ref: 39f18c9d88f52a22373790dec5721fa3521d3f00
dir: /sys/man/3/usb/
.TH USB 3
.EQ
delim $$
.EN
.SH NAME
usb \- USB Host Controller Interface
.SH SYNOPSIS
.nf
.B bind -a #u /dev
.PP
.nf
.B /dev/usb
.B /dev/usb/ctl
.BI /dev/usb/ep N . M
.BI /dev/usb/ep N . M /data
.BI /dev/usb/ep N . M /ctl
\&...
.fi
.SH DESCRIPTION
The Universal Serial Bus is a complex yet popular bus
for connecting all kind of devices to a computer.
It is a four-wire tree-shaped bus that provides both communication and (limited)
power to devices.
Branching points in the tree are provided by devices called
.IR hubs .
Hubs provide ports where USB devices (also hubs) can be attached.
.PP
Most PCs have one or more USB controllers called
.I host
controllers.
Each one has a built-in hub called a
.I "root hub"
providing several ports.
In some cases, more hubs are built-in
and attached to a root hub port.
The topology of the network is a tree with at most
127 nodes, counting both internal and leaf nodes.
.PP
Host controllers come in four flavours:
UHCI and OHCI for USB 1 (up to 12 Mb/s),
EHCI for USB 2 (up to 480 Mb/s)
and
XHCI for USB 3 (up to 5 Gb/s).
We currently support all but XHCI, which is still quite new.
.PP
The USB bus is fully controlled by the host; all devices are polled.
Hubs are passive in the sense that they do not poll the devices attached
to them.
The host polls those devices and the hubs merely route the messages.
.PP
Devices may be added to or removed from the bus at any time.
When a device is attached, the host queries it to determine its type and speed.
The querying process is standardized.
The first level of querying is the same for all devices,
the next is somewhat specialized
for particular classes of devices (such as mice, keyboards, or audio devices).
Specialization continues as subclasses and subsubclasses are explored.
.PP
Enumeration of the bus and initial configuration of devices is done
by a user level program,
.IR usbd .
Device drivers are implemented by separate user programs, although
some of them may be statically linked into
.IR usbd .
.PP
The kernel device described in this page is responsible for providing
I/O for using the devices through so called
.IR endpoints .
Access to the host controller is hidden from user programs, which see
just a set of endpoints.
After system initialization, some endpoints
are created by the device to permit I/O to root hubs.
All other devices must be configured by
.IR usbd .
.SS Devices and Endpoints
A device includes one or more functions (e.g., audio output,
volume control buttons, mouse input, etc.)
Communication with device functions is performed
by some combination of
issuing control requests to, sending data to, and receiving data from
device
.IR endpoints .
Endpoints can be understood as addresses in the bus.
There are several types:
.TF "\fIIsochronous
.TP
.I Control
Their main use is to configure devices.
Writing a message with a specific format
(specified in the USB specification)
issues a request to the device.
If the request implies a reply,
a read can be made next to retrieve the requested data (if the write succeeded).
.TP
.I Interrupt
Used to send and receive messages to or from a specific device function
(e.g., to read events from a mouse).
.TP
.I Bulk
Used to send and receive larger amounts of data through streams
(e.g., to write blocks to a disk).
.TP
.I Isochronous
Used to send and receive data in a timely manner
(e.g., to write audio samples to a speaker).
.PD
.PP
All USB devices include at least
a control endpoint to perform device configuration.
This is called the
.I setup
endpoint or
.IR "endpoint zero" .
After configuring a device, other endpoints may be created
as dictated by the device to perform actual I/O.
.SS Operation
Bus enumeration and device configuration is performed by
.IR usbd
and not by this driver.
The driver provides an interface
to access existing endpoints (initially those for the built-in root hubs),
to create and configure other ones, and to perform I/O through them.
.PP
Each directory
.BI /dev/usb/ep N . M
represents an endpoint, where
.I N
is a number identifying a device and
.I M
is a number identifying one of its endpoints.
.PP
For each device attached to the bus, and configured by
.IR usbd ,
an endpoint zero (a
.I setup
endpoint)
is provided at
.BI /dev/usb/ep N .0
for configuring the device.
This is always a control endpoint and represents the device itself.
.PP
The device driver may use the setup endpoint
to issue control requests and perhaps to create more endpoints for the device.
Each new endpoint created has its own directory as said above.
For example, if the driver for the device
.BI /dev/usb/ep N . 0
creates the endpoint number 3 for that device, a directory
.BI /dev/usb/ep N .3
will be available to access that endpoint.
.PP
All endpoint directories contain two files:
.B data
and
.BR ctl .
