usb.rst: move documentation from proc_usb_info.txt to USB ReST book

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.

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::

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

-  ``/dev/bus/usb//BBB/DDD`` ... magic files exposing the each device's

-  ``/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

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. "usbfs" doesn't (yet) expose those IDs.

second server. Pleast note that it doesn't (yet) expose those IDs.

/dev/bus/usb//BBB/DDD

---------------------

/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

- *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

  descriptors for interfaces, altsettings, endpoints, and maybe additional

  class descriptors.

*Perform USB operations* using *ioctl()* requests to make endpoint I/O

- *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

  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

  *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,

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

    as identified in the endpoint descriptor), with USB_DIR_IN added

    if the device's endpoint sends data to the host.

	**Warning**

    .. Warning::

	*Avoid using this request. It should probably be removed.* Using

	it typically means the device and driver will lose toggle

		void          *data;

	};

    The "ep" value identifies a bulk endpoint number (1 to 15, as

    The ``ep`` value identifies a bulk endpoint number (1 to 15, as

    identified in an endpoint descriptor), masked with USB_DIR_IN when

    referring to an endpoint which sends data to the host from the

    device. The length of the data buffer is identified by "len"; Recent

    device. The length of the data buffer is identified by ``len``; Recent

    kernels support requests up to about 128KBytes. *FIXME say how read

    length is returned, and how short reads are handled.*.

    which sends data to the host from the device.

    Use this on bulk or interrupt endpoints which have stalled,

    returning *-EPIPE* status to a data transfer request. Do not issue

    returning ``-EPIPE`` status to a data transfer request. Do not issue

    the control request directly, since that could invalidate the host's

    record of the data toggle.

debug purposes. It would be more appropriate to use programs such as

udev or HAL to initialize a device or start a user-mode helper program,

for instance.

In this file, each device's output has multiple lines of ASCII output.

I made it ASCII instead of binary on purpose, so that someone

can obtain some useful data from it without the use of an

auxiliary program.  However, with an auxiliary program, the numbers

in the first 4 columns of each ``T:`` line (topology info:

Lev, Prnt, Port, Cnt) can be used to build a USB topology diagram.

Each line is tagged with a one-character ID for that line::

	T = Topology (etc.)

	B = Bandwidth (applies only to USB host controllers, which are

	virtualized as root hubs)

	D = Device descriptor info.

	P = Product ID info. (from Device descriptor, but they won't fit

	together on one line)

	S = String descriptors.

	C = Configuration descriptor info. (* = active configuration)

	I = Interface descriptor info.

	E = Endpoint descriptor info.

/sys/kernel/debug/usb/devices output format

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Legend::

  d = decimal number (may have leading spaces or 0's)

  x = hexadecimal number (may have leading spaces or 0's)

  s = string

Topology info

^^^^^^^^^^^^^

::

	T:  Bus=dd Lev=dd Prnt=dd Port=dd Cnt=dd Dev#=ddd Spd=dddd MxCh=dd

	|   |      |      |       |       |      |        |        |__MaxChildren

	|   |      |      |       |       |      |        |__Device Speed in Mbps

	|   |      |      |       |       |      |__DeviceNumber

	|   |      |      |       |       |__Count of devices at this level

	|   |      |      |       |__Connector/Port on Parent for this device

	|   |      |      |__Parent DeviceNumber

	|   |      |__Level in topology for this bus

	|   |__Bus number

	|__Topology info tag

Speed may be:

	======= ======================================================

	1.5	Mbit/s for low speed USB

	12	Mbit/s for full speed USB

	480	Mbit/s for high speed USB (added for USB 2.0);

		also used for Wireless USB, which has no fixed speed

	5000	Mbit/s for SuperSpeed USB (added for USB 3.0)

	======= ======================================================

For reasons lost in the mists of time, the Port number is always

too low by 1.  For example, a device plugged into port 4 will

show up with ``Port=03``.

Bandwidth info

^^^^^^^^^^^^^^

::

	B:  Alloc=ddd/ddd us (xx%), #Int=ddd, #Iso=ddd

	|   |                       |         |__Number of isochronous requests

	|   |                       |__Number of interrupt requests

	|   |__Total Bandwidth allocated to this bus

	|__Bandwidth info tag

Bandwidth allocation is an approximation of how much of one frame

(millisecond) is in use.  It reflects only periodic transfers, which

are the only transfers that reserve bandwidth.  Control and bulk

transfers use all other bandwidth, including reserved bandwidth that

is not used for transfers (such as for short packets).

The percentage is how much of the "reserved" bandwidth is scheduled by

those transfers.  For a low or full speed bus (loosely, "USB 1.1"),

90% of the bus bandwidth is reserved.  For a high speed bus (loosely,

"USB 2.0") 80% is reserved.

