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1	<?xml version="1.0" encoding="UTF-8"?>
2	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3		"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
5	<book id="USB-Gadget-API">
6	  <bookinfo>
7	    <title>USB Gadget API for Linux</title>
8	    <date>20 August 2004</date>
9	    <edition>20 August 2004</edition>
11	    <legalnotice>
12	       <para>
13		 This documentation is free software; you can redistribute
14		 it and/or modify it under the terms of the GNU General Public
15		 License as published by the Free Software Foundation; either
16		 version 2 of the License, or (at your option) any later
17		 version.
18	       </para>
20	       <para>
21		 This program is distributed in the hope that it will be
22		 useful, but WITHOUT ANY WARRANTY; without even the implied
24		 See the GNU General Public License for more details.
25	       </para>
27	       <para>
28		 You should have received a copy of the GNU General Public
29		 License along with this program; if not, write to the Free
30		 Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
31		 MA 02111-1307 USA
32	       </para>
34	       <para>
35		 For more details see the file COPYING in the source
36		 distribution of Linux.
37	       </para>
38	    </legalnotice>
39	    <copyright>
40	      <year>2003-2004</year>
41	      <holder>David Brownell</holder>
42	    </copyright>
44	    <author>
45	      <firstname>David</firstname> 
46	      <surname>Brownell</surname>
47	      <affiliation>
48	        <address><email>dbrownell@users.sourceforge.net</email></address>
49	      </affiliation>
50	    </author>
51	  </bookinfo>
53	<toc></toc>
55	<chapter id="intro"><title>Introduction</title>
57	<para>This document presents a Linux-USB "Gadget"
58	kernel mode
59	API, for use within peripherals and other USB devices
60	that embed Linux.
61	It provides an overview of the API structure,
62	and shows how that fits into a system development project.
63	This is the first such API released on Linux to address
64	a number of important problems, including: </para>
66	<itemizedlist>
67	    <listitem><para>Supports USB 2.0, for high speed devices which
68		can stream data at several dozen megabytes per second.
69		</para></listitem>
70	    <listitem><para>Handles devices with dozens of endpoints just as
71		well as ones with just two fixed-function ones.  Gadget drivers
72		can be written so they're easy to port to new hardware.
73		</para></listitem>
74	    <listitem><para>Flexible enough to expose more complex USB device
75		capabilities such as multiple configurations, multiple interfaces,
76		composite devices,
77		and alternate interface settings.
78		</para></listitem>
79	    <listitem><para>USB "On-The-Go" (OTG) support, in conjunction
80		with updates to the Linux-USB host side.
81		</para></listitem>
82	    <listitem><para>Sharing data structures and API models with the
83		Linux-USB host side API.  This helps the OTG support, and
84		looks forward to more-symmetric frameworks (where the same
85		I/O model is used by both host and device side drivers).
86		</para></listitem>
87	    <listitem><para>Minimalist, so it's easier to support new device
88		controller hardware.  I/O processing doesn't imply large
89		demands for memory or CPU resources.
90		</para></listitem>
91	</itemizedlist>
94	<para>Most Linux developers will not be able to use this API, since they
95	have USB "host" hardware in a PC, workstation, or server.
96	Linux users with embedded systems are more likely to
97	have USB peripheral hardware.
98	To distinguish drivers running inside such hardware from the
99	more familiar Linux "USB device drivers",
100	which are host side proxies for the real USB devices,
101	a different term is used:
102	the drivers inside the peripherals are "USB gadget drivers".
103	In USB protocol interactions, the device driver is the master
104	(or "client driver")
105	and the gadget driver is the slave (or "function driver").
106	</para>
108	<para>The gadget API resembles the host side Linux-USB API in that both
109	use queues of request objects to package I/O buffers, and those requests
110	may be submitted or canceled.
111	They share common definitions for the standard USB
112	<emphasis>Chapter 9</emphasis> messages, structures, and constants.
113	Also, both APIs bind and unbind drivers to devices.
114	The APIs differ in detail, since the host side's current
115	URB framework exposes a number of implementation details
116	and assumptions that are inappropriate for a gadget API.
117	While the model for control transfers and configuration
118	management is necessarily different (one side is a hardware-neutral master,
119	the other is a hardware-aware slave), the endpoint I/0 API used here
120	should also be usable for an overhead-reduced host side API.
