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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="Linux-USB-API">
<bookinfo>
<title>The Linux-USB Host Side API</title>
<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>
</bookinfo>
<toc></toc>
<chapter id="intro">
<title>Introduction to USB on Linux</title>
<para>A Universal Serial Bus (USB) is used to connect a host,
such as a PC or workstation, to a number of peripheral
devices. USB uses a tree structure, with the host as the
root (the system's master), hubs as interior nodes, and
peripherals as leaves (and slaves).
Modern PCs support several such trees of USB devices, usually
one USB 2.0 tree (480 Mbit/sec each) with
a few USB 1.1 trees (12 Mbit/sec each) that are used when you
connect a USB 1.1 device directly to the machine's "root hub".
</para>
<para>That master/slave asymmetry was designed-in for a number of
reasons, one being ease of use. It is not physically possible to
assemble (legal) USB cables incorrectly: all upstream "to the host"
connectors are the rectangular type (matching the sockets on
root hubs), and all downstream connectors are the squarish type
(or they are built into the peripheral).
Also, the host software doesn't need to deal with distributed
auto-configuration since the pre-designated master node manages all that.
And finally, at the electrical level, bus protocol overhead is reduced by
eliminating arbitration and moving scheduling into the host software.
</para>
<para>USB 1.0 was announced in January 1996 and was revised
as USB 1.1 (with improvements in hub specification and
support for interrupt-out transfers) in September 1998.
USB 2.0 was released in April 2000, adding high-speed
transfers and transaction-translating hubs (used for USB 1.1
and 1.0 backward compatibility).
</para>
<para>Kernel developers added USB support to Linux early in the 2.2 kernel
series, shortly before 2.3 development forked. Updates from 2.3 were
regularly folded back into 2.2 releases, which improved reliability and
brought <filename>/sbin/hotplug</filename> support as well more drivers.
Such improvements were continued in the 2.5 kernel series, where they added
USB 2.0 support, improved performance, and made the host controller drivers
(HCDs) more consistent. They also simplified the API (to make bugs less
likely) and added internal "kerneldoc" documentation.
</para>
<para>Linux can run inside USB devices as well as on
the hosts that control the devices.
But USB device drivers running inside those peripherals
don't do the same things as the ones running inside hosts,
so they've been given a different name:
<emphasis>gadget drivers</emphasis>.
This document does not cover gadget drivers.
</para>
</chapter>
<chapter id="host">
<title>USB Host-Side API Model</title>
<para>Host-side drivers for USB devices talk to the "usbcore" APIs.
There are two. One is intended for
<emphasis>general-purpose</emphasis> drivers (exposed through
driver frameworks), and the other is for drivers that are
<emphasis>part of the core</emphasis>.
Such core drivers include the <emphasis>hub</emphasis> driver
(which manages trees of USB devices) and several different kinds
of <emphasis>host controller drivers</emphasis>,
which control individual busses.
</para>
<para>The device model seen by USB drivers is relatively complex.
</para>
<itemizedlist>
<listitem><para>USB supports four kinds of data transfers
(control, bulk, interrupt, and isochronous). Two of them (control
and bulk) use bandwidth as it's available,
while the other two (interrupt and isochronous)
are scheduled to provide guaranteed bandwidth.
</para></listitem>
<listitem><para>The device description model includes one or more
"configurations" per device, only one of which is active at a time.
Devices that are capable of high-speed operation must also support
full-speed configurations, along with a way to ask about the
"other speed" configurations which might be used.
</para></listitem>
<listitem><para>Configurations have one or more "interfaces", each
of which may have "alternate settings". Interfaces may be
standardized by USB "Class" specifications, or may be specific to
a vendor or device.</para>
<para>USB device drivers actually bind to interfaces, not devices.
Think of them as "interface drivers", though you
may not see many devices where the distinction is important.
