1<?xml version="1.0" encoding="UTF-8"?>
   2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3        "" []>
   5<book id="USB-Gadget-API">
   6  <bookinfo>
   7    <title>USB Gadget API for Linux</title>
   8    <date>20 August 2004</date>
   9    <edition>20 August 2004</edition>
  11    <legalnotice>
  12       <para>
  13         This documentation is free software; you can redistribute
  14         it and/or modify it under the terms of the GNU General Public
  15         License as published by the Free Software Foundation; either
  16         version 2 of the License, or (at your option) any later
  17         version.
  18       </para>
  20       <para>
  21         This program is distributed in the hope that it will be
  22         useful, but WITHOUT ANY WARRANTY; without even the implied
  24         See the GNU General Public License for more details.
  25       </para>
  27       <para>
  28         You should have received a copy of the GNU General Public
  29         License along with this program; if not, write to the Free
  30         Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  31         MA 02111-1307 USA
  32       </para>
  34       <para>
  35         For more details see the file COPYING in the source
  36         distribution of Linux.
  37       </para>
  38    </legalnotice>
  39    <copyright>
  40      <year>2003-2004</year>
  41      <holder>David Brownell</holder>
  42    </copyright>
  44    <author>
  45      <firstname>David</firstname> 
  46      <surname>Brownell</surname>
  47      <affiliation>
  48        <address><email></email></address>
  49      </affiliation>
  50    </author>
  51  </bookinfo>
  55<chapter id="intro"><title>Introduction</title>
  57<para>This document presents a Linux-USB "Gadget"
  58kernel mode
  59API, for use within peripherals and other USB devices
  60that embed Linux.
  61It provides an overview of the API structure,
  62and shows how that fits into a system development project.
  63This is the first such API released on Linux to address
  64a number of important problems, including: </para>
  67    <listitem><para>Supports USB 2.0, for high speed devices which
  68        can stream data at several dozen megabytes per second.
  69        </para></listitem>
  70    <listitem><para>Handles devices with dozens of endpoints just as
  71        well as ones with just two fixed-function ones.  Gadget drivers
  72        can be written so they're easy to port to new hardware.
  73        </para></listitem>
  74    <listitem><para>Flexible enough to expose more complex USB device
  75        capabilities such as multiple configurations, multiple interfaces,
  76        composite devices,
  77        and alternate interface settings.
  78        </para></listitem>
  79    <listitem><para>USB "On-The-Go" (OTG) support, in conjunction
  80        with updates to the Linux-USB host side.
  81        </para></listitem>
  82    <listitem><para>Sharing data structures and API models with the
  83        Linux-USB host side API.  This helps the OTG support, and
  84        looks forward to more-symmetric frameworks (where the same
  85        I/O model is used by both host and device side drivers).
  86        </para></listitem>
  87    <listitem><para>Minimalist, so it's easier to support new device
  88        controller hardware.  I/O processing doesn't imply large
  89        demands for memory or CPU resources.
  90        </para></listitem>
  94<para>Most Linux developers will not be able to use this API, since they
  95have USB "host" hardware in a PC, workstation, or server.
  96Linux users with embedded systems are more likely to
  97have USB peripheral hardware.
  98To distinguish drivers running inside such hardware from the
  99more familiar Linux "USB device drivers",
 100which are host side proxies for the real USB devices,
 101a different term is used:
 102the drivers inside the peripherals are "USB gadget drivers".
 103In USB protocol interactions, the device driver is the master
 104(or "client driver")
 105and the gadget driver is the slave (or "function driver").
 108<para>The gadget API resembles the host side Linux-USB API in that both
 109use queues of request objects to package I/O buffers, and those requests
 110may be submitted or canceled.
 111They share common definitions for the standard USB
 112<emphasis>Chapter 9</emphasis> messages, structures, and constants.
 113Also, both APIs bind and unbind drivers to devices.
 114The APIs differ in detail, since the host side's current
 115URB framework exposes a number of implementation details
 116and assumptions that are inappropriate for a gadget API.