121	</para>
123	</chapter>
125	<chapter id="structure"><title>Structure of Gadget Drivers</title>
127	<para>A system running inside a USB peripheral
128	normally has at least three layers inside the kernel to handle
129	USB protocol processing, and may have additional layers in
130	user space code.
131	The "gadget" API is used by the middle layer to interact
132	with the lowest level (which directly handles hardware).
133	</para>
135	<para>In Linux, from the bottom up, these layers are:
136	</para>
138	<variablelist>
140	    <varlistentry>
141	        <term><emphasis>USB Controller Driver</emphasis></term>
143		<listitem>
144		<para>This is the lowest software level.
145		It is the only layer that talks to hardware,
146		through registers, fifos, dma, irqs, and the like.
147		The <filename>&lt;linux/usb/gadget.h&gt;</filename> API abstracts
148		the peripheral controller endpoint hardware.
149		That hardware is exposed through endpoint objects, which accept
150		streams of IN/OUT buffers, and through callbacks that interact
151		with gadget drivers.
152		Since normal USB devices only have one upstream
153		port, they only have one of these drivers.
154		The controller driver can support any number of different
155		gadget drivers, but only one of them can be used at a time.
156		</para>
158		<para>Examples of such controller hardware include
159		the PCI-based NetChip 2280 USB 2.0 high speed controller,
160		the SA-11x0 or PXA-25x UDC (found within many PDAs),
161		and a variety of other products.
162		</para>
164		</listitem></varlistentry>
166	    <varlistentry>
167		<term><emphasis>Gadget Driver</emphasis></term>
169		<listitem>
170		<para>The lower boundary of this driver implements hardware-neutral
171		USB functions, using calls to the controller driver.
172		Because such hardware varies widely in capabilities and restrictions,
173		and is used in embedded environments where space is at a premium,
174		the gadget driver is often configured at compile time
175		to work with endpoints supported by one particular controller.
176		Gadget drivers may be portable to several different controllers,
177		using conditional compilation.
178		(Recent kernels substantially simplify the work involved in
179		supporting new hardware, by <emphasis>autoconfiguring</emphasis>
180		endpoints automatically for many bulk-oriented drivers.)
181		Gadget driver responsibilities include:
182		</para>
183		<itemizedlist>
184		    <listitem><para>handling setup requests (ep0 protocol responses)
185			possibly including class-specific functionality
186			</para></listitem>
187		    <listitem><para>returning configuration and string descriptors
188			</para></listitem>
189		    <listitem><para>(re)setting configurations and interface
190			altsettings, including enabling and configuring endpoints
191			</para></listitem>
192		    <listitem><para>handling life cycle events, such as managing
193			bindings to hardware,
194			USB suspend/resume, remote wakeup,
195			and disconnection from the USB host.
196			</para></listitem>
197		    <listitem><para>managing IN and OUT transfers on all currently
198			enabled endpoints
199			</para></listitem>
200		</itemizedlist>
202		<para>
203		Such drivers may be modules of proprietary code, although
204		that approach is discouraged in the Linux community.
205		</para>
206		</listitem></varlistentry>
208	    <varlistentry>
209		<term><emphasis>Upper Level</emphasis></term>
211		<listitem>
212		<para>Most gadget drivers have an upper boundary that connects
213		to some Linux driver or framework in Linux.
214		Through that boundary flows the data which the gadget driver
215		produces and/or consumes through protocol transfers over USB.
216		Examples include:
217		</para>
218		<itemizedlist>
219		    <listitem><para>user mode code, using generic (gadgetfs)
220		        or application specific files in
221			<filename>/dev</filename>
222			</para></listitem>
223		    <listitem><para>networking subsystem (for network gadgets,
224			like the CDC Ethernet Model gadget driver)
225			</para></listitem>
226		    <listitem><para>data capture drivers, perhaps video4Linux or
227			 a scanner driver; or test and measurement hardware.
228			 </para></listitem>
229		    <listitem><para>input subsystem (for HID gadgets)
230			</para></listitem>
231		    <listitem><para>sound subsystem (for audio gadgets)
232			</para></listitem>
233		    <listitem><para>file system (for PTP gadgets)
234			</para></listitem>
235		    <listitem><para>block i/o subsystem (for usb-storage gadgets)
236			</para></listitem>
237		    <listitem><para>... and more </para></listitem>
238		</itemizedlist>
239		</listitem></varlistentry>
241	    <varlistentry>
242		<term><emphasis>Additional Layers</emphasis></term>
244		<listitem>
245		<para>Other layers may exist.
