	        When the interrupt is allocated in
	      <function>snd_mpu401_uart_new()</function>, an exclusive ISA
	      interrupt handler is automatically used, hence you don't have
	      anything else to do than creating the mpu401 stuff.  Otherwise, you
	      have to set <constant>MPU401_INFO_IRQ_HOOK</constant>, and call
	      <function>snd_mpu401_uart_interrupt()</function> explicitly from your
	      own interrupt handler when it has determined that a UART interrupt
	      has occurred.
	      </para>
	
	      <para>
	        In this case, you need to pass the private_data of the
	        returned rawmidi object from
	        <function>snd_mpu401_uart_new()</function> as the second
	        argument of <function>snd_mpu401_uart_interrupt()</function>. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- RawMIDI Interface  -->
	<!-- ****************************************************** -->
	  <chapter id="rawmidi-interface">
	    <title>RawMIDI Interface</title>
	
	    <section id="rawmidi-interface-overview">
	      <title>Overview</title>
	
	      <para>
	      The raw MIDI interface is used for hardware MIDI ports that can
	      be accessed as a byte stream.  It is not used for synthesizer
	      chips that do not directly understand MIDI.
	      </para>
	
	      <para>
	      ALSA handles file and buffer management.  All you have to do is
	      to write some code to move data between the buffer and the
	      hardware.
	      </para>
	
	      <para>
	      The rawmidi API is defined in
	      <filename>&lt;sound/rawmidi.h&gt;</filename>.
	      </para>
	    </section>
	
	    <section id="rawmidi-interface-constructor">
	      <title>Constructor</title>
	
	      <para>
	      To create a rawmidi device, call the
	      <function>snd_rawmidi_new</function> function:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_rawmidi *rmidi;
	  err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
	  if (err < 0)
	          return err;
	  rmidi->private_data = chip;
	  strcpy(rmidi->name, "My MIDI");
	  rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
	                      SNDRV_RAWMIDI_INFO_INPUT |
	                      SNDRV_RAWMIDI_INFO_DUPLEX;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	      The first argument is the card pointer, the second argument is
	      the ID string.
	      </para>
	
	      <para>
	      The third argument is the index of this component.  You can
	      create up to 8 rawmidi devices.
	      </para>
	
	      <para>
	      The fourth and fifth arguments are the number of output and
	      input substreams, respectively, of this device (a substream is
	      the equivalent of a MIDI port).
	      </para>
	
	      <para>
	      Set the <structfield>info_flags</structfield> field to specify
	      the capabilities of the device.
	      Set <constant>SNDRV_RAWMIDI_INFO_OUTPUT</constant> if there is
	      at least one output port,
	      <constant>SNDRV_RAWMIDI_INFO_INPUT</constant> if there is at
	      least one input port,
	      and <constant>SNDRV_RAWMIDI_INFO_DUPLEX</constant> if the device
	      can handle output and input at the same time.
	      </para>
	
	      <para>
	      After the rawmidi device is created, you need to set the
	      operators (callbacks) for each substream.  There are helper
	      functions to set the operators for all the substreams of a device:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
	  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	      The operators are usually defined like this:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static struct snd_rawmidi_ops snd_mymidi_output_ops = {
	          .open =    snd_mymidi_output_open,
	          .close =   snd_mymidi_output_close,
	          .trigger = snd_mymidi_output_trigger,
	  };
	]]>
	          </programlisting>
	        </informalexample>
	      These callbacks are explained in the <link
	      linkend="rawmidi-interface-callbacks"><citetitle>Callbacks</citetitle></link>
	      section.
	      </para>
	
	      <para>
	      If there are more than one substream, you should give a
	      unique name to each of them:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_rawmidi_substream *substream;
	  list_for_each_entry(substream,
	                      &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
	                      list {
	          sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
	  }
	  /* same for SNDRV_RAWMIDI_STREAM_INPUT */
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	    </section>
	
