Documentation / DocBook / gadget.tmpl

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	<?xml version="1.0" encoding="UTF-8"?>
	<!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="USB-Gadget-API">
	  <bookinfo>
	    <title>USB Gadget API for Linux</title>
	    <date>20 August 2004</date>
	    <edition>20 August 2004</edition>
	  
	    <legalnotice>
	       <para>
		 This documentation is free software; 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 program is distributed in the hope that it will be
		 useful, but WITHOUT ANY WARRANTY; without even the implied
		 warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
		 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>
		  
	       <para>
		 For more details see the file COPYING in the source
		 distribution of Linux.
	       </para>
	    </legalnotice>
	    <copyright>
	      <year>2003-2004</year>
	      <holder>David Brownell</holder>
	    </copyright>
	
	    <author>
	      <firstname>David</firstname> 
	      <surname>Brownell</surname>
	      <affiliation>
	        <address><email>dbrownell@users.sourceforge.net</email></address>
	      </affiliation>
	    </author>
	  </bookinfo>
	
	<toc></toc>
	
	<chapter id="intro"><title>Introduction</title>
	
	<para>This document presents a Linux-USB "Gadget"
	kernel mode
	API, for use within peripherals and other USB devices
	that embed Linux.
	It provides an overview of the API structure,
	and shows how that fits into a system development project.
	This is the first such API released on Linux to address
	a number of important problems, including: </para>
	
	<itemizedlist>
	    <listitem><para>Supports USB 2.0, for high speed devices which
		can stream data at several dozen megabytes per second.
		</para></listitem>
	    <listitem><para>Handles devices with dozens of endpoints just as
		well as ones with just two fixed-function ones.  Gadget drivers
		can be written so they're easy to port to new hardware.
		</para></listitem>
	    <listitem><para>Flexible enough to expose more complex USB device
		capabilities such as multiple configurations, multiple interfaces,
		composite devices,
		and alternate interface settings.
		</para></listitem>
	    <listitem><para>USB "On-The-Go" (OTG) support, in conjunction
		with updates to the Linux-USB host side.
		</para></listitem>
	    <listitem><para>Sharing data structures and API models with the
		Linux-USB host side API.  This helps the OTG support, and
		looks forward to more-symmetric frameworks (where the same
		I/O model is used by both host and device side drivers).
		</para></listitem>
	    <listitem><para>Minimalist, so it's easier to support new device
		controller hardware.  I/O processing doesn't imply large
		demands for memory or CPU resources.
		</para></listitem>
	</itemizedlist>
	
	
	<para>Most Linux developers will not be able to use this API, since they
	have USB "host" hardware in a PC, workstation, or server.
	Linux users with embedded systems are more likely to
	have USB peripheral hardware.
	To distinguish drivers running inside such hardware from the
	more familiar Linux "USB device drivers",
	which are host side proxies for the real USB devices,
	a different term is used:
	the drivers inside the peripherals are "USB gadget drivers".
	In USB protocol interactions, the device driver is the master
	(or "client driver")
	and the gadget driver is the slave (or "function driver").
	</para>
	
	<para>The gadget API resembles the host side Linux-USB API in that both
	use queues of request objects to package I/O buffers, and those requests
	may be submitted or canceled.
	They share common definitions for the standard USB
	<emphasis>Chapter 9</emphasis> messages, structures, and constants.
	Also, both APIs bind and unbind drivers to devices.
	The APIs differ in detail, since the host side's current
	URB framework exposes a number of implementation details
	and assumptions that are inappropriate for a gadget API.
	While the model for control transfers and configuration
	management is necessarily different (one side is a hardware-neutral master,
	the other is a hardware-aware slave), the endpoint I/0 API used here
	should also be usable for an overhead-reduced host side API.
	</para>
	
	</chapter>
	
	<chapter id="structure"><title>Structure of Gadget Drivers</title>
	
	<para>A system running inside a USB peripheral
	normally has at least three layers inside the kernel to handle
	USB protocol processing, and may have additional layers in
	user space code.
	The "gadget" API is used by the middle layer to interact
	with the lowest level (which directly handles hardware).
	</para>
	