 117While the model for control transfers and configuration
 118management is necessarily different (one side is a hardware-neutral master,
 119the other is a hardware-aware slave), the endpoint I/0 API used here
 120should also be usable for an overhead-reduced host side API.
 125<chapter id="structure"><title>Structure of Gadget Drivers</title>
 127<para>A system running inside a USB peripheral
 128normally has at least three layers inside the kernel to handle
 129USB protocol processing, and may have additional layers in
 130user space code.
 131The "gadget" API is used by the middle layer to interact
 132with the lowest level (which directly handles hardware).
 135<para>In Linux, from the bottom up, these layers are:
 140    <varlistentry>
 141        <term><emphasis>USB Controller Driver</emphasis></term>
 143        <listitem>
 144        <para>This is the lowest software level.
 145        It is the only layer that talks to hardware,
 146        through registers, fifos, dma, irqs, and the like.
 147        The <filename>&lt;linux/usb/gadget.h&gt;</filename> API abstracts
 148        the peripheral controller endpoint hardware.
 149        That hardware is exposed through endpoint objects, which accept
 150        streams of IN/OUT buffers, and through callbacks that interact
 151        with gadget drivers.
 152        Since normal USB devices only have one upstream
 153        port, they only have one of these drivers.
 154        The controller driver can support any number of different
 155        gadget drivers, but only one of them can be used at a time.
 156        </para>
 158        <para>Examples of such controller hardware include
 159        the PCI-based NetChip 2280 USB 2.0 high speed controller,
 160        the SA-11x0 or PXA-25x UDC (found within many PDAs),
 161        and a variety of other products.
 162        </para>
 164        </listitem></varlistentry>
 166    <varlistentry>
 167        <term><emphasis>Gadget Driver</emphasis></term>
 169        <listitem>
 170        <para>The lower boundary of this driver implements hardware-neutral
 171        USB functions, using calls to the controller driver.
 172        Because such hardware varies widely in capabilities and restrictions,
 173        and is used in embedded environments where space is at a premium,
 174        the gadget driver is often configured at compile time
 175        to work with endpoints supported by one particular controller.
 176        Gadget drivers may be portable to several different controllers,
 177        using conditional compilation.
 178        (Recent kernels substantially simplify the work involved in
 179        supporting new hardware, by <emphasis>autoconfiguring</emphasis>
 180        endpoints automatically for many bulk-oriented drivers.)
 181        Gadget driver responsibilities include:
 182        </para>
 183        <itemizedlist>
 184            <listitem><para>handling setup requests (ep0 protocol responses)
 185                possibly including class-specific functionality
 186                </para></listitem>
 187            <listitem><para>returning configuration and string descriptors
 188                </para></listitem>
 189            <listitem><para>(re)setting configurations and interface
 190                altsettings, including enabling and configuring endpoints
 191                </para></listitem>
 192            <listitem><para>handling life cycle events, such as managing
 193                bindings to hardware,
 194                USB suspend/resume, remote wakeup,
 195                and disconnection from the USB host.
 196                </para></listitem>
 197            <listitem><para>managing IN and OUT transfers on all currently
 198                enabled endpoints
 199                </para></listitem>
 200        </itemizedlist>
 202        <para>
 203        Such drivers may be modules of proprietary code, although
 204        that approach is discouraged in the Linux community.
 205        </para>
 206        </listitem></varlistentry>
 208    <varlistentry>
 209        <term><emphasis>Upper Level</emphasis></term>
 211        <listitem>
 212        <para>Most gadget drivers have an upper boundary that connects
 213        to some Linux driver or framework in Linux.
 214        Through that boundary flows the data which the gadget driver
 215        produces and/or consumes through protocol transfers over USB.
 216        Examples include:
 217        </para>
 218        <itemizedlist>
 219            <listitem><para>user mode code, using generic (gadgetfs)
 220                or application specific files in
 221                <filename>/dev</filename>
 222                </para></listitem>
 223            <listitem><para>networking subsystem (for network gadgets,
 224                like the CDC Ethernet Model gadget driver)
 225                </para></listitem>
 226            <listitem><para>data capture drivers, perhaps video4Linux or
 227                 a scanner driver; or test and measurement hardware.
