.. SPDX-License-Identifier: GPL-2.0
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The Linux USB Video Class (UVC) driver
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======================================
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This file documents some driver-specific aspects of the UVC driver, such as
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driver-specific ioctls and implementation notes.
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Questions and remarks can be sent to the Linux UVC development mailing list at
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linux-uvc-devel@lists.berlios.de.
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Extension Unit (XU) support
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---------------------------
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Introduction
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~~~~~~~~~~~~
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The UVC specification allows for vendor-specific extensions through extension
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units (XUs). The Linux UVC driver supports extension unit controls (XU controls)
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through two separate mechanisms:
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- through mappings of XU controls to V4L2 controls
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- through a driver-specific ioctl interface
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The first one allows generic V4L2 applications to use XU controls by mapping
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certain XU controls onto V4L2 controls, which then show up during ordinary
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control enumeration.
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The second mechanism requires uvcvideo-specific knowledge for the application to
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access XU controls but exposes the entire UVC XU concept to user space for
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maximum flexibility.
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Both mechanisms complement each other and are described in more detail below.
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Control mappings
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~~~~~~~~~~~~~~~~
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The UVC driver provides an API for user space applications to define so-called
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control mappings at runtime. These allow for individual XU controls or byte
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ranges thereof to be mapped to new V4L2 controls. Such controls appear and
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function exactly like normal V4L2 controls (i.e. the stock controls, such as
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brightness, contrast, etc.). However, reading or writing of such a V4L2 controls
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triggers a read or write of the associated XU control.
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The ioctl used to create these control mappings is called UVCIOC_CTRL_MAP.
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Previous driver versions (before 0.2.0) required another ioctl to be used
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beforehand (UVCIOC_CTRL_ADD) to pass XU control information to the UVC driver.
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This is no longer necessary as newer uvcvideo versions query the information
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directly from the device.
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For details on the UVCIOC_CTRL_MAP ioctl please refer to the section titled
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"IOCTL reference" below.
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3. Driver specific XU control interface
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For applications that need to access XU controls directly, e.g. for testing
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purposes, firmware upload, or accessing binary controls, a second mechanism to
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access XU controls is provided in the form of a driver-specific ioctl, namely
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UVCIOC_CTRL_QUERY.
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A call to this ioctl allows applications to send queries to the UVC driver that
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directly map to the low-level UVC control requests.
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In order to make such a request the UVC unit ID of the control's extension unit
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and the control selector need to be known. This information either needs to be
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hardcoded in the application or queried using other ways such as by parsing the
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UVC descriptor or, if available, using the media controller API to enumerate a
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device's entities.
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Unless the control size is already known it is necessary to first make a
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UVC_GET_LEN requests in order to be able to allocate a sufficiently large buffer
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and set the buffer size to the correct value. Similarly, to find out whether
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UVC_GET_CUR or UVC_SET_CUR are valid requests for a given control, a
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UVC_GET_INFO request should be made. The bits 0 (GET supported) and 1 (SET
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supported) of the resulting byte indicate which requests are valid.
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With the addition of the UVCIOC_CTRL_QUERY ioctl the UVCIOC_CTRL_GET and
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UVCIOC_CTRL_SET ioctls have become obsolete since their functionality is a
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subset of the former ioctl. For the time being they are still supported but
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application developers are encouraged to use UVCIOC_CTRL_QUERY instead.
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For details on the UVCIOC_CTRL_QUERY ioctl please refer to the section titled
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"IOCTL reference" below.
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Security
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~~~~~~~~
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The API doesn't currently provide a fine-grained access control facility. The
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UVCIOC_CTRL_ADD and UVCIOC_CTRL_MAP ioctls require super user permissions.
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Suggestions on how to improve this are welcome.
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Debugging
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~~~~~~~~~
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In order to debug problems related to XU controls or controls in general it is
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recommended to enable the UVC_TRACE_CONTROL bit in the module parameter 'trace'.
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This causes extra output to be written into the system log.
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IOCTL reference
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~~~~~~~~~~~~~~~
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UVCIOC_CTRL_MAP - Map a UVC control to a V4L2 control
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Argument: struct uvc_xu_control_mapping
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**Description**:
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This ioctl creates a mapping between a UVC control or part of a UVC
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control and a V4L2 control. Once mappings are defined, userspace
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applications can access vendor-defined UVC control through the V4L2
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control API.
