Documentation/video4linux/uvcvideo.txt

   1 Linux USB Video Class (UVC) driver
   2 ==================================
   3
   4 This file documents some driver-specific aspects of the UVC driver, such as
   5 driver-specific ioctls and implementation notes.
   6
   7 Questions and remarks can be sent to the Linux UVC development mailing list at
   8 linux-uvc-devel@lists.berlios.de.
   9
  10
  11 Extension Unit (XU) support
  12 ---------------------------
  13
  14 1. Introduction
  15
  16 The UVC specification allows for vendor-specific extensions through extension
  17 units (XUs). The Linux UVC driver supports extension unit controls (XU controls)
  18 through two separate mechanisms:
  19
  20   - through mappings of XU controls to V4L2 controls
  21   - through a driver-specific ioctl interface
  22
  23 The first one allows generic V4L2 applications to use XU controls by mapping
  24 certain XU controls onto V4L2 controls, which then show up during ordinary
  25 control enumeration.
  26
  27 The second mechanism requires uvcvideo-specific knowledge for the application to
  28 access XU controls but exposes the entire UVC XU concept to user space for
  29 maximum flexibility.
  30
  31 Both mechanisms complement each other and are described in more detail below.
  32
  33
  34 2. Control mappings
  35
  36 The UVC driver provides an API for user space applications to define so-called
  37 control mappings at runtime. These allow for individual XU controls or byte
  38 ranges thereof to be mapped to new V4L2 controls. Such controls appear and
  39 function exactly like normal V4L2 controls (i.e. the stock controls, such as
  40 brightness, contrast, etc.). However, reading or writing of such a V4L2 controls
  41 triggers a read or write of the associated XU control.
  42
  43 The ioctl used to create these control mappings is called UVCIOC_CTRL_MAP.
  44 Previous driver versions (before 0.2.0) required another ioctl to be used
  45 beforehand (UVCIOC_CTRL_ADD) to pass XU control information to the UVC driver.
  46 This is no longer necessary as newer uvcvideo versions query the information
  47 directly from the device.
  48
  49 For details on the UVCIOC_CTRL_MAP ioctl please refer to the section titled
  50 "IOCTL reference" below.
  51
  52
  53 3. Driver specific XU control interface
  54
  55 For applications that need to access XU controls directly, e.g. for testing
  56 purposes, firmware upload, or accessing binary controls, a second mechanism to
  57 access XU controls is provided in the form of a driver-specific ioctl, namely
  58 UVCIOC_CTRL_QUERY.
  59
  60 A call to this ioctl allows applications to send queries to the UVC driver that
  61 directly map to the low-level UVC control requests.
  62
  63 In order to make such a request the UVC unit ID of the control's extension unit
  64 and the control selector need to be known. This information either needs to be
  65 hardcoded in the application or queried using other ways such as by parsing the
  66 UVC descriptor or, if available, using the media controller API to enumerate a
  67 device's entities.
  68
  69 Unless the control size is already known it is necessary to first make a
  70 UVC_GET_LEN requests in order to be able to allocate a sufficiently large buffer
  71 and set the buffer size to the correct value. Similarly, to find out whether
  72 UVC_GET_CUR or UVC_SET_CUR are valid requests for a given control, a
  73 UVC_GET_INFO request should be made. The bits 0 (GET supported) and 1 (SET
  74 supported) of the resulting byte indicate which requests are valid.
  75
  76 With the addition of the UVCIOC_CTRL_QUERY ioctl the UVCIOC_CTRL_GET and
  77 UVCIOC_CTRL_SET ioctls have become obsolete since their functionality is a
  78 subset of the former ioctl. For the time being they are still supported but
  79 application developers are encouraged to use UVCIOC_CTRL_QUERY instead.
  80
  81 For details on the UVCIOC_CTRL_QUERY ioctl please refer to the section titled
  82 "IOCTL reference" below.
  83
  84
  85 4. Security
  86
  87 The API doesn't currently provide a fine-grained access control facility. The
  88 UVCIOC_CTRL_ADD and UVCIOC_CTRL_MAP ioctls require super user permissions.
  89
  90 Suggestions on how to improve this are welcome.
  91
  92
  93 5. Debugging
  94
  95 In order to debug problems related to XU controls or controls in general it is
  96 recommended to enable the UVC_TRACE_CONTROL bit in the module parameter 'trace'.
  97 This causes extra output to be written into the system log.
  98
  99
 100 6. IOCTL reference
 101
 102 ---- UVCIOC_CTRL_MAP - Map a UVC control to a V4L2 control ----
 103
 104 Argument: struct uvc_xu_control_mapping
 105
 106 Description:
 107         This ioctl creates a mapping between a UVC control or part of a UVC
 108         control and a V4L2 control. Once mappings are defined, userspace
 109         applications can access vendor-defined UVC control through the V4L2
 110         control API.
