（一）.v4l2学习：结构体

1. video_device  是启到承上启下的作用，提供app函数，向下调用ioctl_ops函数
struct video_device
{

	const struct v4l2_file_operations *fops;   // 重点，v4l2的操作方法集

	u32 device_caps;

	/* sysfs */
	struct device dev; 
	struct cdev *cdev;    // 字符设备， 很关键

	struct v4l2_device *v4l2_dev;    //v4l2_dev 设备 很关键  指向v4l2_device [2]中
	struct device *dev_parent;    

	struct v4l2_ctrl_handler *ctrl_handler;    //控制句柄，

	struct vb2_queue *queue;  //缓存队列   很关键

	struct v4l2_prio_state *prio;

	/* device info */
	char name[32];   //设备名 
	enum vfl_devnode_type vfl_type;   //设备类型
	enum vfl_devnode_direction vfl_dir;
	int minor;   //次设备号
	u16 num;
	unsigned long flags;
	int index;

	/* V4L2 file handles */
	spinlock_t		fh_lock;
	struct list_head	fh_list;

	int dev_debug;

	v4l2_std_id tvnorms;

	/* callbacks */
	void (*release)(struct video_device *vdev);
	const struct v4l2_ioctl_ops *ioctl_ops;     // 重点，最终回调的函数
	DECLARE_BITMAP(valid_ioctls, BASE_VIDIOC_PRIVATE);

	struct mutex *lock;
};

struct v4l2_file_operations {
	struct module *owner;   //引用计数
	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
	__poll_t (*poll) (struct file *, struct poll_table_struct *);
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
#ifdef CONFIG_COMPAT
	long (*compat_ioctl32) (struct file *, unsigned int, unsigned long);
#endif
	unsigned long (*get_unmapped_area) (struct file *, unsigned long,
				unsigned long, unsigned long, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct file *);
	int (*release) (struct file *);
};

struct v4l2_ioctl_ops {   //这个结构体很关键  是最终需要调用的 
	/* ioctl callbacks */

	/* VIDIOC_QUERYCAP handler */
	int (*vidioc_querycap)(struct file *file, void *fh,
			       struct v4l2_capability *cap);

	/* VIDIOC_ENUM_FMT handlers */
	int (*vidioc_enum_fmt_vid_cap)(struct file *file, void *fh,
				       struct v4l2_fmtdesc *f);
	int (*vidioc_enum_fmt_vid_overlay)(struct file *file, void *fh,
					   struct v4l2_fmtdesc *f);
	int (*vidioc_enum_fmt_vid_out)(struct file *file, void *fh,
				       struct v4l2_fmtdesc *f);
	int (*vidioc_enum_fmt_vid_cap_mplane)(struct file *file, void *fh,
......

---

2.struct v4l2_device

struct v4l2_device {
	struct device *dev;
	struct media_device *mdev;
	struct list_head subdevs;    // 用来管理v4l2_subdeves 的链表
	spinlock_t lock;
	char name[V4L2_DEVICE_NAME_SIZE];
	void (*notify)(struct v4l2_subdev *sd,
			unsigned int notification, void *arg);
	struct v4l2_ctrl_handler *ctrl_handler;    //
	struct v4l2_prio_state prio;
	struct kref ref;
	void (*release)(struct v4l2_device *v4l2_dev);
};

---

3. struct v4l2_subdev

struct v4l2_subdev {
	struct list_head list;
	struct module *owner;
	bool owner_v4l2_dev;
	u32 flags;
	struct v4l2_device *v4l2_dev;   // 这个就是反过来指向哪个v4l2_device
	const struct v4l2_subdev_ops *ops;     // 重点
	const struct v4l2_subdev_internal_ops *internal_ops;  // 重点
	struct v4l2_ctrl_handler *ctrl_handler;
	char name[V4L2_SUBDEV_NAME_SIZE];
	u32 grp_id;
	void *dev_priv;
	void *host_priv;
	struct video_device *devnode;   
	struct device *dev;
	struct fwnode_handle *fwnode;
	struct list_head async_list;
	struct v4l2_async_subdev *asd;
	struct v4l2_async_notifier *notifier;
	struct v4l2_async_notifier *subdev_notifier;
	struct v4l2_subdev_platform_data *pdata;
};

1. 视频设备通用的操作：初始化，加载FW，上电和reset，等等
2. 视频设备特有的操作：设置帧率，裁剪图像，开关视频，等等
3.摄像头传感器操作

struct v4l2_subdev_ops {
	const struct v4l2_subdev_core_ops	*core;
	const struct v4l2_subdev_tuner_ops	*tuner;
	const struct v4l2_subdev_audio_ops	*audio;
	const struct v4l2_subdev_video_ops	*video;
	const struct v4l2_subdev_vbi_ops	*vbi;
	const struct v4l2_subdev_ir_ops		*ir;
	const struct v4l2_subdev_sensor_ops	*sensor;
	const struct v4l2_subdev_pad_ops	*pad;
};

struct v4l2_subdev_core_ops {
	int (*log_status)(struct v4l2_subdev *sd);
	int (*s_io_pin_config)(struct v4l2_subdev *sd, size_t n,
				      struct v4l2_subde