246		These could include kernel layers, such as network protocol stacks,
247		as well as user mode applications building on standard POSIX
248		system call APIs such as
249		<emphasis>open()</emphasis>, <emphasis>close()</emphasis>,
250		<emphasis>read()</emphasis> and <emphasis>write()</emphasis>.
251		On newer systems, POSIX Async I/O calls may be an option.
252		Such user mode code will not necessarily be subject to
253		the GNU General Public License (GPL).
254		</para>
255		</listitem></varlistentry>
258	</variablelist>
260	<para>OTG-capable systems will also need to include a standard Linux-USB
261	host side stack,
262	with <emphasis>usbcore</emphasis>,
263	one or more <emphasis>Host Controller Drivers</emphasis> (HCDs),
264	<emphasis>USB Device Drivers</emphasis> to support
265	the OTG "Targeted Peripheral List",
266	and so forth.
267	There will also be an <emphasis>OTG Controller Driver</emphasis>,
268	which is visible to gadget and device driver developers only indirectly.
269	That helps the host and device side USB controllers implement the
270	two new OTG protocols (HNP and SRP).
271	Roles switch (host to peripheral, or vice versa) using HNP
272	during USB suspend processing, and SRP can be viewed as a
273	more battery-friendly kind of device wakeup protocol.
274	</para>
276	<para>Over time, reusable utilities are evolving to help make some
277	gadget driver tasks simpler.
278	For example, building configuration descriptors from vectors of
279	descriptors for the configurations interfaces and endpoints is
280	now automated, and many drivers now use autoconfiguration to
281	choose hardware endpoints and initialize their descriptors.
283	A potential example of particular interest
284	is code implementing standard USB-IF protocols for
285	HID, networking, storage, or audio classes.
286	Some developers are interested in KDB or KGDB hooks, to let
287	target hardware be remotely debugged.
288	Most such USB protocol code doesn't need to be hardware-specific,
289	any more than network protocols like X11, HTTP, or NFS are.
290	Such gadget-side interface drivers should eventually be combined,
291	to implement composite devices.
292	</para>
294	</chapter>
297	<chapter id="api"><title>Kernel Mode Gadget API</title>
299	<para>Gadget drivers declare themselves through a
300	<emphasis>struct usb_gadget_driver</emphasis>, which is responsible for
301	most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>.
302	The response to a set_configuration usually involves
303	enabling one or more of the <emphasis>struct usb_ep</emphasis> objects
304	exposed by the gadget, and submitting one or more
305	<emphasis>struct usb_request</emphasis> buffers to transfer data.
306	Understand those four data types, and their operations, and
307	you will understand how this API works.
308	</para> 
310	<note><title>Incomplete Data Type Descriptions</title>
312	<para>This documentation was prepared using the standard Linux
313	kernel <filename>docproc</filename> tool, which turns text
314	and in-code comments into SGML DocBook and then into usable
315	formats such as HTML or PDF.
316	Other than the "Chapter 9" data types, most of the significant
317	data types and functions are described here.
318	</para>
320	<para>However, docproc does not understand all the C constructs
321	that are used, so some relevant information is likely omitted from
322	what you are reading.  
323	One example of such information is endpoint autoconfiguration.
324	You'll have to read the header file, and use example source
325	code (such as that for "Gadget Zero"), to fully understand the API.
326	</para>
328	<para>The part of the API implementing some basic
329	driver capabilities is specific to the version of the
330	Linux kernel that's in use.
331	The 2.6 kernel includes a <emphasis>driver model</emphasis>
332	framework that has no analogue on earlier kernels;
333	so those parts of the gadget API are not fully portable.
334	(They are implemented on 2.4 kernels, but in a different way.)
335	The driver model state is another part of this API that is
336	ignored by the kerneldoc tools.
337	</para>
338	</note>
340	<para>The core API does not expose
341	every possible hardware feature, only the most widely available ones.
342	There are significant hardware features, such as device-to-device DMA
343	(without temporary storage in a memory buffer)
344	that would be added using hardware-specific APIs.
345	</para>
347	<para>This API allows drivers to use conditional compilation to handle
348	endpoint capabilities of different hardware, but doesn't require that.