<emphasis>Most USB devices are simple, with only one configuration,
one interface, and one alternate setting.</emphasis>
</para></listitem>
<listitem><para>Interfaces have one or more "endpoints", each of
which supports one type and direction of data transfer such as
"bulk out" or "interrupt in". The entire configuration may have
up to sixteen endpoints in each direction, allocated as needed
among all the interfaces.
</para></listitem>
<listitem><para>Data transfer on USB is packetized; each endpoint
has a maximum packet size.
Drivers must often be aware of conventions such as flagging the end
of bulk transfers using "short" (including zero length) packets.
</para></listitem>
<listitem><para>The Linux USB API supports synchronous calls for
control and bulk messages.
It also supports asynchnous calls for all kinds of data transfer,
using request structures called "URBs" (USB Request Blocks).
</para></listitem>
</itemizedlist>
<para>Accordingly, the USB Core API exposed to device drivers
covers quite a lot of territory. You'll probably need to consult
the USB 2.0 specification, available online from www.usb.org at
no cost, as well as class or device specifications.
</para>
<para>The only host-side drivers that actually touch hardware
(reading/writing registers, handling IRQs, and so on) are the HCDs.
In theory, all HCDs provide the same functionality through the same
API. In practice, that's becoming more true on the 2.5 kernels,
but there are still differences that crop up especially with
fault handling. Different controllers don't necessarily report
the same aspects of failures, and recovery from faults (including
software-induced ones like unlinking an URB) isn't yet fully
consistent.
Device driver authors should make a point of doing disconnect
testing (while the device is active) with each different host
controller driver, to make sure drivers don't have bugs of
their own as well as to make sure they aren't relying on some
HCD-specific behavior.
(You will need external USB 1.1 and/or
USB 2.0 hubs to perform all those tests.)
</para>
</chapter>
<chapter><title>USB-Standard Types</title>
<para>In <filename><linux/usb_ch9.h></filename> you will find
the USB data types defined in chapter 9 of the USB specification.
These data types are used throughout USB, and in APIs including
this host side API, gadget APIs, and usbfs.
</para>
!Iinclude/linux/usb_ch9.h
</chapter>
<chapter><title>Host-Side Data Types and Macros</title>
<para>The host side API exposes several layers to drivers, some of
which are more necessary than others.
These support lifecycle models for host side drivers
and devices, and support passing buffers through usbcore to
some HCD that performs the I/O for the device driver.
</para>
!Iinclude/linux/usb.h
</chapter>
<chapter><title>USB Core APIs</title>
<para>There are two basic I/O models in the USB API.
The most elemental one is asynchronous: drivers submit requests
in the form of an URB, and the URB's completion callback
handle the next step.
All USB transfer types support that model, although there
are special cases for control URBs (which always have setup
and status stages, but may not have a data stage) and
isochronous URBs (which allow large packets and include
per-packet fault reports).
Built on top of that is synchronous API support, where a
driver calls a routine that allocates one or more URBs,
submits them, and waits until they complete.
There are synchronous wrappers for single-buffer control
and bulk transfers (which are awkward to use in some
driver disconnect scenarios), and for scatterlist based
streaming i/o (bulk or interrupt).
</para>
<para>USB drivers need to provide buffers that can be
used for DMA, although they don't necessarily need to
provide the DMA mapping themselves.
There are APIs to use used when allocating DMA buffers,
which can prevent use of bounce buffers on some systems.
In some cases, drivers may be able to rely on 64bit DMA
to eliminate another kind of bounce buffer.
</para>
!Edrivers/usb/core/urb.c
!Edrivers/usb/core/message.c
!Edrivers/usb/core/file.c
!Edrivers/usb/core/driver.c
!Edrivers/usb/core/usb.c
!Edrivers/usb/core/hub.c
</chapter>
<chapter><title>Host Controller APIs</title>
<para>These APIs are only for use by host controller drivers,
most of which implement standard register interfaces such as
EHCI, OHCI, or UHCI.
UHCI was one of the first interfaces, designed by Intel and
also used by VIA; it doesn't do much in hardware.
