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+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