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To create a mapping, applications fill the uvc_xu_control_mapping
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structure with information about an existing UVC control defined with
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UVCIOC_CTRL_ADD and a new V4L2 control.
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A UVC control can be mapped to several V4L2 controls. For instance,
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a UVC pan/tilt control could be mapped to separate pan and tilt V4L2
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controls. The UVC control is divided into non overlapping fields using
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the 'size' and 'offset' fields and are then independently mapped to
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V4L2 control.
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For signed integer V4L2 controls the data_type field should be set to
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UVC_CTRL_DATA_TYPE_SIGNED. Other values are currently ignored.
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**Return value**:
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On success 0 is returned. On error -1 is returned and errno is set
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appropriately.
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ENOMEM
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Not enough memory to perform the operation.
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EPERM
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Insufficient privileges (super user privileges are required).
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EINVAL
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No such UVC control.
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EOVERFLOW
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The requested offset and size would overflow the UVC control.
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EEXIST
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Mapping already exists.
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**Data types**:
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.. code-block:: none
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* struct uvc_xu_control_mapping
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__u32 id V4L2 control identifier
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__u8 name[32] V4L2 control name
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__u8 entity[16] UVC extension unit GUID
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__u8 selector UVC control selector
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__u8 size V4L2 control size (in bits)
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__u8 offset V4L2 control offset (in bits)
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enum v4l2_ctrl_type
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v4l2_type V4L2 control type
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enum uvc_control_data_type
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data_type UVC control data type
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struct uvc_menu_info
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*menu_info Array of menu entries (for menu controls only)
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__u32 menu_count Number of menu entries (for menu controls only)
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* struct uvc_menu_info
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__u32 value Menu entry value used by the device
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__u8 name[32] Menu entry name
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* enum uvc_control_data_type
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UVC_CTRL_DATA_TYPE_RAW Raw control (byte array)
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UVC_CTRL_DATA_TYPE_SIGNED Signed integer
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UVC_CTRL_DATA_TYPE_UNSIGNED Unsigned integer
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UVC_CTRL_DATA_TYPE_BOOLEAN Boolean
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UVC_CTRL_DATA_TYPE_ENUM Enumeration
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UVC_CTRL_DATA_TYPE_BITMASK Bitmask
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UVCIOC_CTRL_QUERY - Query a UVC XU control
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Argument: struct uvc_xu_control_query
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**Description**:
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This ioctl queries a UVC XU control identified by its extension unit ID
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and control selector.
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There are a number of different queries available that closely
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correspond to the low-level control requests described in the UVC
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specification. These requests are:
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UVC_GET_CUR
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Obtain the current value of the control.
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UVC_GET_MIN
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Obtain the minimum value of the control.
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UVC_GET_MAX
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Obtain the maximum value of the control.
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UVC_GET_DEF
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Obtain the default value of the control.
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UVC_GET_RES
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Query the resolution of the control, i.e. the step size of the
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allowed control values.
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UVC_GET_LEN
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Query the size of the control in bytes.
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UVC_GET_INFO
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Query the control information bitmap, which indicates whether
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get/set requests are supported.
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UVC_SET_CUR
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Update the value of the control.
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Applications must set the 'size' field to the correct length for the
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control. Exceptions are the UVC_GET_LEN and UVC_GET_INFO queries, for
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which the size must be set to 2 and 1, respectively. The 'data' field
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must point to a valid writable buffer big enough to hold the indicated
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number of data bytes.
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Data is copied directly from the device without any driver-side
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processing. Applications are responsible for data buffer formatting,
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including little-endian/big-endian conversion. This is particularly
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important for the result of the UVC_GET_LEN requests, which is always
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returned as a little-endian 16-bit integer by the device.
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**Return value**:
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On success 0 is returned. On error -1 is returned and errno is set
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appropriately.
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ENOENT
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The device does not support the given control or the specified
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extension unit could not be found.
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ENOBUFS
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The specified buffer size is incorrect (too big or too small).
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EINVAL
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An invalid request code was passed.
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EBADRQC
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The given request is not supported by the given control.
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EFAULT
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The data pointer references an inaccessible memory area.
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**Data types**:
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.. code-block:: none
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* struct uvc_xu_control_query
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__u8 unit Extension unit ID
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__u8 selector Control selector
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__u8 query Request code to send to the device
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__u16 size Control data size (in bytes)
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__u8 *data Control value
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