	<para>In Linux, from the bottom up, these layers are:
	</para>
	
	<variablelist>
	
	    <varlistentry>
	        <term><emphasis>USB Controller Driver</emphasis></term>
	
		<listitem>
		<para>This is the lowest software level.
		It is the only layer that talks to hardware,
		through registers, fifos, dma, irqs, and the like.
		The <filename>&lt;linux/usb/gadget.h&gt;</filename> API abstracts
		the peripheral controller endpoint hardware.
		That hardware is exposed through endpoint objects, which accept
		streams of IN/OUT buffers, and through callbacks that interact
		with gadget drivers.
		Since normal USB devices only have one upstream
		port, they only have one of these drivers.
		The controller driver can support any number of different
		gadget drivers, but only one of them can be used at a time.
		</para>
	
		<para>Examples of such controller hardware include
		the PCI-based NetChip 2280 USB 2.0 high speed controller,
		the SA-11x0 or PXA-25x UDC (found within many PDAs),
		and a variety of other products.
		</para>
	
		</listitem></varlistentry>
	
	    <varlistentry>
		<term><emphasis>Gadget Driver</emphasis></term>
	
		<listitem>
		<para>The lower boundary of this driver implements hardware-neutral
		USB functions, using calls to the controller driver.
		Because such hardware varies widely in capabilities and restrictions,
		and is used in embedded environments where space is at a premium,
		the gadget driver is often configured at compile time
		to work with endpoints supported by one particular controller.
		Gadget drivers may be portable to several different controllers,
		using conditional compilation.
		(Recent kernels substantially simplify the work involved in
		supporting new hardware, by <emphasis>autoconfiguring</emphasis>
		endpoints automatically for many bulk-oriented drivers.)
		Gadget driver responsibilities include:
		</para>
		<itemizedlist>
		    <listitem><para>handling setup requests (ep0 protocol responses)
			possibly including class-specific functionality
			</para></listitem>
		    <listitem><para>returning configuration and string descriptors
			</para></listitem>
		    <listitem><para>(re)setting configurations and interface
			altsettings, including enabling and configuring endpoints
			</para></listitem>
		    <listitem><para>handling life cycle events, such as managing
			bindings to hardware,
			USB suspend/resume, remote wakeup,
			and disconnection from the USB host.
			</para></listitem>
		    <listitem><para>managing IN and OUT transfers on all currently
			enabled endpoints
			</para></listitem>
		</itemizedlist>
	
		<para>
		Such drivers may be modules of proprietary code, although
		that approach is discouraged in the Linux community.
		</para>
		</listitem></varlistentry>
	
	    <varlistentry>
		<term><emphasis>Upper Level</emphasis></term>
	
		<listitem>
		<para>Most gadget drivers have an upper boundary that connects
		to some Linux driver or framework in Linux.
		Through that boundary flows the data which the gadget driver
		produces and/or consumes through protocol transfers over USB.
		Examples include:
		</para>
		<itemizedlist>
		    <listitem><para>user mode code, using generic (gadgetfs)
		        or application specific files in
			<filename>/dev</filename>
			</para></listitem>
		    <listitem><para>networking subsystem (for network gadgets,
			like the CDC Ethernet Model gadget driver)
			</para></listitem>
		    <listitem><para>data capture drivers, perhaps video4Linux or
			 a scanner driver; or test and measurement hardware.
			 </para></listitem>
		    <listitem><para>input subsystem (for HID gadgets)
			</para></listitem>
		    <listitem><para>sound subsystem (for audio gadgets)
			</para></listitem>
		    <listitem><para>file system (for PTP gadgets)
			</para></listitem>
		    <listitem><para>block i/o subsystem (for usb-storage gadgets)
			</para></listitem>
		    <listitem><para>... and more </para></listitem>
		</itemizedlist>
		</listitem></varlistentry>
	