	    <section id="rawmidi-interface-callbacks">
	      <title>Callbacks</title>
	
	      <para>
	      In all the callbacks, the private data that you've set for the
	      rawmidi device can be accessed as
	      substream-&gt;rmidi-&gt;private_data.
	      <!-- <code> isn't available before DocBook 4.3 -->
	      </para>
	
	      <para>
	      If there is more than one port, your callbacks can determine the
	      port index from the struct snd_rawmidi_substream data passed to each
	      callback:
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_rawmidi_substream *substream;
	  int index = substream->number;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <section id="rawmidi-interface-op-open">
	      <title><function>open</function> callback</title>
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int snd_xxx_open(struct snd_rawmidi_substream *substream);
	]]>
	          </programlisting>
	        </informalexample>
	
	        <para>
	        This is called when a substream is opened.
	        You can initialize the hardware here, but you shouldn't
	        start transmitting/receiving data yet.
	        </para>
	      </section>
	
	      <section id="rawmidi-interface-op-close">
	      <title><function>close</function> callback</title>
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int snd_xxx_close(struct snd_rawmidi_substream *substream);
	]]>
	          </programlisting>
	        </informalexample>
	
	        <para>
	        Guess what.
	        </para>
	
	        <para>
	        The <function>open</function> and <function>close</function>
	        callbacks of a rawmidi device are serialized with a mutex,
	        and can sleep.
	        </para>
	      </section>
	
	      <section id="rawmidi-interface-op-trigger-out">
	      <title><function>trigger</function> callback for output
	      substreams</title>
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
	]]>
	          </programlisting>
	        </informalexample>
	
	        <para>
	        This is called with a nonzero <parameter>up</parameter>
	        parameter when there is some data in the substream buffer that
	        must be transmitted.
	        </para>
	
	        <para>
	        To read data from the buffer, call
	        <function>snd_rawmidi_transmit_peek</function>.  It will
	        return the number of bytes that have been read; this will be
	        less than the number of bytes requested when there are no more
	        data in the buffer.
	        After the data have been transmitted successfully, call
	        <function>snd_rawmidi_transmit_ack</function> to remove the
	        data from the substream buffer:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  unsigned char data;
	  while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
	          if (snd_mychip_try_to_transmit(data))
	                  snd_rawmidi_transmit_ack(substream, 1);
	          else
	                  break; /* hardware FIFO full */
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	        If you know beforehand that the hardware will accept data, you
	        can use the <function>snd_rawmidi_transmit</function> function
	        which reads some data and removes them from the buffer at once:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  while (snd_mychip_transmit_possible()) {
	          unsigned char data;
	          if (snd_rawmidi_transmit(substream, &data, 1) != 1)
	                  break; /* no more data */
	          snd_mychip_transmit(data);
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	
	        <para>
	        If you know beforehand how many bytes you can accept, you can
	        use a buffer size greater than one with the
	        <function>snd_rawmidi_transmit*</function> functions.
	        </para>
	
	        <para>
	        The <function>trigger</function> callback must not sleep.  If
	        the hardware FIFO is full before the substream buffer has been
	        emptied, you have to continue transmitting data later, either
	        in an interrupt handler, or with a timer if the hardware
	        doesn't have a MIDI transmit interrupt.
	        </para>
	
	        <para>
	        The <function>trigger</function> callback is called with a
	        zero <parameter>up</parameter> parameter when the transmission
	        of data should be aborted.
	        </para>
	      </section>
	
	      <section id="rawmidi-interface-op-trigger-in">
	      <title><function>trigger</function> callback for input
	      substreams</title>
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
	]]>
	          </programlisting>
	        </informalexample>
	
	        <para>
	        This is called with a nonzero <parameter>up</parameter>
	        parameter to enable receiving data, or with a zero
	        <parameter>up</parameter> parameter do disable receiving data.
	        </para>
	
