L4V2框架下APP编写ioctl流程

摄像头L4V2框架编写APP，对获取数据的由抽象硬件到软件ioctl操作有具体流程。

查询设备能力属性

 struct v4l2_capability cap;
    ioctl(fd, VIDIOC_QUERYCAP, &cap);

枚举格式

struct v4l2_fmtdesc fmtdesc;
    fmtdesc.index = fmt_index; 
    fmtdesc.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;  
    ioctl(fd, VIDIOC_ENUM_FMT, &fmtdesc);

枚举帧大小

struct v4l2_frmsizeenum fsenum;
        fsenum.pixel_format = fmtdesc.pixelformat;
        fsenum.index = frame_index;
        ioctl(fd, VIDIOC_ENUM_FRAMESIZES, &fsenum);

设置格式

struct v4l2_format fmt;
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    fmt.fmt.pix.width = 1024;
    fmt.fmt.pix.height = 768;
    fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG;
    fmt.fmt.pix.field = V4L2_FIELD