	    <varlistentry>
		<term><emphasis>Additional Layers</emphasis></term>
	
		<listitem>
		<para>Other layers may exist.
		These could include kernel layers, such as network protocol stacks,
		as well as user mode applications building on standard POSIX
		system call APIs such as
		<emphasis>open()</emphasis>, <emphasis>close()</emphasis>,
		<emphasis>read()</emphasis> and <emphasis>write()</emphasis>.
		On newer systems, POSIX Async I/O calls may be an option.
		Such user mode code will not necessarily be subject to
		the GNU General Public License (GPL).
		</para>
		</listitem></varlistentry>
	
	
	</variablelist>
	
	<para>OTG-capable systems will also need to include a standard Linux-USB
	host side stack,
	with <emphasis>usbcore</emphasis>,
	one or more <emphasis>Host Controller Drivers</emphasis> (HCDs),
	<emphasis>USB Device Drivers</emphasis> to support
	the OTG "Targeted Peripheral List",
	and so forth.
	There will also be an <emphasis>OTG Controller Driver</emphasis>,
	which is visible to gadget and device driver developers only indirectly.
	That helps the host and device side USB controllers implement the
	two new OTG protocols (HNP and SRP).
	Roles switch (host to peripheral, or vice versa) using HNP
	during USB suspend processing, and SRP can be viewed as a
	more battery-friendly kind of device wakeup protocol.
	</para>
	
	<para>Over time, reusable utilities are evolving to help make some
	gadget driver tasks simpler.
	For example, building configuration descriptors from vectors of
	descriptors for the configurations interfaces and endpoints is
	now automated, and many drivers now use autoconfiguration to
	choose hardware endpoints and initialize their descriptors.
	
	A potential example of particular interest
	is code implementing standard USB-IF protocols for
	HID, networking, storage, or audio classes.
	Some developers are interested in KDB or KGDB hooks, to let
	target hardware be remotely debugged.
	Most such USB protocol code doesn't need to be hardware-specific,
	any more than network protocols like X11, HTTP, or NFS are.
	Such gadget-side interface drivers should eventually be combined,
	to implement composite devices.
	</para>
	
	</chapter>
	
	
	<chapter id="api"><title>Kernel Mode Gadget API</title>
	
	<para>Gadget drivers declare themselves through a
	<emphasis>struct usb_gadget_driver</emphasis>, which is responsible for
	most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>.
	The response to a set_configuration usually involves
	enabling one or more of the <emphasis>struct usb_ep</emphasis> objects
	exposed by the gadget, and submitting one or more
	<emphasis>struct usb_request</emphasis> buffers to transfer data.
	Understand those four data types, and their operations, and
	you will understand how this API works.
	</para> 
	
	<note><title>Incomplete Data Type Descriptions</title>
	
	<para>This documentation was prepared using the standard Linux
	kernel <filename>docproc</filename> tool, which turns text
	and in-code comments into SGML DocBook and then into usable
	formats such as HTML or PDF.
	Other than the "Chapter 9" data types, most of the significant
	data types and functions are described here.
	</para>
	
	<para>However, docproc does not understand all the C constructs
	that are used, so some relevant information is likely omitted from
	what you are reading.  
	One example of such information is endpoint autoconfiguration.
	You'll have to read the header file, and use example source
	code (such as that for "Gadget Zero"), to fully understand the API.
	</para>
	
	<para>The part of the API implementing some basic
	driver capabilities is specific to the version of the
	Linux kernel that's in use.
	The 2.6 kernel includes a <emphasis>driver model</emphasis>
	framework that has no analogue on earlier kernels;
	so those parts of the gadget API are not fully portable.
	(They are implemented on 2.4 kernels, but in a different way.)
	The driver model state is another part of this API that is
	ignored by the kerneldoc tools.
	</para>
	</note>
	
	<para>The core API does not expose
	every possible hardware feature, only the most widely available ones.
	There are significant hardware features, such as device-to-device DMA
	(without temporary storage in a memory buffer)
	that would be added using hardware-specific APIs.
	</para>
	
