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

原文地址http://blog.163.com/eng_2005@126/blog/static/17209075820128121054735/