OHCI was designed later, to have the hardware do more work
(bigger transfers, tracking protocol state, and so on).
EHCI was designed with USB 2.0; its design has features that
resemble OHCI (hardware does much more work) as well as
UHCI (some parts of ISO support, TD list processing).
</para>
<para>There are host controllers other than the "big three",
although most PCI based controllers (and a few non-PCI based
ones) use one of those interfaces.
Not all host controllers use DMA; some use PIO, and there
is also a simulator.
</para>
<para>The same basic APIs are available to drivers for all
those controllers.
For historical reasons they are in two layers:
<structname>struct usb_bus</structname> is a rather thin
layer that became available in the 2.2 kernels, while
<structname>struct usb_hcd</structname> is a more featureful
layer (available in later 2.4 kernels and in 2.5) that
lets HCDs share common code, to shrink driver size
and significantly reduce hcd-specific behaviors.
</para>
!Edrivers/usb/core/hcd.c
!Edrivers/usb/core/hcd-pci.c
!Idrivers/usb/core/buffer.c
</chapter>
<chapter>
<title>The USB Filesystem (usbfs)</title>
<para>This chapter presents the Linux <emphasis>usbfs</emphasis>.
You may prefer to avoid writing new kernel code for your
USB driver; that's the problem that usbfs set out to solve.
User mode device drivers are usually packaged as applications
or libraries, and may use usbfs through some programming library
that wraps it. Such libraries include
<ulink url="http://libusb.sourceforge.net">libusb</ulink>
for C/C++, and
<ulink url="http://jUSB.sourceforge.net">jUSB</ulink> for Java.
</para>
<note><title>Unfinished</title>
<para>This particular documentation is incomplete,
especially with respect to the asynchronous mode.
As of kernel 2.5.66 the code and this (new) documentation
need to be cross-reviewed.
</para>
</note>
<para>Configure usbfs into Linux kernels by enabling the
<emphasis>USB filesystem</emphasis> option (CONFIG_USB_DEVICEFS),
and you get basic support for user mode USB device drivers.
Until relatively recently it was often (confusingly) called
<emphasis>usbdevfs</emphasis> although it wasn't solving what
<emphasis>devfs</emphasis> was.
Every USB device will appear in usbfs, regardless of whether or
not it has a kernel driver.
</para>
<sect1>
<title>What files are in "usbfs"?</title>
<para>Conventionally mounted at
<filename>/proc/bus/usb</filename>, usbfs
features include:
<itemizedlist>
<listitem><para><filename>/proc/bus/usb/devices</filename>
... a text file
showing each of the USB devices on known to the kernel,
and their configuration descriptors.
You can also poll() this to learn about new devices.
</para></listitem>
<listitem><para><filename>/proc/bus/usb/BBB/DDD</filename>
... magic files
exposing the each device's configuration descriptors, and
supporting a series of ioctls for making device requests,
including I/O to devices. (Purely for access by programs.)
</para></listitem>
</itemizedlist>
</para>
<para> Each bus is given a number (BBB) based on when it was
enumerated; within each bus, each device is given a similar
number (DDD).
Those BBB/DDD paths are not "stable" identifiers;
expect them to change even if you always leave the devices
plugged in to the same hub port.
<emphasis>Don't even think of saving these in application
configuration files.</emphasis>
Stable identifiers are available, for user mode applications
that want to use them. HID and networking devices expose
these stable IDs, so that for example you can be sure that
you told the right UPS to power down its second server.
"usbfs" doesn't (yet) expose those IDs.
</para>
</sect1>
<sect1>
<title>Mounting and Access Control</title>
<para>There are a number of mount options for usbfs, which will
be of most interest to you if you need to override the default
access control policy.
That policy is that only root may read or write device files
(<filename>/proc/bus/BBB/DDD</filename>) although anyone may read
the <filename>devices</filename>
or <filename>drivers</filename> files.
I/O re
|