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Based on kernel version 2.6.34. Page generated on 2010-05-31 16:02 EST.

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	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
		"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
	
	<book id="libataDevGuide">
	 <bookinfo>
	  <title>libATA Developer's Guide</title>
	  
	  <authorgroup>
	   <author>
	    <firstname>Jeff</firstname>
	    <surname>Garzik</surname>
	   </author>
	  </authorgroup>
	
	  <copyright>
	   <year>2003-2006</year>
	   <holder>Jeff Garzik</holder>
	  </copyright>
	
	  <legalnotice>
	   <para>
	   The contents of this file are subject to the Open
	   Software License version 1.1 that can be found at
	   <ulink url="http://www.opensource.org/licenses/osl-1.1.txt">http://www.opensource.org/licenses/osl-1.1.txt</ulink> and is included herein
	   by reference.
	   </para>
	
	   <para>
	   Alternatively, the contents of this file may be used under the terms
	   of the GNU General Public License version 2 (the "GPL") as distributed
	   in the kernel source COPYING file, in which case the provisions of
	   the GPL are applicable instead of the above.  If you wish to allow
	   the use of your version of this file only under the terms of the
	   GPL and not to allow others to use your version of this file under
	   the OSL, indicate your decision by deleting the provisions above and
	   replace them with the notice and other provisions required by the GPL.
	   If you do not delete the provisions above, a recipient may use your
	   version of this file under either the OSL or the GPL.
	   </para>
	
	  </legalnotice>
	 </bookinfo>
	
	<toc></toc>
	
	  <chapter id="libataIntroduction">
	     <title>Introduction</title>
	  <para>
	  libATA is a library used inside the Linux kernel to support ATA host
	  controllers and devices.  libATA provides an ATA driver API, class
	  transports for ATA and ATAPI devices, and SCSI&lt;-&gt;ATA translation
	  for ATA devices according to the T10 SAT specification.
	  </para>
	  <para>
	  This Guide documents the libATA driver API, library functions, library
	  internals, and a couple sample ATA low-level drivers.
	  </para>
	  </chapter>
	
	  <chapter id="libataDriverApi">
	     <title>libata Driver API</title>
	     <para>
	     struct ata_port_operations is defined for every low-level libata
	     hardware driver, and it controls how the low-level driver
	     interfaces with the ATA and SCSI layers.
	     </para>
	     <para>
	     FIS-based drivers will hook into the system with ->qc_prep() and
	     ->qc_issue() high-level hooks.  Hardware which behaves in a manner
	     similar to PCI IDE hardware may utilize several generic helpers,
	     defining at a bare minimum the bus I/O addresses of the ATA shadow
	     register blocks.
	     </para>
	     <sect1>
	        <title>struct ata_port_operations</title>
	
		<sect2><title>Disable ATA port</title>
		<programlisting>
	void (*port_disable) (struct ata_port *);
		</programlisting>
	
		<para>
		Called from ata_bus_probe() and ata_bus_reset() error paths,
		as well as when unregistering from the SCSI module (rmmod, hot
		unplug).
		This function should do whatever needs to be done to take the
		port out of use.  In most cases, ata_port_disable() can be used
		as this hook.
		</para>
		<para>
		Called from ata_bus_probe() on a failed probe.
		Called from ata_bus_reset() on a failed bus reset.
		Called from ata_scsi_release().
		</para>
	
		</sect2>
	
		<sect2><title>Post-IDENTIFY device configuration</title>
		<programlisting>
	void (*dev_config) (struct ata_port *, struct ata_device *);
		</programlisting>
	
		<para>
		Called after IDENTIFY [PACKET] DEVICE is issued to each device
		found.  Typically used to apply device-specific fixups prior to
		issue of SET FEATURES - XFER MODE, and prior to operation.
		</para>
		<para>
		This entry may be specified as NULL in ata_port_operations.
		</para>
	
		</sect2>
	
		<sect2><title>Set PIO/DMA mode</title>
		<programlisting>
	void (*set_piomode) (struct ata_port *, struct ata_device *);
	void (*set_dmamode) (struct ata_port *, struct ata_device *);
	void (*post_set_mode) (struct ata_port *);
	unsigned int (*mode_filter) (struct ata_port *, struct ata_device *, unsigned int);
		</programlisting>
	
		<para>
		Hooks called prior to the issue of SET FEATURES - XFER MODE
		command.  The optional ->mode_filter() hook is called when libata
		has built a mask of the possible modes. This is passed to the 
		->mode_filter() function which should return a mask of valid modes
		after filtering those unsuitable due to hardware limits. It is not
		valid to use this interface to add modes.
		</para>
		<para>
		dev->pio_mode and dev->dma_mode are guaranteed to be valid when
		->set_piomode() and when ->set_dmamode() is called. The timings for
		any other drive sharing the cable will also be valid at this point.
		That is the library records the decisions for the modes of each
		drive on a channel before it attempts to set any of them.
		</para>
		<para>
		->post_set_mode() is
		called unconditionally, after the SET FEATURES - XFER MODE
		command completes successfully.
		</para>
	
		<para>
		->set_piomode() is always called (if present), but
		->set_dma_mode() is only called if DMA is possible.
		</para>
	
		</sect2>
	
		<sect2><title>Taskfile read/write</title>
		<programlisting>
	void (*sff_tf_load) (struct ata_port *ap, struct ata_taskfile *tf);
	void (*sff_tf_read) (struct ata_port *ap, struct ata_taskfile *tf);
		</programlisting>
	
		<para>
		->tf_load() is called to load the given taskfile into hardware
		registers / DMA buffers.  ->tf_read() is called to read the
		hardware registers / DMA buffers, to obtain the current set of
		taskfile register values.
		Most drivers for taskfile-based hardware (PIO or MMIO) use
		ata_sff_tf_load() and ata_sff_tf_read() for these hooks.
		</para>
	
		</sect2>
	
		<sect2><title>PIO data read/write</title>
		<programlisting>
	void (*sff_data_xfer) (struct ata_device *, unsigned char *, unsigned int, int);
		</programlisting>
	