	<para>This API allows drivers to use conditional compilation to handle
	endpoint capabilities of different hardware, but doesn't require that.
	Hardware tends to have arbitrary restrictions, relating to
	transfer types, addressing, packet sizes, buffering, and availability.
	As a rule, such differences only matter for "endpoint zero" logic
	that handles device configuration and management.
	The API supports limited run-time
	detection of capabilities, through naming conventions for endpoints.
	Many drivers will be able to at least partially autoconfigure
	themselves.
	In particular, driver init sections will often have endpoint
	autoconfiguration logic that scans the hardware's list of endpoints
	to find ones matching the driver requirements
	(relying on those conventions), to eliminate some of the most
	common reasons for conditional compilation.
	</para>
	
	<para>Like the Linux-USB host side API, this API exposes
	the "chunky" nature of USB messages:  I/O requests are in terms
	of one or more "packets", and packet boundaries are visible to drivers.
	Compared to RS-232 serial protocols, USB resembles
	synchronous protocols like HDLC
	(N bytes per frame, multipoint addressing, host as the primary
	station and devices as secondary stations)
	more than asynchronous ones
	(tty style:  8 data bits per frame, no parity, one stop bit).
	So for example the controller drivers won't buffer
	two single byte writes into a single two-byte USB IN packet,
	although gadget drivers may do so when they implement
	protocols where packet boundaries (and "short packets")
	are not significant.
	</para>
	
	<sect1 id="lifecycle"><title>Driver Life Cycle</title>
	
	<para>Gadget drivers make endpoint I/O requests to hardware without
	needing to know many details of the hardware, but driver
	setup/configuration code needs to handle some differences.
	Use the API like this:
	</para>
	
	<orderedlist numeration='arabic'>
	
	<listitem><para>Register a driver for the particular device side
	usb controller hardware,
	such as the net2280 on PCI (USB 2.0),
	sa11x0 or pxa25x as found in Linux PDAs,
	and so on.
	At this point the device is logically in the USB ch9 initial state
	("attached"), drawing no power and not usable
	(since it does not yet support enumeration).
	Any host should not see the device, since it's not
	activated the data line pullup used by the host to
	detect a device, even if VBUS power is available.
	</para></listitem>
	
	<listitem><para>Register a gadget driver that implements some higher level
	device function.  That will then bind() to a usb_gadget, which
	activates the data line pullup sometime after detecting VBUS.
	</para></listitem>
	
	<listitem><para>The hardware driver can now start enumerating.
	The steps it handles are to accept USB power and set_address requests.
	Other steps are handled by the gadget driver.
	If the gadget driver module is unloaded before the host starts to
	enumerate, steps before step 7 are skipped.
	</para></listitem>
	
	<listitem><para>The gadget driver's setup() call returns usb descriptors,
	based both on what the bus interface hardware provides and on the
	functionality being implemented.
	That can involve alternate settings or configurations,
	unless the hardware prevents such operation.
	For OTG devices, each configuration descriptor includes
	an OTG descriptor.
	</para></listitem>
	
	<listitem><para>The gadget driver handles the last step of enumeration,
	when the USB host issues a set_configuration call.
	It enables all endpoints used in that configuration,
	with all interfaces in their default settings.
	That involves using a list of the hardware's endpoints, enabling each
	endpoint according to its descriptor.
	It may also involve using <function>usb_gadget_vbus_draw</function>
	to let more power be drawn from VBUS, as allowed by that configuration.
	For OTG devices, setting a configuration may also involve reporting
	HNP capabilities through a user interface.
	</para></listitem>
	
	<listitem><para>Do real work and perform data transfers, possibly involving
	changes to interface settings or switching to new configurations, until the
	device is disconnect()ed from the host.
	Queue any number of transfer requests to each endpoint.
	It may be suspended and resumed several times before being disconnected.
	On disconnect, the drivers go back to step 3 (above).
	</para></listitem>
	
	<listitem><para>When the gadget driver module is being unloaded,
	the driver unbind() callback is issued.  That lets the controller
	driver be unloaded.
	</para></listitem>
	
