Documentation/hid/hidraw.rst

Source file repositories/reference/linux-study-clean/Documentation/hid/hidraw.rst

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Documentation/hid/hidraw.rst
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================================================================
HIDRAW - Raw Access to USB and Bluetooth Human Interface Devices
================================================================

The hidraw driver provides a raw interface to USB and Bluetooth Human
Interface Devices (HIDs).  It differs from hiddev in that reports sent and
received are not parsed by the HID parser, but are sent to and received from
the device unmodified.

Hidraw should be used if the userspace application knows exactly how to
communicate with the hardware device, and is able to construct the HID
reports manually.  This is often the case when making userspace drivers for
custom HID devices.

Hidraw is also useful for communicating with non-conformant HID devices
which send and receive data in a way that is inconsistent with their report
descriptors.  Because hiddev parses reports which are sent and received
through it, checking them against the device's report descriptor, such
communication with these non-conformant devices is impossible using hiddev.
Hidraw is the only alternative, short of writing a custom kernel driver, for
these non-conformant devices.

A benefit of hidraw is that its use by userspace applications is independent
of the underlying hardware type.  Currently, hidraw is implemented for USB
and Bluetooth.  In the future, as new hardware bus types are developed which
use the HID specification, hidraw will be expanded to add support for these
new bus types.

Hidraw uses a dynamic major number, meaning that udev should be relied on to
create hidraw device nodes.  Udev will typically create the device nodes
directly under /dev (eg: /dev/hidraw0).  As this location is distribution-
and udev rule-dependent, applications should use libudev to locate hidraw
devices attached to the system.  There is a tutorial on libudev with a
working example at::

	http://www.signal11.us/oss/udev/
	https://web.archive.org/web/2019*/www.signal11.us

The HIDRAW API
---------------

read()
-------
read() will read a queued report received from the HID device. On USB
devices, the reports read using read() are the reports sent from the device
on the INTERRUPT IN endpoint.  By default, read() will block until there is
a report available to be read.  read() can be made non-blocking, by passing
the O_NONBLOCK flag to open(), or by setting the O_NONBLOCK flag using
fcntl().

On a device which uses numbered reports, the first byte of the returned data
will be the report number; the report data follows, beginning in the second
byte.  For devices which do not use numbered reports, the report data
will begin at the first byte.

write()
-------
The write() function will write a report to the device. For USB devices, if
the device has an INTERRUPT OUT endpoint, the report will be sent on that
endpoint. If it does not, the report will be sent over the control endpoint,
using a SET_REPORT transfer.

The first byte of the buffer passed to write() should be set to the report
number.  If the device does not use numbered reports, the first byte should
be set to 0. The report data itself should begin at the second byte.

ioctl()
-------
Hidraw supports the following ioctls:

Annotation

Implementation Notes