		<para>
	All bmdma-style drivers must implement this hook.  This is the low-level
	operation that actually copies the data bytes during a PIO data
	transfer.
	Typically the driver will choose one of ata_sff_data_xfer_noirq(),
	ata_sff_data_xfer(), or ata_sff_data_xfer32().
		</para>
	
		</sect2>
	
		<sect2><title>ATA command execute</title>
		<programlisting>
	void (*sff_exec_command)(struct ata_port *ap, struct ata_taskfile *tf);
		</programlisting>
	
		<para>
		causes an ATA command, previously loaded with
		->tf_load(), to be initiated in hardware.
		Most drivers for taskfile-based hardware use ata_sff_exec_command()
		for this hook.
		</para>
	
		</sect2>
	
		<sect2><title>Per-cmd ATAPI DMA capabilities filter</title>
		<programlisting>
	int (*check_atapi_dma) (struct ata_queued_cmd *qc);
		</programlisting>
	
		<para>
	Allow low-level driver to filter ATA PACKET commands, returning a status
	indicating whether or not it is OK to use DMA for the supplied PACKET
	command.
		</para>
		<para>
		This hook may be specified as NULL, in which case libata will
		assume that atapi dma can be supported.
		</para>
	
		</sect2>
	
		<sect2><title>Read specific ATA shadow registers</title>
		<programlisting>
	u8   (*sff_check_status)(struct ata_port *ap);
	u8   (*sff_check_altstatus)(struct ata_port *ap);
		</programlisting>
	
		<para>
		Reads the Status/AltStatus ATA shadow register from
		hardware.  On some hardware, reading the Status register has
		the side effect of clearing the interrupt condition.
		Most drivers for taskfile-based hardware use
		ata_sff_check_status() for this hook.
		</para>
	
		</sect2>
	
		<sect2><title>Select ATA device on bus</title>
		<programlisting>
	void (*sff_dev_select)(struct ata_port *ap, unsigned int device);
		</programlisting>
	
		<para>
		Issues the low-level hardware command(s) that causes one of N
		hardware devices to be considered 'selected' (active and
		available for use) on the ATA bus.  This generally has no
		meaning on FIS-based devices.
		</para>
		<para>
		Most drivers for taskfile-based hardware use
		ata_sff_dev_select() for this hook.
		</para>
	
		</sect2>
	
		<sect2><title>Private tuning method</title>
		<programlisting>
	void (*set_mode) (struct ata_port *ap);
		</programlisting>
	
		<para>
		By default libata performs drive and controller tuning in
		accordance with the ATA timing rules and also applies blacklists
		and cable limits. Some controllers need special handling and have
		custom tuning rules, typically raid controllers that use ATA
		commands but do not actually do drive timing.
		</para>
	
		<warning>
		<para>
		This hook should not be used to replace the standard controller
		tuning logic when a controller has quirks. Replacing the default
		tuning logic in that case would bypass handling for drive and
		bridge quirks that may be important to data reliability. If a
		controller needs to filter the mode selection it should use the
		mode_filter hook instead.
		</para>
		</warning>
	
		</sect2>
	
		<sect2><title>Control PCI IDE BMDMA engine</title>
		<programlisting>
	void (*bmdma_setup) (struct ata_queued_cmd *qc);
	void (*bmdma_start) (struct ata_queued_cmd *qc);
	void (*bmdma_stop) (struct ata_port *ap);
	u8   (*bmdma_status) (struct ata_port *ap);
		</programlisting>
	
		<para>
	When setting up an IDE BMDMA transaction, these hooks arm
	(->bmdma_setup), fire (->bmdma_start), and halt (->bmdma_stop)
	the hardware's DMA engine.  ->bmdma_status is used to read the standard
	PCI IDE DMA Status register.
		</para>
	
		<para>
	These hooks are typically either no-ops, or simply not implemented, in
	FIS-based drivers.
		</para>
		<para>
	Most legacy IDE drivers use ata_bmdma_setup() for the bmdma_setup()
	hook.  ata_bmdma_setup() will write the pointer to the PRD table to
	the IDE PRD Table Address register, enable DMA in the DMA Command
	register, and call exec_command() to begin the transfer.
		</para>
		<para>
	Most legacy IDE drivers use ata_bmdma_start() for the bmdma_start()
	hook.  ata_bmdma_start() will write the ATA_DMA_START flag to the DMA
	Command register.
		</para>
		<para>
	Many legacy IDE drivers use ata_bmdma_stop() for the bmdma_stop()
	hook.  ata_bmdma_stop() clears the ATA_DMA_START flag in the DMA
	command register.
		</para>
		<para>
	Many legacy IDE drivers use ata_bmdma_status() as the bmdma_status() hook.
		</para>
	
		</sect2>
	
		<sect2><title>High-level taskfile hooks</title>
		<programlisting>
	void (*qc_prep) (struct ata_queued_cmd *qc);
	int (*qc_issue) (struct ata_queued_cmd *qc);
		</programlisting>
	
		<para>
		Higher-level hooks, these two hooks can potentially supercede
		several of the above taskfile/DMA engine hooks.  ->qc_prep is
		called after the buffers have been DMA-mapped, and is typically
		used to populate the hardware's DMA scatter-gather table.
		Most drivers use the standard ata_qc_prep() helper function, but
		more advanced drivers roll their own.
		</para>
		<para>
		->qc_issue is used to make a command active, once the hardware
		and S/G tables have been prepared.  IDE BMDMA drivers use the
		helper function ata_qc_issue_prot() for taskfile protocol-based
		dispatch.  More advanced drivers implement their own ->qc_issue.
		</para>
		<para>
		ata_qc_issue_prot() calls ->tf_load(), ->bmdma_setup(), and
		->bmdma_start() as necessary to initiate a transfer.
		</para>
	
		</sect2>
	
		<sect2><title>Exception and probe handling (EH)</title>
		<programlisting>
	void (*eng_timeout) (struct ata_port *ap);
	void (*phy_reset) (struct ata_port *ap);
		</programlisting>
	
		<para>
	Deprecated.  Use ->error_handler() instead.
		</para>
	
		<programlisting>
	void (*freeze) (struct ata_port *ap);
	void (*thaw) (struct ata_port *ap);
		</programlisting>
	