 228                 </para></listitem>
 229            <listitem><para>input subsystem (for HID gadgets)
 230                </para></listitem>
 231            <listitem><para>sound subsystem (for audio gadgets)
 232                </para></listitem>
 233            <listitem><para>file system (for PTP gadgets)
 234                </para></listitem>
 235            <listitem><para>block i/o subsystem (for usb-storage gadgets)
 236                </para></listitem>
 237            <listitem><para>... and more </para></listitem>
 238        </itemizedlist>
 239        </listitem></varlistentry>
 241    <varlistentry>
 242        <term><emphasis>Additional Layers</emphasis></term>
 244        <listitem>
 245        <para>Other layers may exist.
 246        These could include kernel layers, such as network protocol stacks,
 247        as well as user mode applications building on standard POSIX
 248        system call APIs such as
 249        <emphasis>open()</emphasis>, <emphasis>close()</emphasis>,
 250        <emphasis>read()</emphasis> and <emphasis>write()</emphasis>.
 251        On newer systems, POSIX Async I/O calls may be an option.
 252        Such user mode code will not necessarily be subject to
 253        the GNU General Public License (GPL).
 254        </para>
 255        </listitem></varlistentry>
 260<para>OTG-capable systems will also need to include a standard Linux-USB
 261host side stack,
 262with <emphasis>usbcore</emphasis>,
 263one or more <emphasis>Host Controller Drivers</emphasis> (HCDs),
 264<emphasis>USB Device Drivers</emphasis> to support
 265the OTG "Targeted Peripheral List",
 266and so forth.
 267There will also be an <emphasis>OTG Controller Driver</emphasis>,
 268which is visible to gadget and device driver developers only indirectly.
 269That helps the host and device side USB controllers implement the
 270two new OTG protocols (HNP and SRP).
 271Roles switch (host to peripheral, or vice versa) using HNP
 272during USB suspend processing, and SRP can be viewed as a
 273more battery-friendly kind of device wakeup protocol.
 276<para>Over time, reusable utilities are evolving to help make some
 277gadget driver tasks simpler.
 278For example, building configuration descriptors from vectors of
 279descriptors for the configurations interfaces and endpoints is
 280now automated, and many drivers now use autoconfiguration to
 281choose hardware endpoints and initialize their descriptors.
 283A potential example of particular interest
 284is code implementing standard USB-IF protocols for
 285HID, networking, storage, or audio classes.
 286Some developers are interested in KDB or KGDB hooks, to let
 287target hardware be remotely debugged.
 288Most such USB protocol code doesn't need to be hardware-specific,
 289any more than network protocols like X11, HTTP, or NFS are.
 290Such gadget-side interface drivers should eventually be combined,
 291to implement composite devices.
 297<chapter id="api"><title>Kernel Mode Gadget API</title>
 299<para>Gadget drivers declare themselves through a
 300<emphasis>struct usb_gadget_driver</emphasis>, which is responsible for
 301most parts of enumeration for a <emphasis>struct usb_gadget</emphasis>.
 302The response to a set_configuration usually involves
 303enabling one or more of the <emphasis>struct usb_ep</emphasis> objects
 304exposed by the gadget, and submitting one or more
 305<emphasis>struct usb_request</emphasis> buffers to transfer data.
 306Understand those four data types, and their operations, and
 307you will understand how this API works.
 310<note><title>Incomplete Data Type Descriptions</title>
 312<para>This documentation was prepared using the standard Linux
 313kernel <filename>docproc</filename> tool, which turns text
 314and in-code comments into SGML DocBook and then into usable
 315formats such as HTML or PDF.
 316Other than the "Chapter 9" data types, most of the significant
 317data types and functions are described here.
 320<para>However, docproc does not understand all the C constructs
 321that are used, so some relevant information is likely omitted from
 322what you are reading.  
 323One example of such information is endpoint autoconfiguration.
 324You'll have to read the header file, and use example source
 325code (such as that for "Gadget Zero"), to fully understand the API.
 328<para>The part of the API implementing some basic
 329driver capabilities is specific to the version of the
 330Linux kernel that's in use.