Device descriptor info & Product ID info

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

	D:  Ver=x.xx Cls=xx(s) Sub=xx Prot=xx MxPS=dd #Cfgs=dd

	P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx

where::

	D:  Ver=x.xx Cls=xx(sssss) Sub=xx Prot=xx MxPS=dd #Cfgs=dd

	|   |        |             |      |       |       |__NumberConfigurations

	|   |        |             |      |       |__MaxPacketSize of Default Endpoint

	|   |        |             |      |__DeviceProtocol

	|   |        |             |__DeviceSubClass

	|   |        |__DeviceClass

	|   |__Device USB version

	|__Device info tag #1

where::

	P:  Vendor=xxxx ProdID=xxxx Rev=xx.xx

	|   |           |           |__Product revision number

	|   |           |__Product ID code

	|   |__Vendor ID code

	|__Device info tag #2

String descriptor info

^^^^^^^^^^^^^^^^^^^^^^

::

	S:  Manufacturer=ssss

	|   |__Manufacturer of this device as read from the device.

	|      For USB host controller drivers (virtual root hubs) this may

	|      be omitted, or (for newer drivers) will identify the kernel

	|      version and the driver which provides this hub emulation.

	|__String info tag

	S:  Product=ssss

	|   |__Product description of this device as read from the device.

	|      For older USB host controller drivers (virtual root hubs) this

	|      indicates the driver; for newer ones, it's a product (and vendor)

	|      description that often comes from the kernel's PCI ID database.

	|__String info tag

	S:  SerialNumber=ssss

	|   |__Serial Number of this device as read from the device.

	|      For USB host controller drivers (virtual root hubs) this is

	|      some unique ID, normally a bus ID (address or slot name) that

	|      can't be shared with any other device.

	|__String info tag

Configuration descriptor info

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

	C:* #Ifs=dd Cfg#=dd Atr=xx MPwr=dddmA

	| | |       |       |      |__MaxPower in mA

	| | |       |       |__Attributes

	| | |       |__ConfiguratioNumber

	| | |__NumberOfInterfaces

	| |__ "*" indicates the active configuration (others are " ")

	|__Config info tag

USB devices may have multiple configurations, each of which act

rather differently.  For example, a bus-powered configuration

might be much less capable than one that is self-powered.  Only

one device configuration can be active at a time; most devices

have only one configuration.

Each configuration consists of one or more interfaces.  Each

interface serves a distinct "function", which is typically bound

to a different USB device driver.  One common example is a USB

speaker with an audio interface for playback, and a HID interface

for use with software volume control.

Interface descriptor info (can be multiple per Config)

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

	I:* If#=dd Alt=dd #EPs=dd Cls=xx(sssss) Sub=xx Prot=xx Driver=ssss

	| | |      |      |       |             |      |       |__Driver name

	| | |      |      |       |             |      |          or "(none)"

	| | |      |      |       |             |      |__InterfaceProtocol

	| | |      |      |       |             |__InterfaceSubClass

	| | |      |      |       |__InterfaceClass

	| | |      |      |__NumberOfEndpoints

	| | |      |__AlternateSettingNumber

	| | |__InterfaceNumber

	| |__ "*" indicates the active altsetting (others are " ")

	|__Interface info tag

A given interface may have one or more "alternate" settings.

For example, default settings may not use more than a small

amount of periodic bandwidth.  To use significant fractions

of bus bandwidth, drivers must select a non-default altsetting.

Only one setting for an interface may be active at a time, and

only one driver may bind to an interface at a time.  Most devices

have only one alternate setting per interface.

Endpoint descriptor info (can be multiple per Interface)

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

	E:  Ad=xx(s) Atr=xx(ssss) MxPS=dddd Ivl=dddss

	|   |        |            |         |__Interval (max) between transfers

	|   |        |            |__EndpointMaxPacketSize

	|   |        |__Attributes(EndpointType)

	|   |__EndpointAddress(I=In,O=Out)

	|__Endpoint info tag

The interval is nonzero for all periodic (interrupt or isochronous)

endpoints.  For high speed endpoints the transfer interval may be

measured in microseconds rather than milliseconds.

For high speed periodic endpoints, the ``EndpointMaxPacketSize`` reflects

the per-microframe data transfer size.  For "high bandwidth"

endpoints, that can reflect two or three packets (for up to

3KBytes every 125 usec) per endpoint.

With the Linux-USB stack, periodic bandwidth reservations use the

transfer intervals and sizes provided by URBs, which can be less

than those found in endpoint descriptor.