		<para>
	ata_port_freeze() is called when HSM violations or some other
	condition disrupts normal operation of the port.  A frozen port
	is not allowed to perform any operation until the port is
	thawed, which usually follows a successful reset.
		</para>
	
		<para>
	The optional ->freeze() callback can be used for freezing the port
	hardware-wise (e.g. mask interrupt and stop DMA engine).  If a
	port cannot be frozen hardware-wise, the interrupt handler
	must ack and clear interrupts unconditionally while the port
	is frozen.
		</para>
		<para>
	The optional ->thaw() callback is called to perform the opposite of ->freeze():
	prepare the port for normal operation once again.  Unmask interrupts,
	start DMA engine, etc.
		</para>
	
		<programlisting>
	void (*error_handler) (struct ata_port *ap);
		</programlisting>
	
		<para>
	->error_handler() is a driver's hook into probe, hotplug, and recovery
	and other exceptional conditions.  The primary responsibility of an
	implementation is to call ata_do_eh() or ata_bmdma_drive_eh() with a set
	of EH hooks as arguments:
		</para>
	
		<para>
	'prereset' hook (may be NULL) is called during an EH reset, before any other actions
	are taken.
		</para>
	
		<para>
	'postreset' hook (may be NULL) is called after the EH reset is performed.  Based on
	existing conditions, severity of the problem, and hardware capabilities,
		</para>
	
		<para>
	Either 'softreset' (may be NULL) or 'hardreset' (may be NULL) will be
	called to perform the low-level EH reset.
		</para>
	
		<programlisting>
	void (*post_internal_cmd) (struct ata_queued_cmd *qc);
		</programlisting>
	
		<para>
	Perform any hardware-specific actions necessary to finish processing
	after executing a probe-time or EH-time command via ata_exec_internal().
		</para>
	
		</sect2>
	
		<sect2><title>Hardware interrupt handling</title>
		<programlisting>
	irqreturn_t (*irq_handler)(int, void *, struct pt_regs *);
	void (*irq_clear) (struct ata_port *);
		</programlisting>
	
		<para>
		->irq_handler is the interrupt handling routine registered with
		the system, by libata.  ->irq_clear is called during probe just
		before the interrupt handler is registered, to be sure hardware
		is quiet.
		</para>
		<para>
		The second argument, dev_instance, should be cast to a pointer
		to struct ata_host_set.
		</para>
		<para>
		Most legacy IDE drivers use ata_sff_interrupt() for the
		irq_handler hook, which scans all ports in the host_set,
		determines which queued command was active (if any), and calls
		ata_sff_host_intr(ap,qc).
		</para>
		<para>
		Most legacy IDE drivers use ata_sff_irq_clear() for the
		irq_clear() hook, which simply clears the interrupt and error
		flags in the DMA status register.
		</para>
	
		</sect2>
	
		<sect2><title>SATA phy read/write</title>
		<programlisting>
	int (*scr_read) (struct ata_port *ap, unsigned int sc_reg,
			 u32 *val);
	int (*scr_write) (struct ata_port *ap, unsigned int sc_reg,
	                   u32 val);
		</programlisting>
	
		<para>
		Read and write standard SATA phy registers.  Currently only used
		if ->phy_reset hook called the sata_phy_reset() helper function.
		sc_reg is one of SCR_STATUS, SCR_CONTROL, SCR_ERROR, or SCR_ACTIVE.
		</para>
	
		</sect2>
	
		<sect2><title>Init and shutdown</title>
		<programlisting>
	int (*port_start) (struct ata_port *ap);
	void (*port_stop) (struct ata_port *ap);
	void (*host_stop) (struct ata_host_set *host_set);
		</programlisting>
	
		<para>
		->port_start() is called just after the data structures for each
		port are initialized.  Typically this is used to alloc per-port
		DMA buffers / tables / rings, enable DMA engines, and similar
		tasks.  Some drivers also use this entry point as a chance to
		allocate driver-private memory for ap->private_data.
		</para>
		<para>
		Many drivers use ata_port_start() as this hook or call
		it from their own port_start() hooks.  ata_port_start()
		allocates space for a legacy IDE PRD table and returns.
		</para>
		<para>
		->port_stop() is called after ->host_stop().  It's sole function
		is to release DMA/memory resources, now that they are no longer
		actively being used.  Many drivers also free driver-private
		data from port at this time.
		</para>
		<para>
		->host_stop() is called after all ->port_stop() calls
	have completed.  The hook must finalize hardware shutdown, release DMA
	and other resources, etc.
		This hook may be specified as NULL, in which case it is not called.
		</para>
	
		</sect2>
	
	     </sect1>
	  </chapter>
	
	  <chapter id="libataEH">
	        <title>Error handling</title>
	
		<para>
		This chapter describes how errors are handled under libata.
		Readers are advised to read SCSI EH
		(Documentation/scsi/scsi_eh.txt) and ATA exceptions doc first.
		</para>
	
		<sect1><title>Origins of commands</title>
		<para>
		In libata, a command is represented with struct ata_queued_cmd
		or qc.  qc's are preallocated during port initialization and
		repetitively used for command executions.  Currently only one
		qc is allocated per port but yet-to-be-merged NCQ branch
		allocates one for each tag and maps each qc to NCQ tag 1-to-1.
		</para>
		<para>
		libata commands can originate from two sources - libata itself
		and SCSI midlayer.  libata internal commands are used for
		initialization and error handling.  All normal blk requests
		and commands for SCSI emulation are passed as SCSI commands
		through queuecommand callback of SCSI host template.
		</para>
		</sect1>
	