	</orderedlist>
	
	<para>Drivers will normally be arranged so that just loading the
	gadget driver module (or statically linking it into a Linux kernel)
	allows the peripheral device to be enumerated, but some drivers
	will defer enumeration until some higher level component (like
	a user mode daemon) enables it.
	Note that at this lowest level there are no policies about how
	ep0 configuration logic is implemented,
	except that it should obey USB specifications.
	Such issues are in the domain of gadget drivers,
	including knowing about implementation constraints
	imposed by some USB controllers
	or understanding that composite devices might happen to
	be built by integrating reusable components.
	</para>
	
	<para>Note that the lifecycle above can be slightly different
	for OTG devices.
	Other than providing an additional OTG descriptor in each
	configuration, only the HNP-related differences are particularly
	visible to driver code.
	They involve reporting requirements during the SET_CONFIGURATION
	request, and the option to invoke HNP during some suspend callbacks.
	Also, SRP changes the semantics of
	<function>usb_gadget_wakeup</function>
	slightly.
	</para>
	
	</sect1>
	
	<sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title>
	
	<para>Gadget drivers
	rely on common USB structures and constants
	defined in the
	<filename>&lt;linux/usb/ch9.h&gt;</filename>
	header file, which is standard in Linux 2.6 kernels.
	These are the same types and constants used by host
	side drivers (and usbcore).
	</para>
	
	!Iinclude/linux/usb/ch9.h
	</sect1>
	
	<sect1 id="core"><title>Core Objects and Methods</title>
	
	<para>These are declared in
	<filename>&lt;linux/usb/gadget.h&gt;</filename>,
	and are used by gadget drivers to interact with
	USB peripheral controller drivers.
	</para>
	
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	!Iinclude/linux/usb/gadget.h
	</sect1>
	
	<sect1 id="utils"><title>Optional Utilities</title>
	
	<para>The core API is sufficient for writing a USB Gadget Driver,
	but some optional utilities are provided to simplify common tasks.
	These utilities include endpoint autoconfiguration.
	</para>
	
	!Edrivers/usb/gadget/usbstring.c
	!Edrivers/usb/gadget/config.c
	<!-- !Edrivers/usb/gadget/epautoconf.c -->
	</sect1>
	
	<sect1 id="composite"><title>Composite Device Framework</title>
	
	<para>The core API is sufficient for writing drivers for composite
	USB devices (with more than one function in a given configuration),
	and also multi-configuration devices (also more than one function,
	but not necessarily sharing a given configuration).
	There is however an optional framework which makes it easier to
	reuse and combine functions.
	</para>
	
	<para>Devices using this framework provide a <emphasis>struct
	usb_composite_driver</emphasis>, which in turn provides one or
	more <emphasis>struct usb_configuration</emphasis> instances.
	Each such configuration includes at least one
	<emphasis>struct usb_function</emphasis>, which packages a user
	visible role such as "network link" or "mass storage device".
	Management functions may also exist, such as "Device Firmware
	Upgrade".
	</para>
	
	!Iinclude/linux/usb/composite.h
	!Edrivers/usb/gadget/composite.c
	
	</sect1>
	
	<sect1 id="functions"><title>Composite Device Functions</title>
	
	<para>At this writing, a few of the current gadget drivers have
	been converted to this framework.
	Near-term plans include converting all of them, except for "gadgetfs".
	</para>
	
	!Edrivers/usb/gadget/function/f_acm.c
	!Edrivers/usb/gadget/function/f_ecm.c
	!Edrivers/usb/gadget/function/f_subset.c
	!Edrivers/usb/gadget/function/f_obex.c
	!Edrivers/usb/gadget/function/f_serial.c
	
	</sect1>
	
	
	</chapter>
	
	<chapter id="controllers"><title>Peripheral Controller Drivers</title>
	
	<para>The first hardware supporting this API was the NetChip 2280
	controller, which supports USB 2.0 high speed and is based on PCI.
	This is the <filename>net2280</filename> driver module.
	The driver supports Linux kernel versions 2.4 and 2.6;
	contact NetChip Technologies for development boards and product
	information.
	</para> 
	
