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usb - Universal Serial Bus


Introduction To Usb
Usb Controller Interface
Usb Event Interface
See Also


.Cd "device usb"

.In dev/usb/usb.h
.In dev/usb/usbhid.h


.Fx provides machine-independent bus support and drivers for USB devices.

The usb driver has three layers: the controller, the bus, and the device layer. The controller attaches to a physical bus (like pci(4)). The USB bus attaches to the controller, and the root hub attaches to the controller. Any devices attached to the bus will attach to the root hub or another hub attached to the USB bus.

The uhub device will always be present as it is needed for the root hub.


The USB is a 12 Mb/s serial bus (1.5 Mb/s for low speed devices). Each USB has a host controller that is the master of the bus; all other devices on the bus only speak when spoken to.

There can be up to 127 devices (apart from the host controller) on a bus, each with its own address. The addresses are assigned dynamically by the host when each device is attached to the bus.

Within each device there can be up to 16 endpoints. Each endpoint is individually addressed and the addresses are static. Each of these endpoints will communicate in one of four different modes: control, isochronous, bulk, or interrupt. A device always has at least one endpoint. This endpoint has address 0 and is a control endpoint and is used to give commands to and extract basic data, such as descriptors, from the device. Each endpoint, except the control endpoint, is unidirectional.

The endpoints in a device are grouped into interfaces. An interface is a logical unit within a device; e.g. a compound device with both a keyboard and a trackball would present one interface for each. An interface can sometimes be set into different modes, called alternate settings, which affects how it operates. Different alternate settings can have different endpoints within it.

A device may operate in different configurations. Depending on the configuration, the device may present different sets of endpoints and interfaces.

The bus enumeration of the USB bus proceeds in several steps:

  1. Any device specific driver can attach to the device.
  2. If none is found, any device class specific driver can attach.
  3. If none is found, all configurations are iterated over. For each configuration, all the interfaces are iterated over, and interface drivers can attach. If any interface driver attached in a certain configuration, the iteration over configurations is stopped.
  4. If still no drivers have been found, the generic USB driver can attach.


Use the following to get access to the USB specific structures and defines.

The /dev/usb N can be opened and a few operations can be performed on it. The poll(2) system call will say that I/O is possible on the controller device when a USB device has been connected or disconnected to the bus.

The following ioctl(2) commands are supported on the controller device:

This command will cause a complete bus discovery to be initiated. If any devices attached or detached from the bus they will be processed during this command. This is the only way that new devices are found on the bus.
USB_DEVICEINFO Vt "struct usb_device_info"
This command can be used to retrieve some information about a device on the bus. The udi_addr field should be filled before the call and the other fields will be filled by information about the device on that address. Should no such device exist, an error is reported.
struct usb_device_info {
u_int8_t udi_bus;
u_int8_t udi_addr; /* device address */
usb_event_cookie_t udi_cookie;
char udi_product[USB_MAX_STRING_LEN];
char udi_vendor[USB_MAX_STRING_LEN];
char udi_release[8];
u_int16_t udi_productNo;
u_int16_t udi_vendorNo;
u_int16_t udi_releaseNo;
u_int8_t udi_class;
u_int8_t udi_subclass;
u_int8_t udi_protocol;
u_int8_t udi_config;
u_int8_t udi_speed;
#define USB_SPEED_LOW 1
#define USB_SPEED_FULL 2
#define USB_SPEED_HIGH 3
int udi_power;/* power consumption in mA, 0 if selfpowered */
int udi_nports;
u_int8_t udi_ports[16];/* hub only: addresses of devices on ports */
#define USB_PORT_ENABLED 0xff
#define USB_PORT_POWERED 0xfd
#define USB_PORT_DISABLED 0xfc

udi_bus and udi_addr contain the topological information for the device. udi_devnames contains the device names of the connected drivers. For example, the third USB Zip drive connected will be umass2. The udi_product, udi_vendor and udi_release fields contain self-explanatory descriptions of the device. udi_productNo, udi_vendorNo, udi_releaseNo, udi_class, udi_subclass and udi_protocol contain the corresponding values from the device descriptors. The udi_config field shows the current configuration of the device.

udi_speed indicates whether the device is at low speed (USB_SPEED_LOW), full speed (USB_SPEED_FULL) or high speed (USB_SPEED_HIGH). The udi_power field shows the power consumption in milli-amps drawn at 5 volts, or zero if the device is self powered.

If the device is a hub, the udi_nports field is non-zero, and the udi_ports field contains the addresses of the connected devices. If no device is connected to a port, one of the USB_PORT_* values indicates its status.

USB_DEVICESTATS Vt "struct usb_device_stats"
This command retrieves statistics about the controller.
struct usb_device_stats {
u_long uds_requests[4];

The udi_requests field is indexed by the transfer kind, i.e. UE_*, and indicates how many transfers of each kind that has been completed by the controller.

USB_REQUEST Vt "struct usb_ctl_request"
This command can be used to execute arbitrary requests on the control pipe. This is DANGEROUS and should be used with great care since it can destroy the bus integrity.

The include file
.In dev/usb/usb.h contains definitions for the types used by the various ioctl(2) calls. The naming convention of the fields for the various USB descriptors exactly follows the naming in the USB specification. Byte sized fields can be accessed directly, but word (16 bit) sized fields must be access by the UGETW field and USETW field value macros to handle byte order and alignment properly.

The include file
.In dev/usb/usbhid.h similarly contains the definitions for Human Interface Devices (HID).


All USB events are reported via the /dev/usb device. This devices can be opened for reading and each read(2) will yield an event record (if something has happened). The poll(2) system call can be used to determine if an event record is available for reading.

The event record has the following definition:
struct usb_event {
int ue_type;
struct timespec ue_time;
union {
struct {
int ue_bus;
} ue_ctrlr;
struct usb_device_info ue_device;
struct {
usb_event_cookie_t ue_cookie;
char ue_devname[16];
} ue_driver;
} u;

The ue_type field identifies the type of event that is described. The possible events are attach/detach of a host controller, a device, or a device driver. The union contains information pertinent to the different types of events. Macros, USB_EVENT_IS_ATTACH "ue_type" and USB_EVENT_IS_DETACH "ue_type" can be used to determine if an event was an "attach" or a "detach" request.

The ue_bus contains the number of the USB bus for host controller events.

The ue_device record contains information about the device in a device event event.

The ue_cookie is an opaque value that uniquely determines which device a device driver has been attached to (i.e., it equals the cookie value in the device that the driver attached to).

The ue_devname contains the name of the device (driver) as seen in, e.g., kernel messages.

Note that there is a separation between device and device driver events. A device event is generated when a physical USB device is attached or detached. A single USB device may have zero, one, or many device drivers associated with it.


aue(4), cue(4), ehci(4), kue(4), ohci(4), pci(4), ucom(4), ugen(4), uhci(4), uhid(4), ukbd(4), ulpt(4), umass(4), ums(4), uplcom(4), urio(4), uscanner(4), uvscom(4), usbd(8), usbdevs(8)



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