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-uvc-devel@lists.berlios.de.
  
  
  Extension Unit (XU) support
  ---------------------------
  
  1. 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.
  
  
  2. 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
  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.
  
  
  4. Security
  
  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.
  
  
  5. Debugging
  
  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.
  
  
  6. IOCTL reference
  
  ---- UVCIOC_CTRL_MAP - Map a UVC control to a V4L2 control ----
  
  Argument: struct uvc_xu_control_mapping
  
  Description:
  	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
  	appropriately.
  
  	ENOMEM
  		Not enough memory to perform the operation.
  	EPERM
  		Insufficient privileges (super user privileges are required).
  	EINVAL
  		No such UVC control.
  	EOVERFLOW
  		The requested offset and size would overflow the UVC control.
  	EEXIST
  		Mapping already exists.
  
  Data types:
  	* 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)
  	UVC_CTRL_DATA_TYPE_SIGNED	Signed integer
  	UVC_CTRL_DATA_TYPE_UNSIGNED	Unsigned integer
  	UVC_CTRL_DATA_TYPE_BOOLEAN	Boolean
  	UVC_CTRL_DATA_TYPE_ENUM		Enumeration
  	UVC_CTRL_DATA_TYPE_BITMASK	Bitmask
  
  
  ---- UVCIOC_CTRL_QUERY - Query a UVC XU control ----
  
  Argument: struct uvc_xu_control_query
  
  Description:
  	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:
  
  	UVC_GET_CUR
  		Obtain the current value of the control.
  	UVC_GET_MIN
  		Obtain the minimum value of the control.
  	UVC_GET_MAX
  		Obtain the maximum value of the control.
  	UVC_GET_DEF
  		Obtain the default value of the control.
  	UVC_GET_RES
  		Query the resolution of the control, i.e. the step size of the
  		allowed control values.
  	UVC_GET_LEN
  		Query the size of the control in bytes.
  	UVC_GET_INFO
  		Query the control information bitmap, which indicates whether
  		get/set requests are supported.
  	UVC_SET_CUR
  		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
  	appropriately.
  
  	ENOENT
  		The device does not support the given control or the specified
  		extension unit could not be found.
  	ENOBUFS
  		The specified buffer size is incorrect (too big or too small).
  	EINVAL
  		An invalid request code was passed.
  	EBADRQC
  		The given request is not supported by the given control.
  	EFAULT
  		The data pointer references an inaccessible memory area.
  
  Data types:
  	* 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