	        <para>
	        The <function>trigger</function> callback must not sleep; the
	        actual reading of data from the device is usually done in an
	        interrupt handler.
	        </para>
	
	        <para>
	        When data reception is enabled, your interrupt handler should
	        call <function>snd_rawmidi_receive</function> for all received
	        data:
	          <informalexample>
	            <programlisting>
	<![CDATA[
	  void snd_mychip_midi_interrupt(...)
	  {
	          while (mychip_midi_available()) {
	                  unsigned char data;
	                  data = mychip_midi_read();
	                  snd_rawmidi_receive(substream, &data, 1);
	          }
	  }
	]]>
	            </programlisting>
	          </informalexample>
	        </para>
	      </section>
	
	      <section id="rawmidi-interface-op-drain">
	      <title><function>drain</function> callback</title>
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
	]]>
	          </programlisting>
	        </informalexample>
	
	        <para>
	        This is only used with output substreams.  This function should wait
	        until all data read from the substream buffer have been transmitted.
	        This ensures that the device can be closed and the driver unloaded
	        without losing data.
	        </para>
	
	        <para>
	        This callback is optional. If you do not set
	        <structfield>drain</structfield> in the struct snd_rawmidi_ops
	        structure, ALSA will simply wait for 50&nbsp;milliseconds
	        instead.
	        </para>
	      </section>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- Miscellaneous Devices  -->
	<!-- ****************************************************** -->
	  <chapter id="misc-devices">
	    <title>Miscellaneous Devices</title>
	
	    <section id="misc-devices-opl3">
	      <title>FM OPL3</title>
	      <para>
	        The FM OPL3 is still used in many chips (mainly for backward
	      compatibility). ALSA has a nice OPL3 FM control layer, too. The
	      OPL3 API is defined in
	      <filename>&lt;sound/opl3.h&gt;</filename>. 
	      </para>
	
	      <para>
	        FM registers can be directly accessed through the direct-FM API,
	      defined in <filename>&lt;sound/asound_fm.h&gt;</filename>. In
	      ALSA native mode, FM registers are accessed through
	      the Hardware-Dependent Device direct-FM extension API, whereas in
	      OSS compatible mode, FM registers can be accessed with the OSS
	      direct-FM compatible API in <filename>/dev/dmfmX</filename> device. 
	      </para>
	
	      <para>
	        To create the OPL3 component, you have two functions to
	        call. The first one is a constructor for the <type>opl3_t</type>
	        instance. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_opl3 *opl3;
	  snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
	                  integrated, &opl3);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The first argument is the card pointer, the second one is the
	      left port address, and the third is the right port address. In
	      most cases, the right port is placed at the left port + 2. 
	      </para>
	
	      <para>
	        The fourth argument is the hardware type.
	      </para>
	
	      <para>
	        When the left and right ports have been already allocated by
	      the card driver, pass non-zero to the fifth argument
	      (<parameter>integrated</parameter>). Otherwise, the opl3 module will
	      allocate the specified ports by itself. 
	      </para>
	
	      <para>
	        When the accessing the hardware requires special method
	        instead of the standard I/O access, you can create opl3 instance
	        separately with <function>snd_opl3_new()</function>.
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_opl3 *opl3;
	  snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
		Then set <structfield>command</structfield>,
		<structfield>private_data</structfield> and
		<structfield>private_free</structfield> for the private
		access function, the private data and the destructor.
		The l_port and r_port are not necessarily set.  Only the
		command must be set properly.  You can retrieve the data
		from the opl3-&gt;private_data field.
	      </para>
	
	      <para>
		After creating the opl3 instance via <function>snd_opl3_new()</function>,
		call <function>snd_opl3_init()</function> to initialize the chip to the
		proper state. Note that <function>snd_opl3_create()</function> always
		calls it internally.
	      </para>
	
	      <para>
	        If the opl3 instance is created successfully, then create a
	        hwdep device for this opl3. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_hwdep *opl3hwdep;
	  snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The first argument is the <type>opl3_t</type> instance you
	      created, and the second is the index number, usually 0. 
	      </para>
	
