[media] uvcvideo: Add driver documentation

This adds detailed documentation about the driver's extension unit control
features. Part of it has been adopted from the dynctrl.txt that used to be
available in the driver's previous Subversion repository.

The documentation has been rewritten to match the current implementation, in
particular to capture the new XU control support including UVCIOC_CTRL_QUERY.
Signed-off-by: NMartin Rubli <martin_rubli@logitech.com>
Signed-off-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/video4linux/uvcvideo.txt

+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 independantly 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