v4l2细节

struct v4l2_file_operations {
    long (*ioctl) (struct file *, unsigned int, unsigned long);
    long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
};


struct video_device {
    /* device ops */
    const struct v4l2_file_operations *fops;
    /* ioctl callbacks */
    const struct v4l2_ioctl_ops *ioctl_ops;
}
如果const struct v4l2_ioctl_ops *ioctl_ops不为空的话，(*unlocked_ioctl) 必须为video_ioctl2,  .unlocked_ioctl = video_ioctl2, video_ioctl2会在内部去调用const struct v4l2_ioctl_ops *ioctl_ops结构体里面的函数，如果const struct v4l2_ioctl_ops *ioctl_ops为空，则需要实现函数long (*ioctl) (struct file *, unsigned int, unsigned long)，long (*ioctl) (struct file *, unsigned int, unsigned long)里面会调用由
v4l2_int_device_register
  ->struct v4l2_int_device
    ->static struct v4l2_int_slave
      ->static struct v4l2_int_ioctl_desc

注册的函数

static struct v4l2_int_ioctl_desc 结构体需要由用户自己去填充，该结构体一般是用在video_capture，如摄像头

函数 static inline int __must_check video_register_device(struct video_device *vdev, int type, int nr)的参数type可以设置VFL_TYPE_GRABBER，VFL_TYPE_VBI，VFL_TYPE_RADIO，VFL_TYPE_SUBDEV，常用的为VFL_TYPE_GRABBER对应video_capture、video_out、video_overlay
VFL_TYPE_RADIO对应radio(收音机), 对应的设备节点名字，通过如下代码判断，最终会在video_register_device里面创建字符设备

switch (type) {
    case VFL_TYPE_GRABBER:
        name_base = "video";
        break;
    case VFL_TYPE_VBI:
        name_base = "vbi";
        break;
    case VFL_TYPE_RADIO:
        name_base = "radio";
        break;
    case VFL_TYPE_SUBDEV:
        name_base = "v4l-subdev";
        break;
    default:
        printk(KERN_ERR "%s called with unknown type: %d\n",
               __func__, type);
        return -EINVAL;
}