 331The 2.6 kernel includes a <emphasis>driver model</emphasis>
 332framework that has no analogue on earlier kernels;
 333so those parts of the gadget API are not fully portable.
 334(They are implemented on 2.4 kernels, but in a different way.)
 335The driver model state is another part of this API that is
 336ignored by the kerneldoc tools.
 340<para>The core API does not expose
 341every possible hardware feature, only the most widely available ones.
 342There are significant hardware features, such as device-to-device DMA
 343(without temporary storage in a memory buffer)
 344that would be added using hardware-specific APIs.
 347<para>This API allows drivers to use conditional compilation to handle
 348endpoint capabilities of different hardware, but doesn't require that.
 349Hardware tends to have arbitrary restrictions, relating to
 350transfer types, addressing, packet sizes, buffering, and availability.
 351As a rule, such differences only matter for "endpoint zero" logic
 352that handles device configuration and management.
 353The API supports limited run-time
 354detection of capabilities, through naming conventions for endpoints.
 355Many drivers will be able to at least partially autoconfigure
 357In particular, driver init sections will often have endpoint
 358autoconfiguration logic that scans the hardware's list of endpoints
 359to find ones matching the driver requirements
 360(relying on those conventions), to eliminate some of the most
 361common reasons for conditional compilation.
 364<para>Like the Linux-USB host side API, this API exposes
 365the "chunky" nature of USB messages:  I/O requests are in terms
 366of one or more "packets", and packet boundaries are visible to drivers.
 367Compared to RS-232 serial protocols, USB resembles
 368synchronous protocols like HDLC
 369(N bytes per frame, multipoint addressing, host as the primary
 370station and devices as secondary stations)
 371more than asynchronous ones
 372(tty style:  8 data bits per frame, no parity, one stop bit).
 373So for example the controller drivers won't buffer
 374two single byte writes into a single two-byte USB IN packet,
 375although gadget drivers may do so when they implement
 376protocols where packet boundaries (and "short packets")
 377are not significant.
 380<sect1 id="lifecycle"><title>Driver Life Cycle</title>
 382<para>Gadget drivers make endpoint I/O requests to hardware without
 383needing to know many details of the hardware, but driver
 384setup/configuration code needs to handle some differences.
 385Use the API like this:
 388<orderedlist numeration='arabic'>
 390<listitem><para>Register a driver for the particular device side
 391usb controller hardware,
 392such as the net2280 on PCI (USB 2.0),
 393sa11x0 or pxa25x as found in Linux PDAs,
 394and so on.
 395At this point the device is logically in the USB ch9 initial state
 396("attached"), drawing no power and not usable
 397(since it does not yet support enumeration).
 398Any host should not see the device, since it's not
 399activated the data line pullup used by the host to
 400detect a device, even if VBUS power is available.
 403<listitem><para>Register a gadget driver that implements some higher level
 404device function.  That will then bind() to a usb_gadget, which
 405activates the data line pullup sometime after detecting VBUS.
 408<listitem><para>The hardware driver can now start enumerating.
 409The steps it handles are to accept USB power and set_address requests.
 410Other steps are handled by the gadget driver.
 411If the gadget driver module is unloaded before the host starts to
 412enumerate, steps before step 7 are skipped.
 415<listitem><para>The gadget driver's setup() call returns usb descriptors,
 416based both on what the bus interface hardware provides and on the
 417functionality being implemented.
 418That can involve alternate settings or configurations,
 419unless the hardware prevents such operation.
 420For OTG devices, each configuration descriptor includes
 421an OTG descriptor.
 424<listitem><para>The gadget driver handles the last step of enumeration,
 425when the USB host issues a set_configuration call.
 426It enables all endpoints used in that configuration,
 427with all interfaces in their default settings.
 428That involves using a list of the hardware's endpoints, enabling each
 429endpoint according to its descriptor.
 430It may also involve using <function>usb_gadget_vbus_draw</function>
 431to let more power be drawn from VBUS, as allowed by that configuration.
 432For OTG devices, setting a configuration may also involve reporting
 433HNP capabilities through a user interface.
