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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><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>
+</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>File-backed 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>
+<!--
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+-->