349	Hardware tends to have arbitrary restrictions, relating to
350	transfer types, addressing, packet sizes, buffering, and availability.
351	As a rule, such differences only matter for "endpoint zero" logic
352	that handles device configuration and management.
353	The API supports limited run-time
354	detection of capabilities, through naming conventions for endpoints.
355	Many drivers will be able to at least partially autoconfigure
356	themselves.
357	In particular, driver init sections will often have endpoint
358	autoconfiguration logic that scans the hardware's list of endpoints
359	to find ones matching the driver requirements
360	(relying on those conventions), to eliminate some of the most
361	common reasons for conditional compilation.
362	</para>
364	<para>Like the Linux-USB host side API, this API exposes
365	the "chunky" nature of USB messages:  I/O requests are in terms
366	of one or more "packets", and packet boundaries are visible to drivers.
367	Compared to RS-232 serial protocols, USB resembles
368	synchronous protocols like HDLC
369	(N bytes per frame, multipoint addressing, host as the primary
370	station and devices as secondary stations)
371	more than asynchronous ones
372	(tty style:  8 data bits per frame, no parity, one stop bit).
373	So for example the controller drivers won't buffer
374	two single byte writes into a single two-byte USB IN packet,
375	although gadget drivers may do so when they implement
376	protocols where packet boundaries (and "short packets")
377	are not significant.
378	</para>
380	<sect1 id="lifecycle"><title>Driver Life Cycle</title>
382	<para>Gadget drivers make endpoint I/O requests to hardware without
383	needing to know many details of the hardware, but driver
384	setup/configuration code needs to handle some differences.
385	Use the API like this:
386	</para>
388	<orderedlist numeration='arabic'>
390	<listitem><para>Register a driver for the particular device side
391	usb controller hardware,
392	such as the net2280 on PCI (USB 2.0),
393	sa11x0 or pxa25x as found in Linux PDAs,
394	and so on.
395	At this point the device is logically in the USB ch9 initial state
396	("attached"), drawing no power and not usable
397	(since it does not yet support enumeration).
398	Any host should not see the device, since it's not
399	activated the data line pullup used by the host to
400	detect a device, even if VBUS power is available.
401	</para></listitem>
403	<listitem><para>Register a gadget driver that implements some higher level
404	device function.  That will then bind() to a usb_gadget, which
405	activates the data line pullup sometime after detecting VBUS.
406	</para></listitem>
408	<listitem><para>The hardware driver can now start enumerating.
409	The steps it handles are to accept USB power and set_address requests.
410	Other steps are handled by the gadget driver.
411	If the gadget driver module is unloaded before the host starts to
412	enumerate, steps before step 7 are skipped.
413	</para></listitem>
415	<listitem><para>The gadget driver's setup() call returns usb descriptors,
416	based both on what the bus interface hardware provides and on the
417	functionality being implemented.
418	That can involve alternate settings or configurations,
419	unless the hardware prevents such operation.
420	For OTG devices, each configuration descriptor includes
421	an OTG descriptor.
422	</para></listitem>
424	<listitem><para>The gadget driver handles the last step of enumeration,
425	when the USB host issues a set_configuration call.
426	It enables all endpoints used in that configuration,
427	with all interfaces in their default settings.
428	That involves using a list of the hardware's endpoints, enabling each
429	endpoint according to its descriptor.
430	It may also involve using <function>usb_gadget_vbus_draw</function>
431	to let more power be drawn from VBUS, as allowed by that configuration.
432	For OTG devices, setting a configuration may also involve reporting
433	HNP capabilities through a user interface.
434	</para></listitem>
436	<listitem><para>Do real work and perform data transfers, possibly involving
437	changes to interface settings or switching to new configurations, until the
438	device is disconnect()ed from the host.
439	Queue any number of transfer requests to each endpoint.
440	It may be suspended and resumed several times before being disconnected.
441	On disconnect, the drivers go back to step 3 (above).
442	</para></listitem>
444	<listitem><para>When the gadget driver module is being unloaded,
445	the driver unbind() callback is issued.  That lets the controller
446	driver be unloaded.
447	</para></listitem>
449	</orderedlist>
451	<para>Drivers will normally be arranged so that just loading the
452	gadget driver module (or statically linking it into a Linux kernel)
453	allows the peripheral device to be enumerated, but some drivers
454	will defer enumeration until some higher level component (like
455	a user mode daemon) enables it.
