usb.tmpl

Linux Kernel 2.6.9 for OMAP1710

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN"[]>
<book id="Linux-USB-API">
 <bookinfo>
  <title>The Linux-USB Host Side API</title>
  
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<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 at the
    root (the system's master), hubs as interior nodes, and
    peripheral devices 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 part for
    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 the downstream type are the squarish type
    (or they are built in to the peripheral).
    Software doesn't need to deal with distributed autoconfiguration
    since the pre-designated master node manages all that.
    At the electrical level, bus protocol overhead is reduced by
    eliminating arbitration and moving scheduling into 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, including high speed
    transfers and transaction translating hubs (used for USB 1.1
    and 1.0 backward compatibility).
    </para>

    <para>USB support was added to Linux early in the 2.2 kernel series
    shortly before the 2.3 development forked off.  Updates
    from 2.3 were regularly folded back into 2.2 releases, bringing
    new features such as <filename>/sbin/hotplug</filename> support,
    more drivers, and more robustness.
    The 2.5 kernel series continued such improvements, and also
    worked on USB 2.0 support,
    higher performance,
    better consistency between host controller drivers,
    API simplification (to make bugs less likely),
    and providing internal "kerneldoc" documentation.
    </para>

    <para>Linux can run inside USB devices as well as on
    the hosts that control the devices.
    Because the Linux 2.x USB support evolved to support mass market
    platforms such as Apple Macintosh or PC-compatible systems,
    it didn't address design concerns for those types of USB systems.
    So it can't be used inside mass-market PDAs, or other peripherals.
    USB device drivers running inside those Linux peripherals
    don't do the same things as the ones running inside hosts,
    and so they've been given a different name:
    they're called <emphasis>gadget drivers</emphasis>.
    This document does not present gadget drivers.
    </para>

    </chapter>

<chapter id="host">
    <title>USB Host-Side API Model</title>

    <para>Within the kernel,
    host-side drivers for USB devices talk to the "usbcore" APIs.
    There are two types of public "usbcore" APIs, targetted at two different
    layers of USB driver.  Those are
    <emphasis>general purpose</emphasis> drivers, exposed through
    driver frameworks such as block, character, or network devices;
    and drivers that are <emphasis>part of the core</emphasis>,
    which are involved in managing a USB bus.
    Such core drivers include the <emphasis>hub</emphasis> driver,
    which manages trees of USB devices, and several different kinds
    of <emphasis>host controller driver (HCD)</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 transfer
	(control, bulk, interrupt, and isochronous).  Two transfer
	types use bandwidth as it's available (control and bulk),
	while the other two types of transfer (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 that might be used.
	</para></listitem>

	<listitem><para>Configurations have one or more "interface", 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 messaging.
	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>&lt;linux/usb_ch9.h&gt;</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/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
!Edrivers/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; but only devices with kernel drivers
	show up in devfs.
	</para>