	      <para>
	        The third argument is the index-offset for the sequencer
	      client assigned to the OPL3 port. When there is an MPU401-UART,
	      give 1 for here (UART always takes 0). 
	      </para>
	    </section>
	
	    <section id="misc-devices-hardware-dependent">
	      <title>Hardware-Dependent Devices</title>
	      <para>
	        Some chips need user-space access for special
	      controls or for loading the micro code. In such a case, you can
	      create a hwdep (hardware-dependent) device. The hwdep API is
	      defined in <filename>&lt;sound/hwdep.h&gt;</filename>. You can
	      find examples in opl3 driver or
	      <filename>isa/sb/sb16_csp.c</filename>. 
	      </para>
	
	      <para>
	        The creation of the <type>hwdep</type> instance is done via
	        <function>snd_hwdep_new()</function>. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_hwdep *hw;
	  snd_hwdep_new(card, "My HWDEP", 0, &hw);
	]]>
	          </programlisting>
	        </informalexample>
	
	        where the third argument is the index number.
	      </para>
	
	      <para>
	        You can then pass any pointer value to the
	        <parameter>private_data</parameter>.
	        If you assign a private data, you should define the
	        destructor, too. The destructor function is set in
	        the <structfield>private_free</structfield> field.  
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
	  hw->private_data = p;
	  hw->private_free = mydata_free;
	]]>
	          </programlisting>
	        </informalexample>
	
	        and the implementation of the destructor would be:
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static void mydata_free(struct snd_hwdep *hw)
	  {
	          struct mydata *p = hw->private_data;
	          kfree(p);
	  }
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The arbitrary file operations can be defined for this
	        instance. The file operators are defined in
	        the <parameter>ops</parameter> table. For example, assume that
	        this chip needs an ioctl. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  hw->ops.open = mydata_open;
	  hw->ops.ioctl = mydata_ioctl;
	  hw->ops.release = mydata_release;
	]]>
	          </programlisting>
	        </informalexample>
	
	        And implement the callback functions as you like.
	      </para>
	    </section>
	
	    <section id="misc-devices-IEC958">
	      <title>IEC958 (S/PDIF)</title>
	      <para>
	        Usually the controls for IEC958 devices are implemented via
	      the control interface. There is a macro to compose a name string for
	      IEC958 controls, <function>SNDRV_CTL_NAME_IEC958()</function>
	      defined in <filename>&lt;include/asound.h&gt;</filename>.  
	      </para>
	
	      <para>
	        There are some standard controls for IEC958 status bits. These
	      controls use the type <type>SNDRV_CTL_ELEM_TYPE_IEC958</type>,
	      and the size of element is fixed as 4 bytes array
	      (value.iec958.status[x]). For the <structfield>info</structfield>
	      callback, you don't specify 
	      the value field for this type (the count field must be set,
	      though). 
	      </para>
	
	      <para>
	        <quote>IEC958 Playback Con Mask</quote> is used to return the
	      bit-mask for the IEC958 status bits of consumer mode. Similarly,
	      <quote>IEC958 Playback Pro Mask</quote> returns the bitmask for
	      professional mode. They are read-only controls, and are defined
	      as MIXER controls (iface =
	      <constant>SNDRV_CTL_ELEM_IFACE_MIXER</constant>).  
	      </para>
	
	      <para>
	        Meanwhile, <quote>IEC958 Playback Default</quote> control is
	      defined for getting and setting the current default IEC958
	      bits. Note that this one is usually defined as a PCM control
	      (iface = <constant>SNDRV_CTL_ELEM_IFACE_PCM</constant>),
	      although in some places it's defined as a MIXER control. 
	      </para>
	