 111
 112         To create a mapping, applications fill the uvc_xu_control_mapping
 113         structure with information about an existing UVC control defined with
 114         UVCIOC_CTRL_ADD and a new V4L2 control.
 115
 116         A UVC control can be mapped to several V4L2 controls. For instance,
 117         a UVC pan/tilt control could be mapped to separate pan and tilt V4L2
 118         controls. The UVC control is divided into non overlapping fields using
 119         the 'size' and 'offset' fields and are then independently mapped to
 120         V4L2 control.
 121
 122         For signed integer V4L2 controls the data_type field should be set to
 123         UVC_CTRL_DATA_TYPE_SIGNED. Other values are currently ignored.
 124
 125 Return value:
 126         On success 0 is returned. On error -1 is returned and errno is set
 127         appropriately.
 128
 129         ENOMEM
 130                 Not enough memory to perform the operation.
 131         EPERM
 132                 Insufficient privileges (super user privileges are required).
 133         EINVAL
 134                 No such UVC control.
 135         EOVERFLOW
 136                 The requested offset and size would overflow the UVC control.
 137         EEXIST
 138                 Mapping already exists.
 139
 140 Data types:
 141         * struct uvc_xu_control_mapping
 142
 143         __u32   id              V4L2 control identifier
 144         __u8    name[32]        V4L2 control name
 145         __u8    entity[16]      UVC extension unit GUID
 146         __u8    selector        UVC control selector
 147         __u8    size            V4L2 control size (in bits)
 148         __u8    offset          V4L2 control offset (in bits)
 149         enum v4l2_ctrl_type
 150                 v4l2_type       V4L2 control type
 151         enum uvc_control_data_type
 152                 data_type       UVC control data type
 153         struct uvc_menu_info
 154                 *menu_info      Array of menu entries (for menu controls only)
 155         __u32   menu_count      Number of menu entries (for menu controls only)
 156
 157         * struct uvc_menu_info
 158
 159         __u32   value           Menu entry value used by the device
 160         __u8    name[32]        Menu entry name
 161
 162
 163         * enum uvc_control_data_type
 164
 165         UVC_CTRL_DATA_TYPE_RAW          Raw control (byte array)
 166         UVC_CTRL_DATA_TYPE_SIGNED       Signed integer
 167         UVC_CTRL_DATA_TYPE_UNSIGNED     Unsigned integer
 168         UVC_CTRL_DATA_TYPE_BOOLEAN      Boolean
 169         UVC_CTRL_DATA_TYPE_ENUM         Enumeration
 170         UVC_CTRL_DATA_TYPE_BITMASK      Bitmask
 171
 172
 173 ---- UVCIOC_CTRL_QUERY - Query a UVC XU control ----
 174
 175 Argument: struct uvc_xu_control_query
 176
 177 Description:
 178         This ioctl queries a UVC XU control identified by its extension unit ID
 179         and control selector.
 180
 181         There are a number of different queries available that closely
 182         correspond to the low-level control requests described in the UVC
 183         specification. These requests are:
 184
 185         UVC_GET_CUR
 186                 Obtain the current value of the control.
 187         UVC_GET_MIN
 188                 Obtain the minimum value of the control.
 189         UVC_GET_MAX
 190                 Obtain the maximum value of the control.
 191         UVC_GET_DEF
 192                 Obtain the default value of the control.
 193         UVC_GET_RES
 194                 Query the resolution of the control, i.e. the step size of the
 195                 allowed control values.
 196         UVC_GET_LEN
 197                 Query the size of the control in bytes.
 198         UVC_GET_INFO
 199                 Query the control information bitmap, which indicates whether
 200                 get/set requests are supported.
 201         UVC_SET_CUR
 202                 Update the value of the control.
 203
 204         Applications must set the 'size' field to the correct length for the
 205         control. Exceptions are the UVC_GET_LEN and UVC_GET_INFO queries, for
 206         which the size must be set to 2 and 1, respectively. The 'data' field
 207         must point to a valid writable buffer big enough to hold the indicated
 208         number of data bytes.
 209
 210         Data is copied directly from the device without any driver-side
 211         processing. Applications are responsible for data buffer formatting,
 212         including little-endian/big-endian conversion. This is particularly
 213         important for the result of the UVC_GET_LEN requests, which is always
 214         returned as a little-endian 16-bit integer by the device.
 215
 216 Return value:
 217         On success 0 is returned. On error -1 is returned and errno is set
 218         appropriately.
 219
 220         ENOENT
 221                 The device does not support the given control or the specified
 222                 extension unit could not be found.
 223         ENOBUFS
 224                 The specified buffer size is incorrect (too big or too small).
 225         EINVAL
 226                 An invalid request code was passed.
 227         EBADRQC
 228                 The given request is not supported by the given control.
 229         EFAULT
 230                 The data pointer references an inaccessible memory area.
 231
 232 Data types:
 233         * struct uvc_xu_control_query
 234
 235         __u8    unit            Extension unit ID
 236         __u8    selector        Control selector
 237         __u8    query           Request code to send to the device
 238         __u16   size            Control data size (in bytes)
 239         __u8    *data           Control value