		<sect1><title>How commands are issued</title>
	
		<variablelist>
	
		<varlistentry><term>Internal commands</term>
		<listitem>
		<para>
		First, qc is allocated and initialized using
		ata_qc_new_init().  Although ata_qc_new_init() doesn't
		implement any wait or retry mechanism when qc is not
		available, internal commands are currently issued only during
		initialization and error recovery, so no other command is
		active and allocation is guaranteed to succeed.
		</para>
		<para>
		Once allocated qc's taskfile is initialized for the command to
		be executed.  qc currently has two mechanisms to notify
		completion.  One is via qc->complete_fn() callback and the
		other is completion qc->waiting.  qc->complete_fn() callback
		is the asynchronous path used by normal SCSI translated
		commands and qc->waiting is the synchronous (issuer sleeps in
		process context) path used by internal commands.
		</para>
		<para>
		Once initialization is complete, host_set lock is acquired
		and the qc is issued.
		</para>
		</listitem>
		</varlistentry>
	
		<varlistentry><term>SCSI commands</term>
		<listitem>
		<para>
		All libata drivers use ata_scsi_queuecmd() as
		hostt->queuecommand callback.  scmds can either be simulated
		or translated.  No qc is involved in processing a simulated
		scmd.  The result is computed right away and the scmd is
		completed.
		</para>
		<para>
		For a translated scmd, ata_qc_new_init() is invoked to
		allocate a qc and the scmd is translated into the qc.  SCSI
		midlayer's completion notification function pointer is stored
		into qc->scsidone.
		</para>
		<para>
		qc->complete_fn() callback is used for completion
		notification.  ATA commands use ata_scsi_qc_complete() while
		ATAPI commands use atapi_qc_complete().  Both functions end up
		calling qc->scsidone to notify upper layer when the qc is
		finished.  After translation is completed, the qc is issued
		with ata_qc_issue().
		</para>
		<para>
		Note that SCSI midlayer invokes hostt->queuecommand while
		holding host_set lock, so all above occur while holding
		host_set lock.
		</para>
		</listitem>
		</varlistentry>
	
		</variablelist>
		</sect1>
	
		<sect1><title>How commands are processed</title>
		<para>
		Depending on which protocol and which controller are used,
		commands are processed differently.  For the purpose of
		discussion, a controller which uses taskfile interface and all
		standard callbacks is assumed.
		</para>
		<para>
		Currently 6 ATA command protocols are used.  They can be
		sorted into the following four categories according to how
		they are processed.
		</para>
	
		<variablelist>
		   <varlistentry><term>ATA NO DATA or DMA</term>
		   <listitem>
		   <para>
		   ATA_PROT_NODATA and ATA_PROT_DMA fall into this category.
		   These types of commands don't require any software
		   intervention once issued.  Device will raise interrupt on
		   completion.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>ATA PIO</term>
		   <listitem>
		   <para>
		   ATA_PROT_PIO is in this category.  libata currently
		   implements PIO with polling.  ATA_NIEN bit is set to turn
		   off interrupt and pio_task on ata_wq performs polling and
		   IO.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>ATAPI NODATA or DMA</term>
		   <listitem>
		   <para>
		   ATA_PROT_ATAPI_NODATA and ATA_PROT_ATAPI_DMA are in this
		   category.  packet_task is used to poll BSY bit after
		   issuing PACKET command.  Once BSY is turned off by the
		   device, packet_task transfers CDB and hands off processing
		   to interrupt handler.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>ATAPI PIO</term>
		   <listitem>
		   <para>
		   ATA_PROT_ATAPI is in this category.  ATA_NIEN bit is set
		   and, as in ATAPI NODATA or DMA, packet_task submits cdb.
		   However, after submitting cdb, further processing (data
		   transfer) is handed off to pio_task.
		   </para>
		   </listitem>
		   </varlistentry>
		</variablelist>
	        </sect1>
	
		<sect1><title>How commands are completed</title>
		<para>
		Once issued, all qc's are either completed with
		ata_qc_complete() or time out.  For commands which are handled
		by interrupts, ata_host_intr() invokes ata_qc_complete(), and,
		for PIO tasks, pio_task invokes ata_qc_complete().  In error
		cases, packet_task may also complete commands.
		</para>
		<para>
		ata_qc_complete() does the following.
		</para>
	
		<orderedlist>
	
		<listitem>
		<para>
		DMA memory is unmapped.
		</para>
		</listitem>
	
		<listitem>
		<para>
		ATA_QCFLAG_ACTIVE is clared from qc->flags.
		</para>
		</listitem>
	
		<listitem>
		<para>
		qc->complete_fn() callback is invoked.  If the return value of
		the callback is not zero.  Completion is short circuited and
		ata_qc_complete() returns.
		</para>
		</listitem>
	
		<listitem>
		<para>
		__ata_qc_complete() is called, which does
		   <orderedlist>
	
		   <listitem>
		   <para>
		   qc->flags is cleared to zero.
		   </para>
		   </listitem>
	
		   <listitem>
		   <para>
		   ap->active_tag and qc->tag are poisoned.
		   </para>
		   </listitem>
	
		   <listitem>
		   <para>
		   qc->waiting is claread &amp; completed (in that order).
		   </para>
		   </listitem>
	
		   <listitem>
		   <para>
		   qc is deallocated by clearing appropriate bit in ap->qactive.
		   </para>
		   </listitem>
	
		   </orderedlist>
		</para>
		</listitem>
	
		</orderedlist>
	
		<para>
		So, it basically notifies upper layer and deallocates qc.  One
		exception is short-circuit path in #3 which is used by
		atapi_qc_complete().
		</para>
		<para>
		For all non-ATAPI commands, whether it fails or not, almost
		the same code path is taken and very little error handling
		takes place.  A qc is completed with success status if it
		succeeded, with failed status otherwise.
		</para>
		<para>
		However, failed ATAPI commands require more handling as
		REQUEST SENSE is needed to acquire sense data.  If an ATAPI
		command fails, ata_qc_complete() is invoked with error status,
		which in turn invokes atapi_qc_complete() via
		qc->complete_fn() callback.
		</para>
		<para>
		This makes atapi_qc_complete() set scmd->result to
		SAM_STAT_CHECK_CONDITION, complete the scmd and return 1.  As
		the sense data is empty but scmd->result is CHECK CONDITION,
		SCSI midlayer will invoke EH for the scmd, and returning 1
		makes ata_qc_complete() to return without deallocating the qc.
		This leads us to ata_scsi_error() with partially completed qc.
		</para>
	
