Documentation/userspace-api/media/drivers/uvcvideo.rst

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Documentation/userspace-api/media/drivers/uvcvideo.rst
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.. SPDX-License-Identifier: GPL-2.0

The Linux USB Video Class (UVC) driver
======================================

This file documents some driver-specific aspects of the UVC driver, such as
driver-specific ioctls and implementation notes.

Questions and remarks can be sent to the Linux UVC development mailing list at
linux-media@vger.kernel.org.


Extension Unit (XU) support
---------------------------

Introduction
~~~~~~~~~~~~

The UVC specification allows for vendor-specific extensions through extension
units (XUs). The Linux UVC driver supports extension unit controls (XU controls)
through two separate mechanisms:

  - through mappings of XU controls to V4L2 controls
  - through a driver-specific ioctl interface

The first one allows generic V4L2 applications to use XU controls by mapping
certain XU controls onto V4L2 controls, which then show up during ordinary
control enumeration.

The second mechanism requires uvcvideo-specific knowledge for the application to
access XU controls but exposes the entire UVC XU concept to user space for
maximum flexibility.

Both mechanisms complement each other and are described in more detail below.


Control mappings
~~~~~~~~~~~~~~~~

The UVC driver provides an API for user space applications to define so-called
control mappings at runtime. These allow for individual XU controls or byte
ranges thereof to be mapped to new V4L2 controls. Such controls appear and
function exactly like normal V4L2 controls (i.e. the stock controls, such as
brightness, contrast, etc.). However, reading or writing of such a V4L2 controls
triggers a read or write of the associated XU control.

The ioctl used to create these control mappings is called UVCIOC_CTRL_MAP.
Previous driver versions (before 0.2.0) required another ioctl to be used
beforehand (UVCIOC_CTRL_ADD) to pass XU control information to the UVC driver.
This is no longer necessary as newer uvcvideo versions query the information
directly from the device.

For details on the UVCIOC_CTRL_MAP ioctl please refer to the section titled
"IOCTL reference" below.


3. Driver specific XU control interface

For applications that need to access XU controls directly, e.g. for testing
purposes, firmware upload, or accessing binary controls, a second mechanism to
access XU controls is provided in the form of a driver-specific ioctl, namely
UVCIOC_CTRL_QUERY.

A call to this ioctl allows applications to send queries to the UVC driver that
directly map to the low-level UVC control requests.

In order to make such a request the UVC unit ID of the control's extension unit
and the control selector need to be known. This information either needs to be
hardcoded in the application or queried using other ways such as by parsing the
UVC descriptor or, if available, using the media controller API to enumerate a

Annotation

Implementation Notes