	<para>Other hardware working in the "gadget" framework includes:
	Intel's PXA 25x and IXP42x series processors
	(<filename>pxa2xx_udc</filename>),
	Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>),
	Renesas SH7705/7727 (<filename>sh_udc</filename>),
	MediaQ 11xx (<filename>mq11xx_udc</filename>),
	Hynix HMS30C7202 (<filename>h7202_udc</filename>),
	National 9303/4 (<filename>n9604_udc</filename>),
	Texas Instruments OMAP (<filename>omap_udc</filename>),
	Sharp LH7A40x (<filename>lh7a40x_udc</filename>),
	and more.
	Most of those are full speed controllers.
	</para>
	
	<para>At this writing, there are people at work on drivers in
	this framework for several other USB device controllers,
	with plans to make many of them be widely available.
	</para>
	
	<!-- !Edrivers/usb/gadget/net2280.c -->
	
	<para>A partial USB simulator,
	the <filename>dummy_hcd</filename> driver, is available.
	It can act like a net2280, a pxa25x, or an sa11x0 in terms
	of available endpoints and device speeds; and it simulates
	control, bulk, and to some extent interrupt transfers.
	That lets you develop some parts of a gadget driver on a normal PC,
	without any special hardware, and perhaps with the assistance
	of tools such as GDB running with User Mode Linux.
	At least one person has expressed interest in adapting that
	approach, hooking it up to a simulator for a microcontroller.
	Such simulators can help debug subsystems where the runtime hardware
	is unfriendly to software development, or is not yet available.
	</para>
	
	<para>Support for other controllers is expected to be developed
	and contributed
	over time, as this driver framework evolves.
	</para>
	
	</chapter>
	
	<chapter id="gadget"><title>Gadget Drivers</title>
	
	<para>In addition to <emphasis>Gadget Zero</emphasis>
	(used primarily for testing and development with drivers
	for usb controller hardware), other gadget drivers exist.
	</para>
	
	<para>There's an <emphasis>ethernet</emphasis> gadget
	driver, which implements one of the most useful
	<emphasis>Communications Device Class</emphasis> (CDC) models.  
	One of the standards for cable modem interoperability even
	specifies the use of this ethernet model as one of two
	mandatory options.
	Gadgets using this code look to a USB host as if they're
	an Ethernet adapter.
	It provides access to a network where the gadget's CPU is one host,
	which could easily be bridging, routing, or firewalling
	access to other networks.
	Since some hardware can't fully implement the CDC Ethernet
	requirements, this driver also implements a "good parts only"
	subset of CDC Ethernet.
	(That subset doesn't advertise itself as CDC Ethernet,
	to avoid creating problems.)
	</para>
	
	<para>Support for Microsoft's <emphasis>RNDIS</emphasis>
	protocol has been contributed by Pengutronix and Auerswald GmbH.
	This is like CDC Ethernet, but it runs on more slightly USB hardware
	(but less than the CDC subset).
	However, its main claim to fame is being able to connect directly to
	recent versions of Windows, using drivers that Microsoft bundles
	and supports, making it much simpler to network with Windows.
	</para>
	
	<para>There is also support for user mode gadget drivers,
	using <emphasis>gadgetfs</emphasis>.
	This provides a <emphasis>User Mode API</emphasis> that presents
	each endpoint as a single file descriptor.  I/O is done using
	normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls.
	Familiar tools like GDB and pthreads can be used to
	develop and debug user mode drivers, so that once a robust
	controller driver is available many applications for it
	won't require new kernel mode software.
	Linux 2.6 <emphasis>Async I/O (AIO)</emphasis>
	support is available, so that user mode software
	can stream data with only slightly more overhead
	than a kernel driver.
	</para>
	
	<para>There's a USB Mass Storage class driver, which provides
	a different solution for interoperability with systems such
	as MS-Windows and MacOS.
	That <emphasis>Mass Storage</emphasis> driver uses a
	file or block device as backing store for a drive,
	like the <filename>loop</filename> driver.
	The USB host uses the BBB, CB, or CBI versions of the mass
	storage class specification, using transparent SCSI commands
	to access the data from the backing store.
	</para>
	
