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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
Extension Unit (XU) support
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
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
device's entities.
Unless the control size is already known it is necessary to first make a
UVC_GET_LEN requests in order to be able to allocate a sufficiently large buffer
and set the buffer size to the correct value. Similarly, to find out whether
UVC_GET_CUR or UVC_SET_CUR are valid requests for a given control, a
UVC_GET_INFO request should be made. The bits 0 (GET supported) and 1 (SET
supported) of the resulting byte indicate which requests are valid.
With the addition of the UVCIOC_CTRL_QUERY ioctl the UVCIOC_CTRL_GET and
UVCIOC_CTRL_SET ioctls have become obsolete since their functionality is a
subset of the former ioctl. For the time being they are still supported but
application developers are encouraged to use UVCIOC_CTRL_QUERY instead.
For details on the UVCIOC_CTRL_QUERY ioctl please refer to the section titled
"IOCTL reference" below.
The API doesn't currently provide a fine-grained access control facility. The
UVCIOC_CTRL_ADD and UVCIOC_CTRL_MAP ioctls require super user permissions.
Suggestions on how to improve this are welcome.
In order to debug problems related to XU controls or controls in general it is
recommended to enable the UVC_TRACE_CONTROL bit in the module parameter 'trace'.
This causes extra output to be written into the system log.
IOCTL reference
UVCIOC_CTRL_MAP - Map a UVC control to a V4L2 control
Argument: struct uvc_xu_control_mapping
This ioctl creates a mapping between a UVC control or part of a UVC
control and a V4L2 control. Once mappings are defined, userspace
applications can access vendor-defined UVC control through the V4L2
control API.
To create a mapping, applications fill the uvc_xu_control_mapping
structure with information about an existing UVC control defined with
UVCIOC_CTRL_ADD and a new V4L2 control.
A UVC control can be mapped to several V4L2 controls. For instance,
a UVC pan/tilt control could be mapped to separate pan and tilt V4L2
controls. The UVC control is divided into non overlapping fields using
the 'size' and 'offset' fields and are then independently mapped to
V4L2 control.
For signed integer V4L2 controls the data_type field should be set to
UVC_CTRL_DATA_TYPE_SIGNED. Other values are currently ignored.
**Return value**:
On success 0 is returned. On error -1 is returned and errno is set
Not enough memory to perform the operation.
Insufficient privileges (super user privileges are required).
No such UVC control.
The requested offset and size would overflow the UVC control.
Mapping already exists.
**Data types**:
.. code-block:: none
* struct uvc_xu_control_mapping
__u32 id V4L2 control identifier
__u8 name[32] V4L2 control name
__u8 entity[16] UVC extension unit GUID
__u8 selector UVC control selector
__u8 size V4L2 control size (in bits)
__u8 offset V4L2 control offset (in bits)
enum v4l2_ctrl_type
v4l2_type V4L2 control type
enum uvc_control_data_type
data_type UVC control data type
struct uvc_menu_info
*menu_info Array of menu entries (for menu controls only)
__u32 menu_count Number of menu entries (for menu controls only)
* struct uvc_menu_info
__u32 value Menu entry value used by the device
__u8 name[32] Menu entry name
* enum uvc_control_data_type
UVC_CTRL_DATA_TYPE_RAW Raw control (byte array)
UVCIOC_CTRL_QUERY - Query a UVC XU control
Argument: struct uvc_xu_control_query
This ioctl queries a UVC XU control identified by its extension unit ID
and control selector.
There are a number of different queries available that closely
correspond to the low-level control requests described in the UVC
specification. These requests are:
Obtain the current value of the control.
Obtain the minimum value of the control.
Obtain the maximum value of the control.
Obtain the default value of the control.
Query the resolution of the control, i.e. the step size of the
allowed control values.
Query the size of the control in bytes.
Query the control information bitmap, which indicates whether
get/set requests are supported.
Update the value of the control.
Applications must set the 'size' field to the correct length for the
control. Exceptions are the UVC_GET_LEN and UVC_GET_INFO queries, for
which the size must be set to 2 and 1, respectively. The 'data' field
must point to a valid writable buffer big enough to hold the indicated
number of data bytes.
Data is copied directly from the device without any driver-side
processing. Applications are responsible for data buffer formatting,
including little-endian/big-endian conversion. This is particularly
important for the result of the UVC_GET_LEN requests, which is always
returned as a little-endian 16-bit integer by the device.
**Return value**:
On success 0 is returned. On error -1 is returned and errno is set
The device does not support the given control or the specified
extension unit could not be found.
The specified buffer size is incorrect (too big or too small).
An invalid request code was passed.
The given request is not supported by the given control.
The data pointer references an inaccessible memory area.
**Data types**:
.. code-block:: none
* struct uvc_xu_control_query
__u8 unit Extension unit ID
__u8 selector Control selector
__u8 query Request code to send to the device
__u16 size Control data size (in bytes)
__u8 *data Control value