path: root/Documentation/driver-api/usb/usb.rst

.. _usb-hostside-api:

===========================
The Linux-USB Host Side API
===========================

Introduction to USB on Linux
============================

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
a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
USB 2.0 (480 MBit/s) busses just in case.

That master/slave asymmetry was designed-in for a number of reasons, one
being ease of use. It is not physically possible to mistake upstream and
downstream or it does not matter with a type C plug (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.

Kernel developers added USB support to Linux early in the 2.2 kernel
series and have been developing it further since then. Besides support
for each new generation of USB, various host controllers gained support,
new drivers for peripherals have been added and advanced features for latency
measurement and improved power management introduced.

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: *gadget drivers*. This document does not
cover gadget drivers.

USB Host-Side API Model
=======================

Host-side drivers for USB devices talk to the "usbcore" APIs. There are
two. One is intended for *general-purpose* drivers (exposed through
driver frameworks), and the other is for drivers that are *part of the
core*. Such core drivers include the *hub* driver (which manages trees
of USB devices) and several different kinds of *host controller
drivers*, which control individual busses.

The device model seen by USB drivers is relatively complex.

-  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.

-  The device description model includes one or more "configurations"
   per device, only one of which is active at a time. Devices are supposed
   to be capable of operating at lower than their top
   speeds and may provide a BOS descriptor showing the lowest speed they
   remain fully operational at.

-  From USB 3.0 on configurations have one or more "functions", which
   provide a common functionality and are grouped together for purposes
   of power management.

-  Configurations or functions 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.

   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. *Most USB devices are simple,
   with only one function, one configuration, one interface, and one alternate
   setting.*

-  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.

-  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.

-  The Linux USB API supports synchronous calls for control and bulk
   messages. It also supports asynchronous calls for all kinds of data
   transfer, using request structures called "URBs" (USB Request
   Blocks).

Accordingly, the USB Core API exposed to device drivers covers quite a
lot of territory. You'll probably need to consult the USB 3.0
specification, available online from www.usb.org at no cost, as well as
class or device specifications.

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, but there are still differences
that crop up especially with fault handling on the less common controllers.
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.

.. _usb_chapter9:

USB-Standard Types
==================

In ``<linux/usb/ch9.h>`` 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, usb character
devices and debugfs interfaces.

.. kernel-doc:: include/linux/usb/ch9.h
   :internal:

.. _usb_header:

Host-Side Data Types and Macros
===============================

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.

.. kernel-doc:: include/linux/usb.h
   :internal:

USB Core APIs
=============

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 handles 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).

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.

.. kernel-doc:: drivers/usb/core/urb.c
   :export:

.. kernel-doc:: drivers/usb/core/message.c
   :export:

.. kernel-doc:: drivers/usb/core/file.c
   :export:

.. kernel-doc:: drivers/usb/core/driver.c
   :export:

.. kernel-doc:: drivers/usb/core/usb.c
   :export:

.. kernel-doc:: drivers/usb/core/hub.c
   :export:

Host Controller APIs
====================

These APIs are only for use by host controller drivers, most of which
implement standard register interfaces such as XHCI, 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). XHCI was designed with USB 3.0. It
continues to shift support for functionality into hardware.

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 and a virtual host controller to pipe USB over the network.

The same basic APIs are available to drivers for all those controllers.
For historical reasons they are in two layers: :c:type:`struct
usb_bus <usb_bus>` is a rather thin layer that became available
in the 2.2 kernels, while :c:type:`struct usb_hcd <usb_hcd>`
is a more featureful layer
that lets HCDs share common code, to shrink driver size and
significantly reduce hcd-specific behaviors.

.. kernel-doc:: drivers/usb/core/hcd.c
   :export:

.. kernel-doc:: drivers/usb/core/hcd-pci.c
   :export:

.. kernel-doc:: drivers/usb/core/buffer.c
   :internal:

The USB character device nodes
==============================

This chapter presents the Linux character device nodes. You may prefer
to avoid writing new kernel code for your USB driver. User mode device
drivers are usually packaged as applications or libraries, and may use
character devices through some programming library that wraps it.
Such libraries include:

 - `libusb <http://libusb.sourceforge.net>`__ for C/C++, and
 - `jUSB <http://jUSB.sourceforge.net>`__ for Java.

Some old information about it can be seen at the "USB Device Filesystem"
section of the USB Guide. The latest copy of the USB Guide can be found
at http://www.linux-usb.org/

.. note::

  - They were used to be implemented via *usbfs*, but this is not part of
    the sysfs debug interface.

   - 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.

What files are in "devtmpfs"?
-----------------------------

Conventionally mounted at ``/dev/bus/usb/``, usbfs features include:

-  ``/dev/bus/usb/BBB/DDD`` ... 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.)

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. *Don't even
think of saving these in application configuration files.* 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. Pleast note that it doesn't (yet) expose those IDs.