		</sect1>
	
		<sect1><title>ata_scsi_error()</title>
		<para>
		ata_scsi_error() is the current transportt->eh_strategy_handler()
		for libata.  As discussed above, this will be entered in two
		cases - timeout and ATAPI error completion.  This function
		calls low level libata driver's eng_timeout() callback, the
		standard callback for which is ata_eng_timeout().  It checks
		if a qc is active and calls ata_qc_timeout() on the qc if so.
		Actual error handling occurs in ata_qc_timeout().
		</para>
		<para>
		If EH is invoked for timeout, ata_qc_timeout() stops BMDMA and
		completes the qc.  Note that as we're currently in EH, we
		cannot call scsi_done.  As described in SCSI EH doc, a
		recovered scmd should be either retried with
		scsi_queue_insert() or finished with scsi_finish_command().
		Here, we override qc->scsidone with scsi_finish_command() and
		calls ata_qc_complete().
		</para>
		<para>
		If EH is invoked due to a failed ATAPI qc, the qc here is
		completed but not deallocated.  The purpose of this
		half-completion is to use the qc as place holder to make EH
		code reach this place.  This is a bit hackish, but it works.
		</para>
		<para>
		Once control reaches here, the qc is deallocated by invoking
		__ata_qc_complete() explicitly.  Then, internal qc for REQUEST
		SENSE is issued.  Once sense data is acquired, scmd is
		finished by directly invoking scsi_finish_command() on the
		scmd.  Note that as we already have completed and deallocated
		the qc which was associated with the scmd, we don't need
		to/cannot call ata_qc_complete() again.
		</para>
	
		</sect1>
	
		<sect1><title>Problems with the current EH</title>
	
		<itemizedlist>
	
		<listitem>
		<para>
		Error representation is too crude.  Currently any and all
		error conditions are represented with ATA STATUS and ERROR
		registers.  Errors which aren't ATA device errors are treated
		as ATA device errors by setting ATA_ERR bit.  Better error
		descriptor which can properly represent ATA and other
		errors/exceptions is needed.
		</para>
		</listitem>
	
		<listitem>
		<para>
		When handling timeouts, no action is taken to make device
		forget about the timed out command and ready for new commands.
		</para>
		</listitem>
	
		<listitem>
		<para>
		EH handling via ata_scsi_error() is not properly protected
		from usual command processing.  On EH entrance, the device is
		not in quiescent state.  Timed out commands may succeed or
		fail any time.  pio_task and atapi_task may still be running.
		</para>
		</listitem>
	
		<listitem>
		<para>
		Too weak error recovery.  Devices / controllers causing HSM
		mismatch errors and other errors quite often require reset to
		return to known state.  Also, advanced error handling is
		necessary to support features like NCQ and hotplug.
		</para>
		</listitem>
	
		<listitem>
		<para>
		ATA errors are directly handled in the interrupt handler and
		PIO errors in pio_task.  This is problematic for advanced
		error handling for the following reasons.
		</para>
		<para>
		First, advanced error handling often requires context and
		internal qc execution.
		</para>
		<para>
		Second, even a simple failure (say, CRC error) needs
		information gathering and could trigger complex error handling
		(say, resetting &amp; reconfiguring).  Having multiple code
		paths to gather information, enter EH and trigger actions
		makes life painful.
		</para>
		<para>
		Third, scattered EH code makes implementing low level drivers
		difficult.  Low level drivers override libata callbacks.  If
		EH is scattered over several places, each affected callbacks
		should perform its part of error handling.  This can be error
		prone and painful.
		</para>
		</listitem>
	
		</itemizedlist>
		</sect1>
	  </chapter>
	
	  <chapter id="libataExt">
	     <title>libata Library</title>
	!Edrivers/ata/libata-core.c
	  </chapter>
	
	  <chapter id="libataInt">
	     <title>libata Core Internals</title>
	!Idrivers/ata/libata-core.c
	  </chapter>
	
	  <chapter id="libataScsiInt">
	     <title>libata SCSI translation/emulation</title>
	!Edrivers/ata/libata-scsi.c
	!Idrivers/ata/libata-scsi.c
	  </chapter>
	
	  <chapter id="ataExceptions">
	     <title>ATA errors and exceptions</title>
	
	  <para>
	  This chapter tries to identify what error/exception conditions exist
	  for ATA/ATAPI devices and describe how they should be handled in
	  implementation-neutral way.
	  </para>
	
	  <para>
	  The term 'error' is used to describe conditions where either an
	  explicit error condition is reported from device or a command has
	  timed out.
	  </para>
	
	  <para>
	  The term 'exception' is either used to describe exceptional
	  conditions which are not errors (say, power or hotplug events), or
	  to describe both errors and non-error exceptional conditions.  Where
	  explicit distinction between error and exception is necessary, the
	  term 'non-error exception' is used.
	  </para>
	
	  <sect1 id="excat">
	     <title>Exception categories</title>
	     <para>
	     Exceptions are described primarily with respect to legacy
	     taskfile + bus master IDE interface.  If a controller provides
	     other better mechanism for error reporting, mapping those into
	     categories described below shouldn't be difficult.
	     </para>
	
	     <para>
	     In the following sections, two recovery actions - reset and
	     reconfiguring transport - are mentioned.  These are described
	     further in <xref linkend="exrec"/>.
	     </para>
	
	     <sect2 id="excatHSMviolation">
	        <title>HSM violation</title>
	        <para>
	        This error is indicated when STATUS value doesn't match HSM
	        requirement during issuing or excution any ATA/ATAPI command.
	        </para>
	
		<itemizedlist>
		<title>Examples</title>
	
	        <listitem>
		<para>
		ATA_STATUS doesn't contain !BSY &amp;&amp; DRDY &amp;&amp; !DRQ while trying
		to issue a command.
	        </para>
		</listitem>
	
	        <listitem>
		<para>
		!BSY &amp;&amp; !DRQ during PIO data transfer.
	        </para>
		</listitem>
	