The former has mode bit
.B DMEXCL
set and can be open by only one process at a time.
.SS data
.PP
The
.B data
file is used to perform actual I/O.
In general, reading from it retrieves
data from the endpoint and writing into it sends data to the endpoint.
For control endpoints, writing to this file issues a control request
(which may include data); if the request retrieves data from the device,
a following read on the file will provide such data.
.PP
USB errors reported by the endpoint upon I/O failures
are passed to the user process through the error string.
I/O stalls not resulting from an error, usually
an indication from the device, are reported by indicating that the
number of bytes transferred has been zero.
In most cases, the correct course of action after noticing the stall
is for the device driver to issue a `clear halt' request (see
.I unstall
in
.IR nusb (2))
to resume I/O.
The most common error is
.L crc/timeout
indicating problems in communication with the device (eg., a physical
detach of the device or a wiring problem).
.PP
For control and isochronous transfers, there is an implicit
timeout performed by the kernel and it is not necessary for applications
to place their own timers.
For other transfer types, the kernel will not time out any operation
by default
(but see the
.L timeout
control request).
.SS "ctl and status"
.PP
The
.B ctl
file can be read to learn about the endpoint.
It contains information that can be used
to locate a particular device (or endpoint).
It also accepts writes with textual control requests described later.
.PP
This may result from the read of an endpoint control file:
.IP
.EX
.I "(the first line is wrapped to make it fit here)"
.ft L
enabled control rw speed full maxpkt 64 pollival 0
samplesz 0 hz 0 hub 1 port 3 busy
storage csp 0x500608 vid 0x951 did 0x1613 Kingston 'DT 101 II'
.ft
.EE
.LP
The first line contains status information.
The rest is information supplied by
.IR usbd
as an aid to locate devices.
The status information includes:
.TF "\fREndpoint mode
.PD
.TP
Device state
One of
.BR config ,
.BR enabled ,
and
.BR detached .
An endpoint starts in the
.B config
state, and accepts control commands written to its
.B ctl
file to configure the endpoint.
When configured, the
state is
.B enabled
and the
.B data
file is used as described above (several control requests can still
be issued to its
.B ctl
file, but most will not be accepted from now on).
Upon severe errors, perhaps a physical
detachment from the bus, the endpoint enters the
.B detached
state and no further I/O is accepted on it.
Files for an endpoint (including its directory)
vanish when the device is detached and its files are no longer open.
Root hubs may not be detached.
.TP
Endpoint type
.BR control ,
.BR iso ,
.BR interrupt ,
or
.BR bulk ,
indicating the type of transfer supported by the endpoint.
.TP
Endpoint mode
One of
.BR r ,
.BR w ,
or
.BR rw ,
depending on the direction of the endpoint (in, out, or inout).
.TP
Speed
.BR low
(1.5 Mb/s),
.BR full
(12 Mb/s),
or
.BR high
(480 Mb/s).
.TP
Maximum packet size
Used when performing I/O on the data file.
.TP
Polling interval
The polling period expressed as a number of µframes
(for high-speed endpoints) or frames (for low- and full-speed endpoints).
Note that a µframe takes 125 µs while a frame takes 1 ms.
This is only of relevance for interrupt and isochronous endpoints.
This value determines how often I/O happens.
Note that the control request adjusting the polling interval does
.I not
use these units, to make things easier for USB device drivers.
.TP
Sample size
Number of bytes per I/O sample (isochronous endpoints only).
.TP
Frequency
Number of samples per second (Hertz).
.TP
Hub address
Device address of the hub where the device is attached.
.TP
Port number
Port number (in the hub) where the device is attached.
.TP
Usage
.L busy
while the data file is open and
.L idle
otherwise.
This is useful to avoid disturbing endpoints already run
by a device driver.
.LP
The second line contains information describing the device:
.TF "\fRDevice strings
.PD
.TP
Class name
As provided by the device itself.
.TP
CSP
Class, Subclass, and Protocol for the device.
If the device contains different functions and has more CSPs,
all of them will be listed.
The first one is that of the device itself.
For example,
a mouse and keyboard combo may identify itself as a keyboard but
then include two CSPs, one for the keyboard and another one for the mouse.
.TP
Vid and Did
Vendor and device identifiers.
.TP
Device strings
Provided by the device and identifying the manufacturer and type of device.
.LP
For example, to find a mouse not yet in use by a driver, scan the
.B ctl
files for
.BR enabled ,
.BR idle ,
and
.BR "csp 0x020103" .
A mouse belongs to class 3 (in the least significant byte),
.IR "human interface device" ,
subclass 1,
.IR boot ,
protocol 2,
.I mouse
(protocol 1 would be the keyboard).