	      <para>
	        In addition, you can define the control switches to
	      enable/disable or to set the raw bit mode. The implementation
	      will depend on the chip, but the control should be named as
	      <quote>IEC958 xxx</quote>, preferably using
	      the <function>SNDRV_CTL_NAME_IEC958()</function> macro. 
	      </para>
	
	      <para>
	        You can find several cases, for example,
	      <filename>pci/emu10k1</filename>,
	      <filename>pci/ice1712</filename>, or
	      <filename>pci/cmipci.c</filename>.  
	      </para>
	    </section>
	
	  </chapter>
	
	
	<!-- ****************************************************** -->
	<!-- Buffer and Memory Management  -->
	<!-- ****************************************************** -->
	  <chapter id="buffer-and-memory">
	    <title>Buffer and Memory Management</title>
	
	    <section id="buffer-and-memory-buffer-types">
	      <title>Buffer Types</title>
	      <para>
	        ALSA provides several different buffer allocation functions
	      depending on the bus and the architecture. All these have a
	      consistent API. The allocation of physically-contiguous pages is
	      done via 
	      <function>snd_malloc_xxx_pages()</function> function, where xxx
	      is the bus type. 
	      </para>
	
	      <para>
	        The allocation of pages with fallback is
	      <function>snd_malloc_xxx_pages_fallback()</function>. This
	      function tries to allocate the specified pages but if the pages
	      are not available, it tries to reduce the page sizes until
	      enough space is found.
	      </para>
	
	      <para>
	      The release the pages, call
	      <function>snd_free_xxx_pages()</function> function. 
	      </para>
	
	      <para>
	      Usually, ALSA drivers try to allocate and reserve
	       a large contiguous physical space
	       at the time the module is loaded for the later use.
	       This is called <quote>pre-allocation</quote>.
	       As already written, you can call the following function at 
	       pcm instance construction time (in the case of PCI bus). 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
	                                        snd_dma_pci_data(pci), size, max);
	]]>
	          </programlisting>
	        </informalexample>
	
	        where <parameter>size</parameter> is the byte size to be
	      pre-allocated and the <parameter>max</parameter> is the maximum
	      size to be changed via the <filename>prealloc</filename> proc file.
	      The allocator will try to get an area as large as possible
	      within the given size. 
	      </para>
	
	      <para>
	      The second argument (type) and the third argument (device pointer)
	      are dependent on the bus.
	      In the case of the ISA bus, pass <function>snd_dma_isa_data()</function>
	      as the third argument with <constant>SNDRV_DMA_TYPE_DEV</constant> type.
	      For the continuous buffer unrelated to the bus can be pre-allocated
	      with <constant>SNDRV_DMA_TYPE_CONTINUOUS</constant> type and the
	      <function>snd_dma_continuous_data(GFP_KERNEL)</function> device pointer,
	      where <constant>GFP_KERNEL</constant> is the kernel allocation flag to
	      use.
	      For the PCI scatter-gather buffers, use
	      <constant>SNDRV_DMA_TYPE_DEV_SG</constant> with
	      <function>snd_dma_pci_data(pci)</function>
	      (see the 
	          <link linkend="buffer-and-memory-non-contiguous"><citetitle>Non-Contiguous Buffers
	          </citetitle></link> section).
	      </para>
	
	      <para>
	        Once the buffer is pre-allocated, you can use the
	        allocator in the <structfield>hw_params</structfield> callback: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  snd_pcm_lib_malloc_pages(substream, size);
	]]>
	          </programlisting>
	        </informalexample>
	
	        Note that you have to pre-allocate to use this function.
	      </para>
	    </section>
	
	    <section id="buffer-and-memory-external-hardware">
	      <title>External Hardware Buffers</title>
	      <para>
	        Some chips have their own hardware buffers and the DMA
	      transfer from the host memory is not available. In such a case,
	      you need to either 1) copy/set the audio data directly to the
	      external hardware buffer, or 2) make an intermediate buffer and
	      copy/set the data from it to the external hardware buffer in
	      interrupts (or in tasklets, preferably).
	      </para>
	