456	Note that at this lowest level there are no policies about how
457	ep0 configuration logic is implemented,
458	except that it should obey USB specifications.
459	Such issues are in the domain of gadget drivers,
460	including knowing about implementation constraints
461	imposed by some USB controllers
462	or understanding that composite devices might happen to
463	be built by integrating reusable components.
464	</para>
466	<para>Note that the lifecycle above can be slightly different
467	for OTG devices.
468	Other than providing an additional OTG descriptor in each
469	configuration, only the HNP-related differences are particularly
470	visible to driver code.
471	They involve reporting requirements during the SET_CONFIGURATION
472	request, and the option to invoke HNP during some suspend callbacks.
473	Also, SRP changes the semantics of
474	<function>usb_gadget_wakeup</function>
475	slightly.
476	</para>
478	</sect1>
480	<sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title>
482	<para>Gadget drivers
483	rely on common USB structures and constants
484	defined in the
485	<filename>&lt;linux/usb/ch9.h&gt;</filename>
486	header file, which is standard in Linux 2.6 kernels.
487	These are the same types and constants used by host
488	side drivers (and usbcore).
489	</para>
491	!Iinclude/linux/usb/ch9.h
492	</sect1>
494	<sect1 id="core"><title>Core Objects and Methods</title>
496	<para>These are declared in
497	<filename>&lt;linux/usb/gadget.h&gt;</filename>,
498	and are used by gadget drivers to interact with
499	USB peripheral controller drivers.
500	</para>
502		<!-- yeech, this is ugly in nsgmls PDF output.
504		     the PDF bookmark and refentry output nesting is wrong,
505		     and the member/argument documentation indents ugly.
507		     plus something (docproc?) adds whitespace before the
508		     descriptive paragraph text, so it can't line up right
509		     unless the explanations are trivial.
510		  -->
512	!Iinclude/linux/usb/gadget.h
513	</sect1>
515	<sect1 id="utils"><title>Optional Utilities</title>
517	<para>The core API is sufficient for writing a USB Gadget Driver,
518	but some optional utilities are provided to simplify common tasks.
519	These utilities include endpoint autoconfiguration.
520	</para>
522	!Edrivers/usb/gadget/usbstring.c
523	!Edrivers/usb/gadget/config.c
524	<!-- !Edrivers/usb/gadget/epautoconf.c -->
525	</sect1>
527	<sect1 id="composite"><title>Composite Device Framework</title>
529	<para>The core API is sufficient for writing drivers for composite
530	USB devices (with more than one function in a given configuration),
531	and also multi-configuration devices (also more than one function,
532	but not necessarily sharing a given configuration).
533	There is however an optional framework which makes it easier to
534	reuse and combine functions.
535	</para>
537	<para>Devices using this framework provide a <emphasis>struct
538	usb_composite_driver</emphasis>, which in turn provides one or
539	more <emphasis>struct usb_configuration</emphasis> instances.
540	Each such configuration includes at least one
541	<emphasis>struct usb_function</emphasis>, which packages a user
542	visible role such as "network link" or "mass storage device".
543	Management functions may also exist, such as "Device Firmware
544	Upgrade".
545	</para>
547	!Iinclude/linux/usb/composite.h
548	!Edrivers/usb/gadget/composite.c
550	</sect1>
552	<sect1 id="functions"><title>Composite Device Functions</title>
554	<para>At this writing, a few of the current gadget drivers have
555	been converted to this framework.
556	Near-term plans include converting all of them, except for "gadgetfs".
557	</para>
559	!Edrivers/usb/gadget/f_acm.c
560	!Edrivers/usb/gadget/f_ecm.c
561	!Edrivers/usb/gadget/f_subset.c
562	!Edrivers/usb/gadget/f_obex.c
563	!Edrivers/usb/gadget/f_serial.c
565	</sect1>
568	</chapter>
570	<chapter id="controllers"><title>Peripheral Controller Drivers</title>
572	<para>The first hardware supporting this API was the NetChip 2280
573	controller, which supports USB 2.0 high speed and is based on PCI.
574	This is the <filename>net2280</filename> driver module.
575	The driver supports Linux kernel versions 2.4 and 2.6;
576	contact NetChip Technologies for development boards and product
577	information.