	        <listitem>
		<para>
		DRQ on command completion.
	        </para>
		</listitem>
	
	        <listitem>
		<para>
		!BSY &amp;&amp; ERR after CDB tranfer starts but before the
	        last byte of CDB is transferred.  ATA/ATAPI standard states
	        that &quot;The device shall not terminate the PACKET command
	        with an error before the last byte of the command packet has
	        been written&quot; in the error outputs description of PACKET
	        command and the state diagram doesn't include such
	        transitions.
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		In these cases, HSM is violated and not much information
		regarding the error can be acquired from STATUS or ERROR
		register.  IOW, this error can be anything - driver bug,
		faulty device, controller and/or cable.
		</para>
	
		<para>
		As HSM is violated, reset is necessary to restore known state.
		Reconfiguring transport for lower speed might be helpful too
		as transmission errors sometimes cause this kind of errors.
		</para>
	     </sect2>
	     
	     <sect2 id="excatDevErr">
	        <title>ATA/ATAPI device error (non-NCQ / non-CHECK CONDITION)</title>
	
		<para>
		These are errors detected and reported by ATA/ATAPI devices
		indicating device problems.  For this type of errors, STATUS
		and ERROR register values are valid and describe error
		condition.  Note that some of ATA bus errors are detected by
		ATA/ATAPI devices and reported using the same mechanism as
		device errors.  Those cases are described later in this
		section.
		</para>
	
		<para>
		For ATA commands, this type of errors are indicated by !BSY
		&amp;&amp; ERR during command execution and on completion.
		</para>
	
		<para>For ATAPI commands,</para>
	
		<itemizedlist>
	
		<listitem>
		<para>
		!BSY &amp;&amp; ERR &amp;&amp; ABRT right after issuing PACKET
		indicates that PACKET command is not supported and falls in
		this category.
		</para>
		</listitem>
	
		<listitem>
		<para>
		!BSY &amp;&amp; ERR(==CHK) &amp;&amp; !ABRT after the last
		byte of CDB is transferred indicates CHECK CONDITION and
		doesn't fall in this category.
		</para>
		</listitem>
	
		<listitem>
		<para>
		!BSY &amp;&amp; ERR(==CHK) &amp;&amp; ABRT after the last byte
	        of CDB is transferred *probably* indicates CHECK CONDITION and
	        doesn't fall in this category.
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		Of errors detected as above, the followings are not ATA/ATAPI
		device errors but ATA bus errors and should be handled
		according to <xref linkend="excatATAbusErr"/>.
		</para>
	
		<variablelist>
	
		   <varlistentry>
		   <term>CRC error during data transfer</term>
		   <listitem>
		   <para>
		   This is indicated by ICRC bit in the ERROR register and
		   means that corruption occurred during data transfer.  Upto
		   ATA/ATAPI-7, the standard specifies that this bit is only
		   applicable to UDMA transfers but ATA/ATAPI-8 draft revision
		   1f says that the bit may be applicable to multiword DMA and
		   PIO.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry>
		   <term>ABRT error during data transfer or on completion</term>
		   <listitem>
		   <para>
		   Upto ATA/ATAPI-7, the standard specifies that ABRT could be
		   set on ICRC errors and on cases where a device is not able
		   to complete a command.  Combined with the fact that MWDMA
		   and PIO transfer errors aren't allowed to use ICRC bit upto
		   ATA/ATAPI-7, it seems to imply that ABRT bit alone could
		   indicate tranfer errors.
		   </para>
		   <para>
		   However, ATA/ATAPI-8 draft revision 1f removes the part
		   that ICRC errors can turn on ABRT.  So, this is kind of
		   gray area.  Some heuristics are needed here.
		   </para>
		   </listitem>
		   </varlistentry>
	
		</variablelist>
	
		<para>
		ATA/ATAPI device errors can be further categorized as follows.
		</para>
	
		<variablelist>
	
		   <varlistentry>
		   <term>Media errors</term>
		   <listitem>
		   <para>
		   This is indicated by UNC bit in the ERROR register.  ATA
		   devices reports UNC error only after certain number of
		   retries cannot recover the data, so there's nothing much
		   else to do other than notifying upper layer.
		   </para>
		   <para>
		   READ and WRITE commands report CHS or LBA of the first
		   failed sector but ATA/ATAPI standard specifies that the
		   amount of transferred data on error completion is
		   indeterminate, so we cannot assume that sectors preceding
		   the failed sector have been transferred and thus cannot
		   complete those sectors successfully as SCSI does.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry>
		   <term>Media changed / media change requested error</term>
		   <listitem>
		   <para>
		   &lt;&lt;TODO: fill here&gt;&gt;
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>Address error</term>
		   <listitem>
		   <para>
		   This is indicated by IDNF bit in the ERROR register.
		   Report to upper layer.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>Other errors</term>
		   <listitem>
		   <para>
		   This can be invalid command or parameter indicated by ABRT
		   ERROR bit or some other error condition.  Note that ABRT
		   bit can indicate a lot of things including ICRC and Address
		   errors.  Heuristics needed.
		   </para>
		   </listitem>
		   </varlistentry>
	
		</variablelist>
	
		<para>
		Depending on commands, not all STATUS/ERROR bits are
		applicable.  These non-applicable bits are marked with
		&quot;na&quot; in the output descriptions but upto ATA/ATAPI-7
		no definition of &quot;na&quot; can be found.  However,
		ATA/ATAPI-8 draft revision 1f describes &quot;N/A&quot; as
		follows.
		</para>
	
		<blockquote>
		<variablelist>
		   <varlistentry><term>3.2.3.3a N/A</term>
		   <listitem>
		   <para>
		   A keyword the indicates a field has no defined value in
		   this standard and should not be checked by the host or
		   device. N/A fields should be cleared to zero.
		   </para>
		   </listitem>
		   </varlistentry>
		</variablelist>
		</blockquote>
	
		<para>
		So, it seems reasonable to assume that &quot;na&quot; bits are
		cleared to zero by devices and thus need no explicit masking.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatATAPIcc">
	        <title>ATAPI device CHECK CONDITION</title>
	