	      <para>
	        The first case works fine if the external hardware buffer is large
	      enough.  This method doesn't need any extra buffers and thus is
	      more effective. You need to define the
	      <structfield>copy</structfield> and
	      <structfield>silence</structfield> callbacks for 
	      the data transfer. However, there is a drawback: it cannot
	      be mmapped. The examples are GUS's GF1 PCM or emu8000's
	      wavetable PCM. 
	      </para>
	
	      <para>
	        The second case allows for mmap on the buffer, although you have
	      to handle an interrupt or a tasklet to transfer the data
	      from the intermediate buffer to the hardware buffer. You can find an
	      example in the vxpocket driver. 
	      </para>
	
	      <para>
	        Another case is when the chip uses a PCI memory-map
	      region for the buffer instead of the host memory. In this case,
	      mmap is available only on certain architectures like the Intel one.
	      In non-mmap mode, the data cannot be transferred as in the normal
	      way. Thus you need to define the <structfield>copy</structfield> and
	      <structfield>silence</structfield> callbacks as well, 
	      as in the cases above. The examples are found in
	      <filename>rme32.c</filename> and <filename>rme96.c</filename>. 
	      </para>
	
	      <para>
	        The implementation of the <structfield>copy</structfield> and
	        <structfield>silence</structfield> callbacks depends upon 
	        whether the hardware supports interleaved or non-interleaved
	        samples. The <structfield>copy</structfield> callback is
	        defined like below, a bit 
	        differently depending whether the direction is playback or
	        capture: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int playback_copy(struct snd_pcm_substream *substream, int channel,
	               snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
	  static int capture_copy(struct snd_pcm_substream *substream, int channel,
	               snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        In the case of interleaved samples, the second argument
	      (<parameter>channel</parameter>) is not used. The third argument
	      (<parameter>pos</parameter>) points the 
	      current position offset in frames. 
	      </para>
	
	      <para>
	        The meaning of the fourth argument is different between
	      playback and capture. For playback, it holds the source data
	      pointer, and for capture, it's the destination data pointer. 
	      </para>
	
	      <para>
	        The last argument is the number of frames to be copied.
	      </para>
	
	      <para>
	        What you have to do in this callback is again different
	        between playback and capture directions. In the
	        playback case, you copy the given amount of data
	        (<parameter>count</parameter>) at the specified pointer
	        (<parameter>src</parameter>) to the specified offset
	        (<parameter>pos</parameter>) on the hardware buffer. When
	        coded like memcpy-like way, the copy would be like: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
	            frames_to_bytes(runtime, count));
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        For the capture direction, you copy the given amount of
	        data (<parameter>count</parameter>) at the specified offset
	        (<parameter>pos</parameter>) on the hardware buffer to the
	        specified pointer (<parameter>dst</parameter>). 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
	            frames_to_bytes(runtime, count));
	]]>
	          </programlisting>
	        </informalexample>
	
	        Note that both the position and the amount of data are given
	      in frames. 
	      </para>
	
	      <para>
	        In the case of non-interleaved samples, the implementation
	      will be a bit more complicated. 
	      </para>
	
	      <para>
	        You need to check the channel argument, and if it's -1, copy
	      the whole channels. Otherwise, you have to copy only the
	      specified channel. Please check
	      <filename>isa/gus/gus_pcm.c</filename> as an example. 
	      </para>
	
	      <para>
	        The <structfield>silence</structfield> callback is also
	        implemented in a similar way. 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  static int silence(struct snd_pcm_substream *substream, int channel,
	                     snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        The meanings of arguments are the same as in the
	      <structfield>copy</structfield> 
	      callback, although there is no <parameter>src/dst</parameter>
	      argument. In the case of interleaved samples, the channel
	      argument has no meaning, as well as on
	      <structfield>copy</structfield> callback.  
	      </para>
	