578	</para> 
580	<para>Other hardware working in the "gadget" framework includes:
581	Intel's PXA 25x and IXP42x series processors
582	(<filename>pxa2xx_udc</filename>),
583	Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>),
584	Renesas SH7705/7727 (<filename>sh_udc</filename>),
585	MediaQ 11xx (<filename>mq11xx_udc</filename>),
586	Hynix HMS30C7202 (<filename>h7202_udc</filename>),
587	National 9303/4 (<filename>n9604_udc</filename>),
588	Texas Instruments OMAP (<filename>omap_udc</filename>),
589	Sharp LH7A40x (<filename>lh7a40x_udc</filename>),
590	and more.
591	Most of those are full speed controllers.
592	</para>
594	<para>At this writing, there are people at work on drivers in
595	this framework for several other USB device controllers,
596	with plans to make many of them be widely available.
597	</para>
599	<!-- !Edrivers/usb/gadget/net2280.c -->
601	<para>A partial USB simulator,
602	the <filename>dummy_hcd</filename> driver, is available.
603	It can act like a net2280, a pxa25x, or an sa11x0 in terms
604	of available endpoints and device speeds; and it simulates
605	control, bulk, and to some extent interrupt transfers.
606	That lets you develop some parts of a gadget driver on a normal PC,
607	without any special hardware, and perhaps with the assistance
608	of tools such as GDB running with User Mode Linux.
609	At least one person has expressed interest in adapting that
610	approach, hooking it up to a simulator for a microcontroller.
611	Such simulators can help debug subsystems where the runtime hardware
612	is unfriendly to software development, or is not yet available.
613	</para>
615	<para>Support for other controllers is expected to be developed
616	and contributed
617	over time, as this driver framework evolves.
618	</para>
620	</chapter>
622	<chapter id="gadget"><title>Gadget Drivers</title>
624	<para>In addition to <emphasis>Gadget Zero</emphasis>
625	(used primarily for testing and development with drivers
626	for usb controller hardware), other gadget drivers exist.
627	</para>
629	<para>There's an <emphasis>ethernet</emphasis> gadget
630	driver, which implements one of the most useful
631	<emphasis>Communications Device Class</emphasis> (CDC) models.  
632	One of the standards for cable modem interoperability even
633	specifies the use of this ethernet model as one of two
634	mandatory options.
635	Gadgets using this code look to a USB host as if they're
636	an Ethernet adapter.
637	It provides access to a network where the gadget's CPU is one host,
638	which could easily be bridging, routing, or firewalling
639	access to other networks.
640	Since some hardware can't fully implement the CDC Ethernet
641	requirements, this driver also implements a "good parts only"
642	subset of CDC Ethernet.
643	(That subset doesn't advertise itself as CDC Ethernet,
644	to avoid creating problems.)
645	</para>
647	<para>Support for Microsoft's <emphasis>RNDIS</emphasis>
648	protocol has been contributed by Pengutronix and Auerswald GmbH.
649	This is like CDC Ethernet, but it runs on more slightly USB hardware
650	(but less than the CDC subset).
651	However, its main claim to fame is being able to connect directly to
652	recent versions of Windows, using drivers that Microsoft bundles
653	and supports, making it much simpler to network with Windows.
654	</para>
656	<para>There is also support for user mode gadget drivers,
657	using <emphasis>gadgetfs</emphasis>.
658	This provides a <emphasis>User Mode API</emphasis> that presents
659	each endpoint as a single file descriptor.  I/O is done using
660	normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls.
661	Familiar tools like GDB and pthreads can be used to
662	develop and debug user mode drivers, so that once a robust
663	controller driver is available many applications for it
664	won't require new kernel mode software.
665	Linux 2.6 <emphasis>Async I/O (AIO)</emphasis>
666	support is available, so that user mode software
667	can stream data with only slightly more overhead
668	than a kernel driver.
669	</para>
671	<para>There's a USB Mass Storage class driver, which provides
672	a different solution for interoperability with systems such
673	as MS-Windows and MacOS.
674	That <emphasis>Mass Storage</emphasis> driver uses a
675	file or block device as backing store for a drive,
676	like the <filename>loop</filename> driver.
677	The USB host uses the BBB, CB, or CBI versions of the mass
678	storage class specification, using transparent SCSI commands
679	to access the data from the backing store.
680	</para>
682	<para>There's a "serial line" driver, useful for TTY style
683	operation over USB.
