diff options
Diffstat (limited to 'Documentation/video4linux/uvcvideo.txt')
| -rw-r--r-- | Documentation/video4linux/uvcvideo.txt | 239 |
1 files changed, 239 insertions, 0 deletions
diff --git a/Documentation/video4linux/uvcvideo.txt b/Documentation/video4linux/uvcvideo.txt new file mode 100644 index 000000000..35ce19cdd --- /dev/null +++ b/Documentation/video4linux/uvcvideo.txt @@ -0,0 +1,239 @@ +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 |