		<para>
		ATAPI device CHECK CONDITION error is indicated by set CHK bit
		(ERR bit) in the STATUS register after the last byte of CDB is
		transferred for a PACKET command.  For this kind of errors,
		sense data should be acquired to gather information regarding
		the errors.  REQUEST SENSE packet command should be used to
		acquire sense data.
		</para>
	
		<para>
		Once sense data is acquired, this type of errors can be
		handled similary to other SCSI errors.  Note that sense data
		may indicate ATA bus error (e.g. Sense Key 04h HARDWARE ERROR
		&amp;&amp; ASC/ASCQ 47h/00h SCSI PARITY ERROR).  In such
		cases, the error should be considered as an ATA bus error and
		handled according to <xref linkend="excatATAbusErr"/>.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatNCQerr">
	        <title>ATA device error (NCQ)</title>
	
		<para>
		NCQ command error is indicated by cleared BSY and set ERR bit
		during NCQ command phase (one or more NCQ commands
		outstanding).  Although STATUS and ERROR registers will
		contain valid values describing the error, READ LOG EXT is
		required to clear the error condition, determine which command
		has failed and acquire more information.
		</para>
	
		<para>
		READ LOG EXT Log Page 10h reports which tag has failed and
		taskfile register values describing the error.  With this
		information the failed command can be handled as a normal ATA
		command error as in <xref linkend="excatDevErr"/> and all
		other in-flight commands must be retried.  Note that this
		retry should not be counted - it's likely that commands
		retried this way would have completed normally if it were not
		for the failed command.
		</para>
	
		<para>
		Note that ATA bus errors can be reported as ATA device NCQ
		errors.  This should be handled as described in <xref
		linkend="excatATAbusErr"/>.
		</para>
	
		<para>
		If READ LOG EXT Log Page 10h fails or reports NQ, we're
		thoroughly screwed.  This condition should be treated
		according to <xref linkend="excatHSMviolation"/>.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatATAbusErr">
	        <title>ATA bus error</title>
	
		<para>
		ATA bus error means that data corruption occurred during
		transmission over ATA bus (SATA or PATA).  This type of errors
		can be indicated by
		</para>
	
		<itemizedlist>
	
		<listitem>
		<para>
		ICRC or ABRT error as described in <xref linkend="excatDevErr"/>.
		</para>
		</listitem>
	
		<listitem>
		<para>
		Controller-specific error completion with error information
		indicating transmission error.
		</para>
		</listitem>
	
		<listitem>
		<para>
		On some controllers, command timeout.  In this case, there may
		be a mechanism to determine that the timeout is due to
		transmission error.
		</para>
		</listitem>
	
		<listitem>
		<para>
		Unknown/random errors, timeouts and all sorts of weirdities.
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		As described above, transmission errors can cause wide variety
		of symptoms ranging from device ICRC error to random device
		lockup, and, for many cases, there is no way to tell if an
		error condition is due to transmission error or not;
		therefore, it's necessary to employ some kind of heuristic
		when dealing with errors and timeouts.  For example,
		encountering repetitive ABRT errors for known supported
		command is likely to indicate ATA bus error.
		</para>
	
		<para>
		Once it's determined that ATA bus errors have possibly
		occurred, lowering ATA bus transmission speed is one of
		actions which may alleviate the problem.  See <xref
		linkend="exrecReconf"/> for more information.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatPCIbusErr">
	        <title>PCI bus error</title>
	
		<para>
		Data corruption or other failures during transmission over PCI
		(or other system bus).  For standard BMDMA, this is indicated
		by Error bit in the BMDMA Status register.  This type of
		errors must be logged as it indicates something is very wrong
		with the system.  Resetting host controller is recommended.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatLateCompletion">
	        <title>Late completion</title>
	
		<para>
		This occurs when timeout occurs and the timeout handler finds
		out that the timed out command has completed successfully or
		with error.  This is usually caused by lost interrupts.  This
		type of errors must be logged.  Resetting host controller is
		recommended.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatUnknown">
	        <title>Unknown error (timeout)</title>
	
		<para>
		This is when timeout occurs and the command is still
		processing or the host and device are in unknown state.  When
		this occurs, HSM could be in any valid or invalid state.  To
		bring the device to known state and make it forget about the
		timed out command, resetting is necessary.  The timed out
		command may be retried.
		</para>
	
		<para>
		Timeouts can also be caused by transmission errors.  Refer to
		<xref linkend="excatATAbusErr"/> for more details.
		</para>
	
	     </sect2>
	
	     <sect2 id="excatHoplugPM">
	        <title>Hotplug and power management exceptions</title>
	
		<para>
		&lt;&lt;TODO: fill here&gt;&gt;
		</para>
	
	     </sect2>
	
	  </sect1>
	
	  <sect1 id="exrec">
	     <title>EH recovery actions</title>
	
	     <para>
	     This section discusses several important recovery actions.
	     </para>
	
	     <sect2 id="exrecClr">
	        <title>Clearing error condition</title>
	
		<para>
		Many controllers require its error registers to be cleared by
		error handler.  Different controllers may have different
		requirements.
		</para>
	
		<para>
		For SATA, it's strongly recommended to clear at least SError
		register during error handling.
		</para>
	     </sect2>
	
	     <sect2 id="exrecRst">
	        <title>Reset</title>
	
		<para>
		During EH, resetting is necessary in the following cases.
		</para>
	
		<itemizedlist>
	
		<listitem>
		<para>
		HSM is in unknown or invalid state
		</para>
		</listitem>
	
		<listitem>
		<para>
		HBA is in unknown or invalid state
		</para>
		</listitem>
	
		<listitem>
		<para>
		EH needs to make HBA/device forget about in-flight commands
		</para>
		</listitem>
	
		<listitem>
		<para>
		HBA/device behaves weirdly
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		Resetting during EH might be a good idea regardless of error
		condition to improve EH robustness.  Whether to reset both or
		either one of HBA and device depends on situation but the
		following scheme is recommended.
		</para>
	
		<itemizedlist>
	
		<listitem>
		<para>
		When it's known that HBA is in ready state but ATA/ATAPI
		device is in unknown state, reset only device.
		</para>
		</listitem>
	