USB class, subclass and proto codes can be found at
.BR http://www.usb.org .
.SS Control requests
Endpoint control files accept the following requests.
In most cases
the driver does not issue them, leaving the task to either
.IR usbd
or the usb driver library documented in
.IR nusb (2).
.TF "\fLsamplehz\fI n
.TP
.B detach
Prevent further I/O on the device (delete the endpoint)
and remove its file interface as soon as no process is using their files.
.TP
.BI maxpkt " n"
Set the maximum packet size to
.I n
bytes.
.TP
.BI pollival " n"
Only for interrupt and isochronous endpoints.
Set the polling interval as a function of the value
.I n
given by the endpoint descriptor.
The interval value used is the period
.I n
in bus time units for low- and full-speed interrupt endpoints.
Otherwise, the actual interval is
$2 sup n$
and not
.IR n .
Bus time units are 1 ms for low- and full-speed endpoints and 125 µs for
high-speed endpoints.
In most cases, the device driver may ignore
all this and issue the control request supplying the
polling interval value as found
in the endpoint descriptor.
The kernel adjusts the value according
to the endpoint configuration and converts it into the number of
frames or µframes between two consecutive polls.
.TP
.BI samplesz " n"
Use
.I n
as the number of bytes per sample.
.TP
.BI hz " n"
Use
.I n
as the number of samples per second.
.TP
.BI ntds " n"
Use
.I n
as the number of transactions per frame (or µframe), as reported
by the descriptor.
.TP
.B clrhalt
Clear the halt condition for an endpoint.
Used to recover from a stall caused by a device to signal its driver
(usually due to an unknown request or a failure to complete one).
.TP
.BI info " string"
Replaces description information in
.B ctl
with
.IR string .
.IR Usbd (4)
uses this to add device descriptions.
.TP
.B address
Tell this driver that the device has been given an address,
which causes the device to enter the
.I enabled
state.
.TP
.BI name " str"
Generates an additional file name,
.I str ,
for the
.B data
file of the endpoint.
This file name appears in the root directory of the
.L #u
tree.
For example, this is used by the audio device
driver to make the
.B data
file also available as
.BR /dev/audio .
.TP
.BI debug " n"
Enable debugging of the endpoint.
.I N
is an integer;
larger values make diagnostics more verbose.
.L 0
stops debugging diagnostics.
.L 1
causes just problem reports.
Bigger values report almost everything.
.TP
.BI timeout " n"
Enable time-outs for the endpoint.
Transfers are timed out by the kernel after
.I n
ms.
This should not be used for control and isochronous endpoints,
which are always timed out.
.PD
.LP
Setup endpoints
(those represented by
.BI ep N .0
directories)
also accept the following requests:
.TP
.BI new " n type mode"
Creates a new endpoint with number
.I n
of the given
.IR type
(\c
.BR ctl ,
.BR bulk ,
.BR intr ,
or
.BR iso ).
.I Mode
may be
.BR r ,
.BR w ,
or
.BR rw ,
which creates, respectively, an input, output, or input/output endpoint.
.TP
.B "speed {low|full|high}
Set the endpoint speed to full, low, or high, respectively.
.TP
.B hub
Tell this driver that the endpoint corresponds to a hub device.
.PD
.PP
Setup endpoints for hub devices also accept his request:
.TP
.B "newdev {low|full|high} \fIport\fP
Create a new setup endpoint to represent a new device.
The first argument is the device speed.
.I Port
is the port number where the device is attached
(the hub is implied by the endpoint where the control request is issued).
.PD
.PP
The file
.B /dev/usb/ctl
provides all the information provided by the various
.B ctl
files when read.
It accepts several requests that refer to
the entire driver and not to particular endpoints:
.TF "\fLdebug \fIn"
.TP
.B "debug \fIn\fP
Sets the global debug flag to
.IR n .
.TP
.B dump
Dumps data structures for inspection.
.SH FILES
.TF #u/usb
.TP
.B #u/usb
root of the USB interface
.SH SOURCE
.B /sys/src/9/port/usb.h
.br
.B /sys/src/9/port/devusb.c
.br
.B /sys/src/9/*/usb*.c
.SH "SEE ALSO"
.IR nusb (2),
.IR nusb (4),
.IR plan9.ini (8)
.SH BUGS
USB controllers limit the speed of all their ports
to that of the slowest device connected to any one of them.
.PP
Isochronous input streams are not implemented for OHCI.
.PP
Some EHCI controllers drop completion interrupts and so must
be polled, which hurts throughput.