	<para>There's a "serial line" driver, useful for TTY style
	operation over USB.
	The latest version of that driver supports CDC ACM style
	operation, like a USB modem, and so on most hardware it can
	interoperate easily with MS-Windows.
	One interesting use of that driver is in boot firmware (like a BIOS),
	which can sometimes use that model with very small systems without
	real serial lines.
	</para>
	
	<para>Support for other kinds of gadget is expected to
	be developed and contributed
	over time, as this driver framework evolves.
	</para>
	
	</chapter>
	
	<chapter id="otg"><title>USB On-The-GO (OTG)</title>
	
	<para>USB OTG support on Linux 2.6 was initially developed
	by Texas Instruments for
	<ulink url="http://www.omap.com">OMAP</ulink> 16xx and 17xx
	series processors.
	Other OTG systems should work in similar ways, but the
	hardware level details could be very different.
	</para> 
	
	<para>Systems need specialized hardware support to implement OTG,
	notably including a special <emphasis>Mini-AB</emphasis> jack
	and associated transceiver to support <emphasis>Dual-Role</emphasis>
	operation:
	they can act either as a host, using the standard
	Linux-USB host side driver stack,
	or as a peripheral, using this "gadget" framework.
	To do that, the system software relies on small additions
	to those programming interfaces,
	and on a new internal component (here called an "OTG Controller")
	affecting which driver stack connects to the OTG port.
	In each role, the system can re-use the existing pool of
	hardware-neutral drivers, layered on top of the controller
	driver interfaces (<emphasis>usb_bus</emphasis> or
	<emphasis>usb_gadget</emphasis>).
	Such drivers need at most minor changes, and most of the calls
	added to support OTG can also benefit non-OTG products.
	</para>
	
	<itemizedlist>
	    <listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis>
		flag, and use it to determine whether or not to include
		an OTG descriptor in each of their configurations.
		</para></listitem>
	    <listitem><para>Gadget drivers may need changes to support the
		two new OTG protocols, exposed in new gadget attributes
		such as <emphasis>b_hnp_enable</emphasis> flag.
		HNP support should be reported through a user interface
		(two LEDs could suffice), and is triggered in some cases
		when the host suspends the peripheral.
		SRP support can be user-initiated just like remote wakeup,
		probably by pressing the same button.
		</para></listitem>
	    <listitem><para>On the host side, USB device drivers need
		to be taught to trigger HNP at appropriate moments, using
		<function>usb_suspend_device()</function>.
		That also conserves battery power, which is useful even
		for non-OTG configurations.
		</para></listitem>
	    <listitem><para>Also on the host side, a driver must support the
		OTG "Targeted Peripheral List".  That's just a whitelist,
		used to reject peripherals not supported with a given
		Linux OTG host.
		<emphasis>This whitelist is product-specific;
		each product must modify <filename>otg_whitelist.h</filename>
		to match its interoperability specification.
		</emphasis>
		</para>
		<para>Non-OTG Linux hosts, like PCs and workstations,
		normally have some solution for adding drivers, so that
		peripherals that aren't recognized can eventually be supported.
		That approach is unreasonable for consumer products that may
		never have their firmware upgraded, and where it's usually
		unrealistic to expect traditional PC/workstation/server kinds
		of support model to work.
		For example, it's often impractical to change device firmware
		once the product has been distributed, so driver bugs can't
		normally be fixed if they're found after shipment.
		</para></listitem>
	</itemizedlist>
	
	<para>
	Additional changes are needed below those hardware-neutral
	<emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis>
	driver interfaces; those aren't discussed here in any detail.
	Those affect the hardware-specific code for each USB Host or Peripheral
	controller, and how the HCD initializes (since OTG can be active only
	on a single port).
	They also involve what may be called an <emphasis>OTG Controller
	Driver</emphasis>, managing the OTG transceiver and the OTG state
	machine logic as well as much of the root hub behavior for the
	OTG port.
	The OTG controller driver needs to activate and deactivate USB
	controllers depending on the relevant device role.
	Some related changes were needed inside usbcore, so that it
	can identify OTG-capable devices and respond appropriately
	to HNP or SRP protocols.
	</para> 
	
	</chapter>
	
	</book>
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