		<listitem>
		<para>
		If HBA is in unknown state, reset both HBA and device.
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		HBA resetting is implementation specific.  For a controller
		complying to taskfile/BMDMA PCI IDE, stopping active DMA
		transaction may be sufficient iff BMDMA state is the only HBA
		context.  But even mostly taskfile/BMDMA PCI IDE complying
		controllers may have implementation specific requirements and
		mechanism to reset themselves.  This must be addressed by
		specific drivers.
		</para>
	
		<para>
		OTOH, ATA/ATAPI standard describes in detail ways to reset
		ATA/ATAPI devices.
		</para>
	
		<variablelist>
	
		   <varlistentry><term>PATA hardware reset</term>
		   <listitem>
		   <para>
		   This is hardware initiated device reset signalled with
		   asserted PATA RESET- signal.  There is no standard way to
		   initiate hardware reset from software although some
		   hardware provides registers that allow driver to directly
		   tweak the RESET- signal.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>Software reset</term>
		   <listitem>
		   <para>
		   This is achieved by turning CONTROL SRST bit on for at
		   least 5us.  Both PATA and SATA support it but, in case of
		   SATA, this may require controller-specific support as the
		   second Register FIS to clear SRST should be transmitted
		   while BSY bit is still set.  Note that on PATA, this resets
		   both master and slave devices on a channel.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>EXECUTE DEVICE DIAGNOSTIC command</term>
		   <listitem>
		   <para>
		   Although ATA/ATAPI standard doesn't describe exactly, EDD
		   implies some level of resetting, possibly similar level
		   with software reset.  Host-side EDD protocol can be handled
		   with normal command processing and most SATA controllers
		   should be able to handle EDD's just like other commands.
		   As in software reset, EDD affects both devices on a PATA
		   bus.
		   </para>
		   <para>
		   Although EDD does reset devices, this doesn't suit error
		   handling as EDD cannot be issued while BSY is set and it's
		   unclear how it will act when device is in unknown/weird
		   state.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>ATAPI DEVICE RESET command</term>
		   <listitem>
		   <para>
		   This is very similar to software reset except that reset
		   can be restricted to the selected device without affecting
		   the other device sharing the cable.
		   </para>
		   </listitem>
		   </varlistentry>
	
		   <varlistentry><term>SATA phy reset</term>
		   <listitem>
		   <para>
		   This is the preferred way of resetting a SATA device.  In
		   effect, it's identical to PATA hardware reset.  Note that
		   this can be done with the standard SCR Control register.
		   As such, it's usually easier to implement than software
		   reset.
		   </para>
		   </listitem>
		   </varlistentry>
	
		</variablelist>
	
		<para>
		One more thing to consider when resetting devices is that
		resetting clears certain configuration parameters and they
		need to be set to their previous or newly adjusted values
		after reset.
		</para>
	
		<para>
		Parameters affected are.
		</para>
	
		<itemizedlist>
	
		<listitem>
		<para>
		CHS set up with INITIALIZE DEVICE PARAMETERS (seldomly used)
		</para>
		</listitem>
	
		<listitem>
		<para>
		Parameters set with SET FEATURES including transfer mode setting
		</para>
		</listitem>
	
		<listitem>
		<para>
		Block count set with SET MULTIPLE MODE
		</para>
		</listitem>
	
		<listitem>
		<para>
		Other parameters (SET MAX, MEDIA LOCK...)
		</para>
		</listitem>
	
		</itemizedlist>
	
		<para>
		ATA/ATAPI standard specifies that some parameters must be
		maintained across hardware or software reset, but doesn't
		strictly specify all of them.  Always reconfiguring needed
		parameters after reset is required for robustness.  Note that
		this also applies when resuming from deep sleep (power-off).
		</para>
	
		<para>
		Also, ATA/ATAPI standard requires that IDENTIFY DEVICE /
		IDENTIFY PACKET DEVICE is issued after any configuration
		parameter is updated or a hardware reset and the result used
		for further operation.  OS driver is required to implement
		revalidation mechanism to support this.
		</para>
	
	     </sect2>
	
	     <sect2 id="exrecReconf">
	        <title>Reconfigure transport</title>
	
		<para>
		For both PATA and SATA, a lot of corners are cut for cheap
		connectors, cables or controllers and it's quite common to see
		high transmission error rate.  This can be mitigated by
		lowering transmission speed.
		</para>
	
		<para>
		The following is a possible scheme Jeff Garzik suggested.
		</para>
	
		<blockquote>
		<para>
		If more than $N (3?) transmission errors happen in 15 minutes,
		</para>	
		<itemizedlist>
		<listitem>
		<para>
		if SATA, decrease SATA PHY speed.  if speed cannot be decreased,
		</para>
		</listitem>
		<listitem>
		<para>
		decrease UDMA xfer speed.  if at UDMA0, switch to PIO4,
		</para>
		</listitem>
		<listitem>
		<para>
		decrease PIO xfer speed.  if at PIO3, complain, but continue
		</para>
		</listitem>
		</itemizedlist>
		</blockquote>
	
	     </sect2>
	
	  </sect1>
	
	  </chapter>
	
	  <chapter id="PiixInt">
	     <title>ata_piix Internals</title>
	!Idrivers/ata/ata_piix.c
	  </chapter>
	
	  <chapter id="SILInt">
	     <title>sata_sil Internals</title>
	!Idrivers/ata/sata_sil.c
	  </chapter>
	
	  <chapter id="libataThanks">
	     <title>Thanks</title>
	  <para>
	  The bulk of the ATA knowledge comes thanks to long conversations with
	  Andre Hedrick (www.linux-ide.org), and long hours pondering the ATA
	  and SCSI specifications.
	  </para>
	  <para>
	  Thanks to Alan Cox for pointing out similarities 
	  between SATA and SCSI, and in general for motivation to hack on
	  libata.
	  </para>
	  <para>
	  libata's device detection
	  method, ata_pio_devchk, and in general all the early probing was
	  based on extensive study of Hale Landis's probe/reset code in his
	  ATADRVR driver (www.ata-atapi.com).
	  </para>
	  </chapter>
	
	</book>

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