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usb – USB Host Controller Interface

bind –a #u /dev


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 hubs. Hubs provide ports where USB devices (also hubs) can be attached.

Most PCs have one or more USB controllers called host controllers. Each one has a built–in hub called a 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.

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.

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.

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.

Enumeration of the bus and initial configuration of devices is done by a user level program, usbd. Device drivers are implemented by separate user programs, although some of them may be statically linked into usbd.

The kernel device described in this page is responsible for providing I/O for using the devices through so called 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 usbd.

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 endpoints. Endpoints can be understood as addresses in the bus. There are several types:
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).
Interrupt    Used to send and receive messages to or from a specific device function (e.g., to read events from a mouse).
Bulk       Used to send and receive larger amounts of data through streams (e.g., to write blocks to a disk).
IsochronousUsed to send and receive data in a timely manner (e.g., to write audio samples to a speaker).

All USB devices include at least a control endpoint to perform device configuration. This is called the setup endpoint or endpoint zero. After configuring a device, other endpoints may be created as dictated by the device to perform actual I/O.

Bus enumeration and device configuration is performed by 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.

Each directory /dev/usb/epN.M represents an endpoint, where N is a number identifying a device and M is a number identifying one of its endpoints.

For each device attached to the bus, and configured by usbd, an endpoint zero (a setup endpoint) is provided at /dev/usb/epN.0 for configuring the device. This is always a control endpoint and represents the device itself.

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 /dev/usb/epN.0 creates the endpoint number 3 for that device, a directory /dev/usb/epN.3 will be available to access that endpoint.

All endpoint directories contain two files: data and ctl. The former has mode bit DMEXCL set and can be open by only one process at a time.


The 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.

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 unstall in nusb(2)) to resume I/O. The most common error is crc/timeout indicating problems in communication with the device (eg., a physical detach of the device or a wiring problem).

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 timeout control request).

ctl and status

The 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.

This may result from the read of an endpoint control file:
(the first line is wrapped to make it fit here)
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'

The first line contains status information. The rest is information supplied by usbd as an aid to locate devices. The status information includes:
Device state    One of config, enabled, and detached. An endpoint starts in the config state, and accepts control commands written to its ctl file to configure the endpoint. When configured, the state is enabled and the data file is used as described above (several control requests can still be issued to
its 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 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.
Endpoint type   control, iso, interrupt, or bulk, indicating the type of transfer supported by the endpoint.
Endpoint modeOne of r, w, or rw, depending on the direction of the endpoint (in, out, or inout).
Speed        low (1.5 Mb/s), full (12 Mb/s), or high (480 Mb/s).
Maximum packet size
Used when performing I/O on the data file.
Polling intervalThe 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 not use these units, to make things easier for USB device drivers.
Sample size    Number of bytes per I/O sample (isochronous endpoints only).
Frequency     Number of samples per second (Hertz).
Hub address   Device address of the hub where the device is attached.
Port number   Port number (in the hub) where the device is attached.
Usage        busy while the data file is open and idle otherwise. This is useful to avoid disturbing endpoints already run by a device driver.

The second line contains information describing the device:
Class name    As provided by the device itself.
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.
Vid and Did    Vendor and device identifiers.
Device stringsProvided by the device and identifying the manufacturer and type of device.

For example, to find a mouse not yet in use by a driver, scan the ctl files for enabled, idle, and csp 0x020103. A mouse belongs to class 3 (in the least significant byte), human interface device, subclass 1, boot, protocol 2, mouse (protocol 1 would be the keyboard). USB class, subclass and proto codes can be found at

Control requests
Endpoint control files accept the following requests. In most cases the driver does not issue them, leaving the task to either usbd or the usb driver library documented in nusb(2).
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.
maxpkt n    Set the maximum packet size to n bytes.
pollival nOnly for interrupt and isochronous endpoints. Set the polling interval as a function of the value n given by the endpoint descriptor. The interval value used is the period n in bus time units for low– and full–speed interrupt endpoints. Otherwise, the actual interval is $2 sup n$ and not 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.
samplesz nUse n as the number of bytes per sample.
hz n       Use n as the number of samples per second.
ntds n      Use n as the number of transactions per frame (or µframe), as reported by the descriptor.
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).
info string   Replaces description information in ctl with string. Usbd uses this to add device descriptions.
address    Tell this driver that the device has been given an address, which causes the device to enter the enabled state.
name str     Generates an additional file name, str , for the data file of the endpoint. This file name appears in the root directory of the #u tree. For example, this is used by the audio device driver to make the data file also available as /dev/audio.
debug n     Enable debugging of the endpoint. N is an integer; larger values make diagnostics more verbose. 0 stops debugging diagnostics. 1 causes just problem reports. Bigger values report almost everything.
timeout n   Enable time–outs for the endpoint. Transfers are timed out by the kernel after n ms. This should not be used for control and isochronous endpoints, which are always timed out.

Setup endpoints (those represented by epN.0 directories) also accept the following requests:
new n type mode
Creates a new endpoint with number n of the given type (ctl, bulk, intr, or iso). Mode may be r, w, or rw, which creates, respectively, an input, output, or input/output endpoint.
speed {low|full|high}
Set the endpoint speed to full, low, or high, respectively.
hub   Tell this driver that the endpoint corresponds to a hub device.

Setup endpoints for hub devices also accept his request:
newdev {low|full|high} port
Create a new setup endpoint to represent a new device. The first argument is the device speed. Port is the port number where the device is attached (the hub is implied by the endpoint where the control request is issued).

The file /dev/usb/ctl provides all the information provided by the various ctl files when read. It accepts several requests that refer to the entire driver and not to particular endpoints:
debug nSets the global debug flag to n.
dump     Dumps data structures for inspection.

#u/usb   root of the USB interface


nusb(2), nusb(4), plan9.ini(8)

USB controllers limit the speed of all their ports to that of the slowest device connected to any one of them.

Isochronous input streams are not implemented for OHCI.

Some EHCI controllers drop completion interrupts and so must be polled, which hurts throughput.