<?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>
  
  	<!-- yeech, this is ugly in nsgmls PDF output.
  
  	     the PDF bookmark and refentry output nesting is wrong,
  	     and the member/argument documentation indents ugly.
  
  	     plus something (docproc?) adds whitespace before the
  	     descriptive paragraph text, so it can't line up right
  	     unless the explanations are trivial.
  	  -->
  
  !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/f_acm.c
  !Edrivers/usb/gadget/f_ecm.c
  !Edrivers/usb/gadget/f_subset.c
  !Edrivers/usb/gadget/f_obex.c
  !Edrivers/usb/gadget/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 transciever 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>
  <!--
  	vim:syntax=sgml:sw=4
  -->