 436<listitem><para>Do real work and perform data transfers, possibly involving
 437changes to interface settings or switching to new configurations, until the
 438device is disconnect()ed from the host.
 439Queue any number of transfer requests to each endpoint.
 440It may be suspended and resumed several times before being disconnected.
 441On disconnect, the drivers go back to step 3 (above).
 444<listitem><para>When the gadget driver module is being unloaded,
 445the driver unbind() callback is issued.  That lets the controller
 446driver be unloaded.
 451<para>Drivers will normally be arranged so that just loading the
 452gadget driver module (or statically linking it into a Linux kernel)
 453allows the peripheral device to be enumerated, but some drivers
 454will defer enumeration until some higher level component (like
 455a user mode daemon) enables it.
 456Note that at this lowest level there are no policies about how
 457ep0 configuration logic is implemented,
 458except that it should obey USB specifications.
 459Such issues are in the domain of gadget drivers,
 460including knowing about implementation constraints
 461imposed by some USB controllers
 462or understanding that composite devices might happen to
 463be built by integrating reusable components.
 466<para>Note that the lifecycle above can be slightly different
 467for OTG devices.
 468Other than providing an additional OTG descriptor in each
 469configuration, only the HNP-related differences are particularly
 470visible to driver code.
 471They involve reporting requirements during the SET_CONFIGURATION
 472request, and the option to invoke HNP during some suspend callbacks.
 473Also, SRP changes the semantics of
 480<sect1 id="ch9"><title>USB 2.0 Chapter 9 Types and Constants</title>
 482<para>Gadget drivers
 483rely on common USB structures and constants
 484defined in the
 486header file, which is standard in Linux 2.6 kernels.
 487These are the same types and constants used by host
 488side drivers (and usbcore).
 494<sect1 id="core"><title>Core Objects and Methods</title>
 496<para>These are declared in
 498and are used by gadget drivers to interact with
 499USB peripheral controller drivers.
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 515<sect1 id="utils"><title>Optional Utilities</title>
 517<para>The core API is sufficient for writing a USB Gadget Driver,
 518but some optional utilities are provided to simplify common tasks.
 519These utilities include endpoint autoconfiguration.
 524<!-- !Edrivers/usb/gadget/epautoconf.c -->
 527<sect1 id="composite"><title>Composite Device Framework</title>
 529<para>The core API is sufficient for writing drivers for composite
 530USB devices (with more than one function in a given configuration),
 531and also multi-configuration devices (also more than one function,
 532but not necessarily sharing a given configuration).
 533There is however an optional framework which makes it easier to
 534reuse and combine functions.
 537<para>Devices using this framework provide a <emphasis>struct
 538usb_composite_driver</emphasis>, which in turn provides one or
 539more <emphasis>struct usb_configuration</emphasis> instances.
 540Each such configuration includes at least one
 541<emphasis>struct usb_function</emphasis>, which packages a user
 542visible role such as "network link" or "mass storage device".
 543Management functions may also exist, such as "Device Firmware
 552<sect1 id="functions"><title>Composite Device Functions</title>
 554<para>At this writing, a few of the current gadget drivers have
 555been converted to this framework.
 556Near-term plans include converting all of them, except for "gadgetfs".
 570<chapter id="controllers"><title>Peripheral Controller Drivers</title>
 572<para>The first hardware supporting this API was the NetChip 2280
 573controller, which supports USB 2.0 high speed and is based on PCI.
 574This is the <filename>net2280</filename> driver module.
 575The driver supports Linux kernel versions 2.4 and 2.6;
 576contact NetChip Technologies for development boards and product
 580<para>Other hardware working in the "gadget" framework includes:
 581Intel's PXA 25x and IXP42x series processors
 583Toshiba TC86c001 "Goku-S" (<filename>goku_udc</filename>),
 584Renesas SH7705/7727 (<filename>sh_udc</filename>),
 585MediaQ 11xx (<filename>mq11xx_udc</filename>),
 586Hynix HMS30C7202 (<filename>h7202_udc</filename>),
 587National 9303/4 (<filename>n9604_udc</filename>),
 588Texas Instruments OMAP (<filename>omap_udc</filename>),
 589Sharp LH7A40x (<filename>lh7a40x_udc</filename>),
 590and more.
 591Most of those are full speed controllers.
 594<para>At this writing, there are people at work on drivers in
 595this framework for several other USB device controllers,
 596with plans to make many of them be widely available.
 599<!-- !Edrivers/usb/gadget/net2280.c -->
 601<para>A partial USB simulator,
 602the <filename>dummy_hcd</filename> driver, is available.
 603It can act like a net2280, a pxa25x, or an sa11x0 in terms
 604of available endpoints and device speeds; and it simulates
 605control, bulk, and to some extent interrupt transfers.
 606That lets you develop some parts of a gadget driver on a normal PC,
 607without any special hardware, and perhaps with the assistance
 608of tools such as GDB running with User Mode Linux.
 609At least one person has expressed interest in adapting that
 610approach, hooking it up to a simulator for a microcontroller.
 611Such simulators can help debug subsystems where the runtime hardware
 612is unfriendly to software development, or is not yet available.
 615<para>Support for other controllers is expected to be developed
 616and contributed
 617over time, as this driver framework evolves.
 622<chapter id="gadget"><title>Gadget Drivers</title>
 624<para>In addition to <emphasis>Gadget Zero</emphasis>
 625(used primarily for testing and development with drivers
 626for usb controller hardware), other gadget drivers exist.
 629<para>There's an <emphasis>ethernet</emphasis> gadget
 630driver, which implements one of the most useful
 631<emphasis>Communications Device Class</emphasis> (CDC) models.  
 632One of the standards for cable modem interoperability even
 633specifies the use of this ethernet model as one of two
 634mandatory options.
 635Gadgets using this code look to a USB host as if they're
 636an Ethernet adapter.
 637It provides access to a network where the gadget's CPU is one host,
 638which could easily be bridging, routing, or firewalling
 639access to other networks.
 640Since some hardware can't fully implement the CDC Ethernet
 641requirements, this driver also implements a "good parts only"
 642subset of CDC Ethernet.
 643(That subset doesn't advertise itself as CDC Ethernet,
 644to avoid creating problems.)
 647<para>Support for Microsoft's <emphasis>RNDIS</emphasis>
 648protocol has been contributed by Pengutronix and Auerswald GmbH.
 649This is like CDC Ethernet, but it runs on more slightly USB hardware
 650(but less than the CDC subset).
 651However, its main claim to fame is being able to connect directly to
 652recent versions of Windows, using drivers that Microsoft bundles
 653and supports, making it much simpler to network with Windows.
 656<para>There is also support for user mode gadget drivers,
 657using <emphasis>gadgetfs</emphasis>.
 658This provides a <emphasis>User Mode API</emphasis> that presents
 659each endpoint as a single file descriptor.  I/O is done using
 660normal <emphasis>read()</emphasis> and <emphasis>read()</emphasis> calls.
 661Familiar tools like GDB and pthreads can be used to
 662develop and debug user mode drivers, so that once a robust
 663controller driver is available many applications for it
 664won't require new kernel mode software.
 665Linux 2.6 <emphasis>Async I/O (AIO)</emphasis>
 666support is available, so that user mode software
 667can stream data with only slightly more overhead
 668than a kernel driver.
 671<para>There's a USB Mass Storage class driver, which provides
 672a different solution for interoperability with systems such
 673as MS-Windows and MacOS.
 674That <emphasis>File-backed Storage</emphasis> driver uses a
 675file or block device as backing store for a drive,
 676like the <filename>loop</filename> driver.
 677The USB host uses the BBB, CB, or CBI versions of the mass
 678storage class specification, using transparent SCSI commands
 679to access the data from the backing store.
 682<para>There's a "serial line" driver, useful for TTY style
 683operation over USB.
 684The latest version of that driver supports CDC ACM style
 685operation, like a USB modem, and so on most hardware it can
 686interoperate easily with MS-Windows.
 687One interesting use of that driver is in boot firmware (like a BIOS),
 688which can sometimes use that model with very small systems without
 689real serial lines.
