看看Linux为相机提供的驱动框架

前言

前面我们了解了一些相机的基础知识《相机基础知识》。下面来看看Linux为相机提供的驱动框架。

V4L2简介

V4L2 （Video Linux Two），是为支持Linux内核设计的驱动框架驱动框架。为应用设置的操作接口层（ioctl），是在提供更广泛的时候它的设备，它们只有在原件上，才是真正的视频设备，所以它们才是真正的摄像头设计。

V4L2框架

 

在应用层，可以在 /dev 显示设备节点，程序打开设备设备进行我们的设备目录发现应用程序0、视频画面。video1、video2...这些设备节点是在核心层注册。

v4l2-dev.c) 起承上启下的作用，它会为每一个核心层注册中的设备设置一个统一的 v4l2_fops ，这些统一驱动的接口最终将调用到驱动的 video_device 的 fops 。

重要结构体

视频设备

视频设备

//表示一个视频设备

struct video_device {

#if defined(CONFIG_MEDIA_CONTROLLER);

  struct media_entity entity;

  struct media_intf_devnode *intf_devnode;

  struct media_pipeline pipe;

#endif;

  const struct v4l2_file_operations *fops; //文件操作接口(dev/videoX)

  u32 device_caps; //设备功能，用于v4l2_capabilities(应用层定义的结构体)

  struct device dev;

  struct cdev *cdev; //字符设备

  struct v4l2_device *v4l2_dev; //V4L2设备

  struct device *dev_parent;

  struct v4l2_ctrl_handler *ctrl_handler; //设备节点对应的控制句柄

  struct vb2_queue *queue;

  struct v4l2_prio_state *prio;

  char name[32];  //Video设备名称

  enum vfl_devnode_type vfl_type; //V4L设备类型

  enum vfl_devnode_direction vfl_dir; //V4L 接收者/发送者/m2m

  int minor; //子设备号，主设备为81

  u16 num; 

  unsigned long flags;

  int index;

  spinlock_t fh_lock;

  struct list_head        fh_list;

  int dev_debug;

  v4l2_std_id tvnorms;

  void (*release)(struct video_device *vdev); //video_device release()回调

  const struct v4l2_ioctl_ops *ioctl_ops; //IOCTL回调

  unsigned long valid_ioctls[BITS_TO_LONGS(BASE_VIDIOC_PRIVATE)];

  struct mutex *lock;

};

v4l2_device

struct v4l2_device {

  struct device *dev;

  struct media_device *mdev;

  struct list_head subdevs; //用于追踪已注册的subdev

  spinlock_t lock;

  char name[V4L2_DEVICE_NAME_SIZE]; //设备名

  void (*notify)(struct v4l2_subdev *sd, unsigned int notification, void *arg);

  struct v4l2_ctrl_handler *ctrl_handler; //控制句柄

  struct v4l2_prio_state prio; //设备的优先状态

  struct kref ref; //引用

  void (*release)(struct v4l2_device *v4l2_dev);//引用计数为0后调用

};

嵌入到video_device中，表示一个v4l2设备实例。

v4l2_subdev

struct v4l2_subdev {

#if defined(CONFIG_MEDIA_CONTROLLER);

  struct media_entity entity;

#endif;

  struct list_head list;  //subdev列表

  struct module *owner;

  bool owner_v4l2_dev;

  u32 flags;

  struct v4l2_device *v4l2_dev;  //依附的v4l2_device

  const struct v4l2_subdev_ops *ops; //subdev操作函数

  const struct v4l2_subdev_internal_ops *internal_ops;

  struct v4l2_ctrl_handler *ctrl_handler; //控制句柄

  char name[V4L2_SUBDEV_NAME_SIZE]; //subdev名称

  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;//subdev平台数据

};

依附在一个v4l2_device下面，并表示一个v4l2设备的子设备，v4l2_device下可以有多个sub_device。

v4l2_fh

struct v4l2_fh {

  struct list_head        list; //文件句柄列表

  struct video_device     *vdev; //依附的video_device

  struct v4l2_ctrl_handler *ctrl_handler;

  enum v4l2_priority      prio; //文件句柄的优先级

  wait_queue_head_t wait;

  struct mutex            subscribe_lock;

  struct list_head        subscribed; //订阅的事件列表

  struct list_head        available; //可用的事件

  unsigned int            navailable; //可用的事件数

  u32 sequence;

  struct v4l2_m2m_ctx     *m2m_ctx;

};

追踪的文件句柄用于

v4l2_device和v4l2_subdev的关系：

subdev 的设计目的是为了多路复用，就是控制使用一个v4l2_device可以挂接多个v4l2_subdev。这样的多路复用就是一个摄像头来多个摄像头。手机和后置摄像头。