	      <para>
	        The role of <structfield>silence</structfield> callback is to
	        set the given amount 
	        (<parameter>count</parameter>) of silence data at the
	        specified offset (<parameter>pos</parameter>) on the hardware
	        buffer. Suppose that the data format is signed (that is, the
	        silent-data is 0), and the implementation using a memset-like
	        function would be like: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
	            frames_to_bytes(runtime, count));
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        In the case of non-interleaved samples, again, the
	      implementation becomes a bit more complicated. See, for example,
	      <filename>isa/gus/gus_pcm.c</filename>. 
	      </para>
	    </section>
	
	    <section id="buffer-and-memory-non-contiguous">
	      <title>Non-Contiguous Buffers</title>
	      <para>
	        If your hardware supports the page table as in emu10k1 or the
	      buffer descriptors as in via82xx, you can use the scatter-gather
	      (SG) DMA. ALSA provides an interface for handling SG-buffers.
	      The API is provided in <filename>&lt;sound/pcm.h&gt;</filename>. 
	      </para>
	
	      <para>
	        For creating the SG-buffer handler, call
	        <function>snd_pcm_lib_preallocate_pages()</function> or
	        <function>snd_pcm_lib_preallocate_pages_for_all()</function>
	        with <constant>SNDRV_DMA_TYPE_DEV_SG</constant>
		in the PCM constructor like other PCI pre-allocator.
	        You need to pass <function>snd_dma_pci_data(pci)</function>,
	        where pci is the struct <structname>pci_dev</structname> pointer
	        of the chip as well.
	        The <type>struct snd_sg_buf</type> instance is created as
	        substream-&gt;dma_private. You can cast
	        the pointer like: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	
	      <para>
	        Then call <function>snd_pcm_lib_malloc_pages()</function>
	      in the <structfield>hw_params</structfield> callback
	      as well as in the case of normal PCI buffer.
	      The SG-buffer handler will allocate the non-contiguous kernel
	      pages of the given size and map them onto the virtually contiguous
	      memory.  The virtual pointer is addressed in runtime-&gt;dma_area.
	      The physical address (runtime-&gt;dma_addr) is set to zero,
	      because the buffer is physically non-contiguous.
	      The physical address table is set up in sgbuf-&gt;table.
	      You can get the physical address at a certain offset via
	      <function>snd_pcm_sgbuf_get_addr()</function>. 
	      </para>
	
	      <para>
	        When a SG-handler is used, you need to set
	      <function>snd_pcm_sgbuf_ops_page</function> as
	      the <structfield>page</structfield> callback.
	      (See <link linkend="pcm-interface-operators-page-callback">
	      <citetitle>page callback section</citetitle></link>.)
	      </para>
	
	      <para>
	        To release the data, call
	      <function>snd_pcm_lib_free_pages()</function> in the
	      <structfield>hw_free</structfield> callback as usual.
	      </para>
	    </section>
	
	    <section id="buffer-and-memory-vmalloced">
	      <title>Vmalloc'ed Buffers</title>
	      <para>
	        It's possible to use a buffer allocated via
	      <function>vmalloc</function>, for example, for an intermediate
	      buffer. Since the allocated pages are not contiguous, you need
	      to set the <structfield>page</structfield> callback to obtain
	      the physical address at every offset. 
	      </para>
	
	      <para>
	        The implementation of <structfield>page</structfield> callback
	        would be like this: 
	
	        <informalexample>
	          <programlisting>
	<![CDATA[
	  #include <linux/vmalloc.h>
	
	  /* get the physical page pointer on the given offset */
	  static struct page *mychip_page(struct snd_pcm_substream *substream,
	                                  unsigned long offset)
	  {
	          void *pageptr = substream->runtime->dma_area + offset;
	          return vmalloc_to_page(pageptr);
	  }
	]]>
	          </programlisting>
	        </informalexample>
	      </para>
	    </section>
	
	  </chapter>
	
	
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