LINUX&UVC控制请求VIDIOC_QUERYCAP设备功能属性信息v4l2_capability

VIDIOC_QUERYCAP请求使用api函数ioctl，最终进行LINUX&UVC驱动中，根据设备节点的不同，执行的回调函数不同。

VIDIOC_QUERYCAP请求用于返回设备支持的功能属性信息,其对应的结构体v4l2_capability。

struct v4l2_capability
{
    u8 driver[16]; // 驱动名字
    u8 card[32]; // 设备名字
    u8 bus_info[32]; // 设备在系统中的位置
    u32 version;// 驱动版本号
    u32 capabilities;// 设备支持的操作
    u32 reserved[4]; // 保留字段
};

UVC驱动

LINUX&UVC驱动中，对于视频节点，其对应的函数为：

const struct v4l2_ioctl_ops uvc_ioctl_ops = {
    .vidioc_querycap = uvc_ioctl_querycap,
    ...
}

具体代码为：

static int uvc_ioctl_querycap(struct file *file, void *fh,
                  struct v4l2_capability *cap)
{
    struct video_device *vdev = video_devdata(file);
    struct uvc_fh *handle = file->private_data;
    struct uvc_video_chain *chain = handle->chain;
    struct uvc_streaming *stream = handle->stream;

    strscpy(cap->driver, "uvcvideo", sizeof(cap->driver));
    strscpy(cap->card, vdev->name, sizeof(cap->card));
    usb_make_path(stream->dev->udev, cap->bus_info, sizeof(cap->bus_info));
    cap->capabilities = V4L2_CAP_DEVICE_CAPS | V4L2_CAP_STREAMING | chain->caps;

    return 0;
}

对于metadata节点，对应的函数为：

static const struct v4l2_ioctl_ops uvc_meta_ioctl_ops = {
    .vidioc_querycap        = uvc_meta_v4l2_querycap,
...
}

具体代码为：

static int uvc_meta_v4l2_querycap(struct file *file, void *fh,
                  struct v4l2_capability *cap)
{
    struct v4l2_fh *vfh = file->private_data;
    struct uvc_streaming *stream = video_get_drvdata(vfh->vdev);
    struct uvc_video_chain *chain = stream->chain;

    strscpy(cap->driver, "uvcvideo", sizeof(cap->driver));
    strscpy(cap->card, vfh->vdev->name, sizeof(cap->card));
    usb_make_path(stream->dev->udev, cap->bus_info, sizeof(cap->bus_info));
    cap->capabilities = V4L2_CAP_DEVICE_CAPS | V4L2_CAP_STREAMING | chain->caps;

    return 0;
}

v4l2_capability

通过分析两个回调函数，可知：

• 对应驱动名称，因其都是usb video class设备节点，故驱动使用的是”uvcvideo”,其对应MakeFile编译后的模块uvcvideo.o

• 对于设备名称，则来源于usbCore的设备名称。

  int uvc_register_video_device(...)
  {
  ...
  strscpy(vdev->name, dev->name, sizeof(vdev->name));
  ...
  }
  

• bus_info:其通过函数usb_make_path实现：

  static inline int usb_make_path(struct usb_device *dev, char *buf, size_t size)
  {
    int actual;
    actual = snprintf(buf, size, "usb-%s-%s", dev->bus->bus_name,dev->devpath);
    return (actual >= (int)size) ? -1 : actual;
  }
  

• capabilities:这里标识了设备的特征信息。其都包含V4L2_CAP_DEVICE_CAPS和V4L2_CAP_STREAMING，对于 chain->caps，则根据视频流描述符是输入还是输出设备不同而不同：

    if (stream->type == V4L2_BUF_TYPE_VIDEO_CAPTURE)
        stream->chain->caps |= V4L2_CAP_VIDEO_CAPTURE| V4L2_CAP_META_CAPTURE;
    else
        stream->chain->caps |= V4L2_CAP_VIDEO_OUTPUT;
  

其它相关的属性信息可详见：https://elixir.bootlin.com/linux/v5.5-rc2/source/include/uapi/linux/videodev2.h#L452

/* Values for 'capabilities' field */
#define V4L2_CAP_VIDEO_CAPTURE        0x00000001  /* Is a video capture device */
#define V4L2_CAP_VIDEO_OUTPUT        0x00000002  /* Is a video output device */
#define V4L2_CAP_VIDEO_OVERLAY        0x00000004  /* Can do video overlay */
#define V4L2_CAP_VBI_CAPTURE        0x00000010  /* Is a raw VBI capture device */
#define V4L2_CAP_VBI_OUTPUT        0x00000020  /* Is a raw VBI output device */
#define V4L2_CAP_SLICED_VBI_CAPTURE    0x00000040  /* Is a sliced VBI capture device */
#define V4L2_CAP_SLICED_VBI_OUTPUT    0x00000080  /* Is a sliced VBI output device */
#define V4L2_CAP_RDS_CAPTURE        0x00000100  /* RDS data capture */
#define V4L2_CAP_VIDEO_OUTPUT_OVERLAY    0x00000200  /* Can do video output overlay */
#define V4L2_CAP_HW_FREQ_SEEK        0x00000400  /* Can do hardware frequency seek  */
#define V4L2_CAP_RDS_OUTPUT        0x00000800  /* Is an RDS encoder */

/* Is a video capture device that supports multiplanar formats */
#define V4L2_CAP_VIDEO_CAPTURE_MPLANE    0x00001000
/* Is a video output device that supports multiplanar formats */
#define V4L2_CAP_VIDEO_OUTPUT_MPLANE    0x00002000
/* Is a video mem-to-mem device that supports multiplanar formats */
#define V4L2_CAP_VIDEO_M2M_MPLANE    0x00004000
/* Is a video mem-to-mem device */
#define V4L2_CAP_VIDEO_M2M        0x00008000

#define V4L2_CAP_TUNER            0x00010000  /* has a tuner */
#define V4L2_CAP_AUDIO            0x00020000  /* has audio support */
#define V4L2_CAP_RADIO            0x00040000  /* is a radio device */
#define V4L2_CAP_MODULATOR        0x00080000  /* has a modulator */

#define V4L2_CAP_SDR_CAPTURE        0x00100000  /* Is a SDR capture device */
#define V4L2_CAP_EXT_PIX_FORMAT        0x00200000  /* Supports the extended pixel format */
#define V4L2_CAP_SDR_OUTPUT        0x00400000  /* Is a SDR output device */
#define V4L2_CAP_META_CAPTURE        0x00800000  /* Is a metadata capture device */

#define V4L2_CAP_READWRITE              0x01000000  /* read/write systemcalls */
#define V4L2_CAP_ASYNCIO                0x02000000  /* async I/O */
#define V4L2_CAP_STREAMING              0x04000000  /* streaming I/O ioctls */
#define V4L2_CAP_META_OUTPUT        0x08000000  /* Is a metadata output device */

#define V4L2_CAP_TOUCH                  0x10000000  /* Is a touch device */

#define V4L2_CAP_DEVICE_CAPS            0x80000000  /* sets device capabilities field */