在V4L2驱动中，使用v4l2_device来表示摄像头摄像头（ISP）。使用v4l2_subdev来表示具体的某个摄像头（Sensor）。

v4l2_file_operations

//V4L2设备操作函数

struct v4l2_file_operations {

  struct module *owner;

  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 *);

};

v4l2_ioctl_ops

//IOCTL操作函数

struct v4l2_ioctl_ops {

    ......

    int (*vidioc_overlay)(struct file *file, void *fh, unsigned int i);

    int (*vidioc_g_fbuf)(struct file *file, void *fh, struct v4l2_framebuffer *a);

    int (*vidioc_s_fbuf)(struct file *file, void *fh, const struct v4l2_framebuffer *a);

    int (*vidioc_streamon)(struct file *file, void *fh, enum v4l2_buf_type i);

    int (*vidioc_streamoff)(struct file *file, void *fh, enum v4l2_buf_type i);

    ......

    int (*vidioc_enum_framesizes)(struct file *file, void *fh, struct v4l2_frmsizeenum *fsize);

    int (*vidioc_enum_frameintervals)(struct file *file, void *fh, struct v4l2_frmivalenum *fival);

    int (*vidioc_s_dv_timings)(struct file *file, void *fh, struct v4l2_dv_timings *timings);

    int (*vidioc_g_dv_timings)(struct file *file, void *fh, struct v4l2_dv_timings *timings);

    int (*vidioc_query_dv_timings)(struct file *file, void *fh, struct v4l2_dv_timings *timings);

    int (*vidioc_enum_dv_timings)(struct file *file, void *fh, struct v4l2_enum_dv_timings *timings);

    int (*vidioc_dv_timings_cap)(struct file *file, void *fh, struct v4l2_dv_timings_cap *cap);

    int (*vidioc_g_edid)(struct file *file, void *fh, struct v4l2_edid *edid);

    int (*vidioc_s_edid)(struct file *file, void *fh, struct v4l2_edid *edid);

    int (*vidioc_subscribe_event)(struct v4l2_fh *fh, const struct v4l2_event_subscription *sub);

    int (*vidioc_unsubscribe_event)(struct v4l2_fh *fh, const struct v4l2_event_subscription *sub);

    long (*vidioc_default)(struct file *file, void *fh, bool valid_prio, unsigned int cmd, void *arg);

};

v4l2_subdev_ops

//subdev操作函数

struct v4l2_subdev_ops {

    const struct v4l2_subdev_core_ops       *core;     //subdev核心操作回调

    const struct v4l2_subdev_tuner_ops      *tuner;    //radio模式打开v4l设备时的操作回调

    const struct v4l2_subdev_audio_ops      *audio;    //音频相关设置回调

    const struct v4l2_subdev_video_ops      *video;    //video模式打开v4l设备时的操作回调

    const struct v4l2_subdev_vbi_ops        *vbi;    //通过vbi设备节点以video模式打开v4l设备时的操作回调

    const struct v4l2_subdev_ir_ops         *ir;     //IR(红外)设备操作函数

    const struct v4l2_subdev_sensor_ops     *sensor; //sensor操作函数

    const struct v4l2_subdev_pad_ops        *pad;    //pad操作函数

};

v4l22，所以多部分可以轻松地根据实际设备的需要实现的设备。

上面的_ioctl_ops中实现的部分ioctl最终到v4l2_dev_ops中的调用子函数。

API函数

//注册/注销video_device

int video_register_device(struct video_device *vdev, enum vfl_devnode_type type, int nr)

void video_unregister_device(struct video_device *vdev)

//分配/释放video_device

struct video_device * __must_check video_device_alloc(void);

void video_device_release(struct video_device *vdev)

//注册/注销v4l2_device

int v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev)

void v4l2_device_unregister(struct v4l2_device *v4l2_dev)

//注册/注销v4l2_subdev(关联v4l2_device和v4l2_subdev)

int v4l2_device_register_subdev(struct v4l2_device *v4l2_dev, struct v4l2_subdev *sd)

void v4l2_device_unregister_subdev(struct v4l2_subdev *sd)

//初始化v4l2_subdev

void v4l2_subdev_init(struct v4l2_subdev *sd, const struct v4l2_subdev_ops *ops);

/********************************I2C subdev*************************************/

//初始化v4l2_subdev, 该subdev是I2C设备

void v4l2_i2c_subdev_init(struct v4l2_subdev *sd, struct i2c_client *client,

    const struct v4l2_subdev_ops *ops)

/*******************************SPI subdev************************************************/

//初始化v4l2_subdev, 该subdev是SPI设备

void v4l2_spi_subdev_init(struct v4l2_subdev *sd, struct spi_device *spi,

    const struct v4l2_subdev_ops *ops)



审核编辑：刘清