Usage examples

~~~~~~~~~~~~~~

If a user or script is interested only in Topology info, for

example, use something like ``grep ^T: /sys/kernel/debug/usb/devices``

for only the Topology lines.  A command like

``grep -i ^[tdp]: /sys/kernel/debug/usb/devices`` can be used to list

only the lines that begin with the characters in square brackets,

where the valid characters are TDPCIE.  With a slightly more able

script, it can display any selected lines (for example, only T, D,

and P lines) and change their output format.  (The ``procusb``

Perl script is the beginning of this idea.  It will list only

selected lines [selected from TBDPSCIE] or "All" lines from

``/sys/kernel/debug/usb/devices``.)

The Topology lines can be used to generate a graphic/pictorial

of the USB devices on a system's root hub.  (See more below

on how to do this.)

The Interface lines can be used to determine what driver is

being used for each device, and which altsetting it activated.

The Configuration lines could be used to list maximum power

(in milliamps) that a system's USB devices are using.

For example, ``grep ^C: /sys/kernel/debug/usb/devices``.

Here's an example, from a system which has a UHCI root hub,

an external hub connected to the root hub, and a mouse and

a serial converter connected to the external hub.

::

	T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2

	B:  Alloc= 28/900 us ( 3%), #Int=  2, #Iso=  0

	D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1

	P:  Vendor=0000 ProdID=0000 Rev= 0.00

	S:  Product=USB UHCI Root Hub

	S:  SerialNumber=dce0

	C:* #Ifs= 1 Cfg#= 1 Atr=40 MxPwr=  0mA

	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub

	E:  Ad=81(I) Atr=03(Int.) MxPS=   8 Ivl=255ms

	T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4

	D:  Ver= 1.00 Cls=09(hub  ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1

	P:  Vendor=0451 ProdID=1446 Rev= 1.00

	C:* #Ifs= 1 Cfg#= 1 Atr=e0 MxPwr=100mA

	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub

	E:  Ad=81(I) Atr=03(Int.) MxPS=   1 Ivl=255ms

	T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0

	D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1

	P:  Vendor=04b4 ProdID=0001 Rev= 0.00

	C:* #Ifs= 1 Cfg#= 1 Atr=80 MxPwr=100mA

	I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse

	E:  Ad=81(I) Atr=03(Int.) MxPS=   3 Ivl= 10ms

	T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0

	D:  Ver= 1.00 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1

	P:  Vendor=0565 ProdID=0001 Rev= 1.08

	S:  Manufacturer=Peracom Networks, Inc.

	S:  Product=Peracom USB to Serial Converter

	C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=100mA

	I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial

	E:  Ad=81(I) Atr=02(Bulk) MxPS=  64 Ivl= 16ms

	E:  Ad=01(O) Atr=02(Bulk) MxPS=  16 Ivl= 16ms

	E:  Ad=82(I) Atr=03(Int.) MxPS=   8 Ivl=  8ms

Selecting only the ``T:`` and ``I:`` lines from this (for example, by using

``procusb ti``), we have

::

	T:  Bus=00 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#=  1 Spd=12   MxCh= 2

	T:  Bus=00 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12   MxCh= 4

	I:  If#= 0 Alt= 0 #EPs= 1 Cls=09(hub  ) Sub=00 Prot=00 Driver=hub

	T:  Bus=00 Lev=02 Prnt=02 Port=00 Cnt=01 Dev#=  3 Spd=1.5  MxCh= 0

	I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=01 Prot=02 Driver=mouse

	T:  Bus=00 Lev=02 Prnt=02 Port=02 Cnt=02 Dev#=  4 Spd=12   MxCh= 0

	I:  If#= 0 Alt= 0 #EPs= 3 Cls=00(>ifc ) Sub=00 Prot=00 Driver=serial

Physically this looks like (or could be converted to)::

                      +------------------+

                      |  PC/root_hub (12)|   Dev# = 1

                      +------------------+   (nn) is Mbps.

    Level 0           |  CN.0   |  CN.1  |   [CN = connector/port #]

                      +------------------+

                          /

                         /

            +-----------------------+

  Level 1   | Dev#2: 4-port hub (12)|

            +-----------------------+

            |CN.0 |CN.1 |CN.2 |CN.3 |

            +-----------------------+

                \           \____________________

                 \_____                          \

                       \                          \

               +--------------------+      +--------------------+

  Level 2      | Dev# 3: mouse (1.5)|      | Dev# 4: serial (12)|

               +--------------------+      +--------------------+

Or, in a more tree-like structure (ports [Connectors] without

connections could be omitted)::

	PC:  Dev# 1, root hub, 2 ports, 12 Mbps

	|_ CN.0:  Dev# 2, hub, 4 ports, 12 Mbps

	     |_ CN.0:  Dev #3, mouse, 1.5 Mbps

	     |_ CN.1:

	     |_ CN.2:  Dev #4, serial, 12 Mbps

	     |_ CN.3:

	|_ CN.1: