path: root/Documentation/DocBook/gadget.tmpl

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<!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