 692<para>Support for other kinds of gadget is expected to
 693be developed and contributed
 694over time, as this driver framework evolves.
 699<chapter id="otg"><title>USB On-The-GO (OTG)</title>
 701<para>USB OTG support on Linux 2.6 was initially developed
 702by Texas Instruments for
 703<ulink url="">OMAP</ulink> 16xx and 17xx
 704series processors.
 705Other OTG systems should work in similar ways, but the
 706hardware level details could be very different.
 709<para>Systems need specialized hardware support to implement OTG,
 710notably including a special <emphasis>Mini-AB</emphasis> jack
 711and associated transciever to support <emphasis>Dual-Role</emphasis>
 713they can act either as a host, using the standard
 714Linux-USB host side driver stack,
 715or as a peripheral, using this "gadget" framework.
 716To do that, the system software relies on small additions
 717to those programming interfaces,
 718and on a new internal component (here called an "OTG Controller")
 719affecting which driver stack connects to the OTG port.
 720In each role, the system can re-use the existing pool of
 721hardware-neutral drivers, layered on top of the controller
 722driver interfaces (<emphasis>usb_bus</emphasis> or
 724Such drivers need at most minor changes, and most of the calls
 725added to support OTG can also benefit non-OTG products.
 729    <listitem><para>Gadget drivers test the <emphasis>is_otg</emphasis>
 730        flag, and use it to determine whether or not to include
 731        an OTG descriptor in each of their configurations.
 732        </para></listitem>
 733    <listitem><para>Gadget drivers may need changes to support the
 734        two new OTG protocols, exposed in new gadget attributes
 735        such as <emphasis>b_hnp_enable</emphasis> flag.
 736        HNP support should be reported through a user interface
 737        (two LEDs could suffice), and is triggered in some cases
 738        when the host suspends the peripheral.
 739        SRP support can be user-initiated just like remote wakeup,
 740        probably by pressing the same button.
 741        </para></listitem>
 742    <listitem><para>On the host side, USB device drivers need
 743        to be taught to trigger HNP at appropriate moments, using
 744        <function>usb_suspend_device()</function>.
 745        That also conserves battery power, which is useful even
 746        for non-OTG configurations.
 747        </para></listitem>
 748    <listitem><para>Also on the host side, a driver must support the
 749        OTG "Targeted Peripheral List".  That's just a whitelist,
 750        used to reject peripherals not supported with a given
 751        Linux OTG host.
 752        <emphasis>This whitelist is product-specific;
 753        each product must modify <filename>otg_whitelist.h</filename>
 754        to match its interoperability specification.
 755        </emphasis>
 756        </para>
 757        <para>Non-OTG Linux hosts, like PCs and workstations,
 758        normally have some solution for adding drivers, so that
 759        peripherals that aren't recognized can eventually be supported.
 760        That approach is unreasonable for consumer products that may
 761        never have their firmware upgraded, and where it's usually
 762        unrealistic to expect traditional PC/workstation/server kinds
 763        of support model to work.
 764        For example, it's often impractical to change device firmware
 765        once the product has been distributed, so driver bugs can't
 766        normally be fixed if they're found after shipment.
 767        </para></listitem>
 771Additional changes are needed below those hardware-neutral
 772<emphasis>usb_bus</emphasis> and <emphasis>usb_gadget</emphasis>
 773driver interfaces; those aren't discussed here in any detail.
 774Those affect the hardware-specific code for each USB Host or Peripheral
 775controller, and how the HCD initializes (since OTG can be active only
 776on a single port).
 777They also involve what may be called an <emphasis>OTG Controller
 778Driver</emphasis>, managing the OTG transceiver and the OTG state
 779machine logic as well as much of the root hub behavior for the
 780OTG port.
 781The OTG controller driver needs to activate and deactivate USB
 782controllers depending on the relevant device role.
 783Some related changes were needed inside usbcore, so that it
 784can identify OTG-capable devices and respond appropriately
 785to HNP or SRP protocols.
 792        vim:syntax=sgml:sw=4
 794 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.