/dev/bus/usb/BBB/DDD
--------------------

Use these files in one of these basic ways:

- *They can be read,* producing first the device descriptor (18 bytes) and
  then the descriptors for the current configuration. See the USB 2.0 spec
  for details about those binary data formats. You'll need to convert most
  multibyte values from little endian format to your native host byte
  order, although a few of the fields in the device descriptor (both of
  the BCD-encoded fields, and the vendor and product IDs) will be
  byteswapped for you. Note that configuration descriptors include
  descriptors for interfaces, altsettings, endpoints, and maybe additional
  class descriptors.

- *Perform USB operations* using *ioctl()* requests to make endpoint I/O
  requests (synchronously or asynchronously) or manage the device. These
  requests need the ``CAP_SYS_RAWIO`` capability, as well as filesystem
  access permissions. Only one ioctl request can be made on one of these
  device files at a time. This means that if you are synchronously reading
  an endpoint from one thread, you won't be able to write to a different
  endpoint from another thread until the read completes. This works for
  *half duplex* protocols, but otherwise you'd use asynchronous i/o
  requests.

Each connected USB device has one file.  The ``BBB`` indicates the bus
number.  The ``DDD`` indicates the device address on that bus.  Both
of these numbers are assigned sequentially, and can be reused, so
you can't rely on them for stable access to devices.  For example,
it's relatively common for devices to re-enumerate while they are
still connected (perhaps someone jostled their power supply, hub,
or USB cable), so a device might be ``002/027`` when you first connect
it and ``002/048`` sometime later.

These files can be read as binary data.  The binary data consists
of first the device descriptor, then the descriptors for each
configuration of the device.  Multi-byte fields in the device descriptor
are converted to host endianness by the kernel.  The configuration
descriptors are in bus endian format! The configuration descriptor
are wTotalLength bytes apart. If a device returns less configuration
descriptor data than indicated by wTotalLength there will be a hole in
the file for the missing bytes.  This information is also shown
in text form by the ``/sys/kernel/debug/usb/devices`` file, described later.

These files may also be used to write user-level drivers for the USB
devices.  You would open the ``/dev/bus/usb/BBB/DDD`` file read/write,
read its descriptors to make sure it's the device you expect, and then
bind to an interface (or perhaps several) using an ioctl call.  You
would issue more ioctls to the device to communicate to it using
control, bulk, or other kinds of USB transfers.  The IOCTLs are
listed in the ``<linux/usbdevice_fs.h>`` file, and at this writing the
source code (``linux/drivers/usb/core/devio.c``) is the primary reference
for how to access devices through those files.

Note that since by default these ``BBB/DDD`` files are writable only by
root, only root can write such user mode drivers.  You can selectively
grant read/write permissions to other users by using ``chmod``.  Also,
usbfs mount options such as ``devmode=0666`` may be helpful.


Life Cycle of User Mode Drivers
-------------------------------

Such a driver first needs to find a device file for a device it knows
how to handle. Maybe it was told about it because a ``/sbin/hotplug``
event handling agent chose that driver to handle the new device. Or
maybe it's an application that scans all the ``/dev/bus/usb`` device files,
and ignores most devices. In either case, it should :c:func:`read()`
all the descriptors from the device file, and check them against what it
knows how to handle. It might just reject everything except a particular
vendor and product ID, or need a more complex policy.

Never assume there will only be one such device on the system at a time!
If your code can't handle more than one device at a time, at least
detect when there's more than one, and have your users choose which
device to use.

Once your user mode driver knows what device to use, it interacts with
it in either of two styles. The simple style is to make only control
requests; some devices don't need more complex interactions than those.
(An example might be software using vendor-specific control requests for
some initialization or configuration tasks, with a kernel driver for the
rest.)

More likely, you need a more complex style driver: one using non-control
endpoints, reading or writing data and claiming exclusive use of an
interface. *Bulk* transfers are easiest to use, but only their sibling
*interrupt* transfers work with low speed devices. Both interrupt and
*isochronous* transfers offer service guarantees because their bandwidth
is reserved. Such "periodic" transfers are awkward to use through usbfs,
unless you're using the asynchronous calls. However, interrupt transfers
can also be used in a synchronous "one shot" style.

Your user-mode driver should never need to worry about cleaning up
request state when the device is disconnected, although it should close
its open file descriptors as soon as it starts seeing the ENODEV errors.

The ioctl() Requests
--------------------

To use these ioctls, you need to include the following headers in your
userspace program::

    #include <linux/usb.h>
    #include <linux/usbdevice_fs.h>
    #include <asm/byteorder.h>

The standard USB device model requests, from "Chapter 9" of the USB 2.0
specification, are automatically included from the ``<linux/usb/ch9.h>``
header.

Unless noted otherwise, the ioctl requests described here will update
the modification time on the usbfs file to which they are applied
(unless they fail). A return of zero indicates success; otherwise, a
standard USB error code is returned (These are documented in
:ref:`usb-error-codes`).

Each of these files multiplexes access to several I/O streams, one per
endpoint. Each device has one control endpoint (endpoint zero) which
supports a limited RPC style RPC access. Devices are configured by
hub_wq (in the kernel) setting a device-wide *configuration* that
affects things like power consumption and basic functionality. The
endpoints are part of USB *interfaces*, which may have *altsettings*
affecting things like which endpoints are available. Many devices only
have a single configu