684	The latest version of that driver supports CDC ACM style
685	operation, like a USB modem, and so on most hardware it can
686	interoperate easily with MS-Windows.
687	One interesting use of that driver is in boot firmware (like a BIOS),
688	which can sometimes use that model with very small systems without
689	real serial lines.
690	</para>
692	<para>Support for other kinds of gadget is expected to
693	be developed and contributed
694	over time, as this driver framework evolves.
695	</para>
697	</chapter>
699	<chapter id="otg"><title>USB On-The-GO (OTG)</title>
701	<para>USB OTG support on Linux 2.6 was initially developed
702	by Texas Instruments for
703	<ulink url="http://www.omap.com">OMAP</ulink> 16xx and 17xx
704	series processors.
705	Other OTG systems should work in similar ways, but the
706	hardware level details could be very different.
707	</para> 
709	<para>Systems need specialized hardware support to implement OTG,
710	notably including a special <emphasis>Mini-AB</emphasis> jack
711	and associated transciever to support <emphasis>Dual-Role</emphasis>
712	operation:
713	they can act either as a host, using the standard
714	Linux-USB host side driver stack,
715	or as a peripheral, using this "gadget" framework.
716	To do that, the system software relies on small additions
717	to those programming interfaces,
718	and on a new internal component (here called an "OTG Controller")
719	affecting which driver stack connects to the OTG port.
720	In each role, the system can re-use the existing pool of
721	hardware-neutral drivers, layered on top of the controller
722	driver interfaces (<emphasis>usb_bus</emphasis> or
723	<emphasis>usb_gadget</emphasis>).
724	Such drivers need at most minor changes, and most of the calls
725	added to support OTG can also benefit non-OTG products.
726	</para>
728	<itemizedlist>
729	    <listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis>
730		flag, and use it to determine whether or not to include
731		an OTG descriptor in each of their configurations.
732		</para></listitem>
733	    <listitem><para>Gadget drivers may need changes to support the
734		two new OTG protocols, exposed in new gadget attributes
735		such as <emphasis>b_hnp_enable</emphasis> flag.
736		HNP support should be reported through a user interface
737		(two LEDs could suffice), and is triggered in some cases
738		when the host suspends the peripheral.
739		SRP support can be user-initiated just like remote wakeup,
740		probably by pressing the same button.
741		</para></listitem>
742	    <listitem><para>On the host side, USB device drivers need
743		to be taught to trigger HNP at appropriate moments, using
744		<function>usb_suspend_device()</function>.
745		That also conserves battery power, which is useful even
746		for non-OTG configurations.
747		</para></listitem>
748	    <listitem><para>Also on the host side, a driver must support the
749		OTG "Targeted Peripheral List".  That's just a whitelist,
750		used to reject peripherals not supported with a given
751		Linux OTG host.
752		<emphasis>This whitelist is product-specific;
753		each product must modify <filename>otg_whitelist.h</filename>
754		to match its interoperability specification.
755		</emphasis>
756		</para>
757		<para>Non-OTG Linux hosts, like PCs and workstations,
758		normally have some solution for adding drivers, so that
759		peripherals that aren't recognized can eventually be supported.
760		That approach is unreasonable for consumer products that may
761		never have their firmware upgraded, and where it's usually
762		unrealistic to expect traditional PC/workstation/server kinds
763		of support model to work.
764		For example, it's often impractical to change device firmware
765		once the product has been distributed, so driver bugs can't
766		normally be fixed if they're found after shipment.
767		</para></listitem>
768	</itemizedlist>
770	<para>
771	Additional changes are needed below those hardware-neutral
772	<emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis>
773	driver interfaces; those aren't discussed here in any detail.
774	Those affect the hardware-specific code for each USB Host or Peripheral
775	controller, and how the HCD initializes (since OTG can be active only
776	on a single port).
777	They also involve what may be called an <emphasis>OTG Controller
778	Driver</emphasis>, managing the OTG transceiver and the OTG state
779	machine logic as well as much of the root hub behavior for the
780	OTG port.
781	The OTG controller driver needs to activate and deactivate USB
782	controllers depending on the relevant device role.
783	Some related changes were needed inside usbcore, so that it
784	can identify OTG-capable devices and respond appropriately
785	to HNP or SRP protocols.
786	</para> 
788	</chapter>
790	</book>
791	<!--
792		vim:syntax=sgml:sw=4
793	-->
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