前言

前面我們了解了一些相機的基礎知識《相機基礎知識》。下面來看看Linux為相機提供的驅動框架。

V4L2簡介

V4L2 （Video Linux Two），是為支持Linux內核設計的驅動框架驅動框架。為應用設置的操作接口層（ioctl），是在提供更廣泛的時候它的設備，它們只有在原件上，才是真正的視頻設備，所以它們才是真正的攝像頭設計。

V4L2框架

在應用層，可以在 /dev 顯示設備節點，程序打開設備設備進行我們的設備目錄發現應用程序0、視頻畫面。video1、video2...這些設備節點是在核心層注冊。

v4l2-dev.c) 起承上啟下的作用，它會為每一個核心層注冊中的設備設置一個統一的 v4l2_fops ，這些統一驅動的接口最終將調用到驅動的 video_device 的 fops 。

重要結構體

視頻設備

視頻設備

//表示一個視頻設備

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; //設備功能，用于v4l2_capabilities(應用層定義的結構體)

struct device dev;

struct cdev *cdev; //字符設備

struct v4l2_device *v4l2_dev; //V4L2設備

struct device *dev_parent;

struct v4l2_ctrl_handler *ctrl_handler; //設備節點對應的控制句柄

struct vb2_queue *queue;

struct v4l2_prio_state *prio;

char name[32]; //Video設備名稱

enum vfl_devnode_type vfl_type; //V4L設備類型

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

int minor; //子設備號，主設備為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()回調

const struct v4l2_ioctl_ops *ioctl_ops; //IOCTL回調

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; //用于追蹤已注冊的subdev

spinlock_t lock;

char name[V4L2_DEVICE_NAME_SIZE]; //設備名

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; //設備的優先狀態

struct kref ref; //引用

void (*release)(struct v4l2_device *v4l2_dev);//引用計數為0后調用

};

嵌入到video_device中，表示一個v4l2設備實例。

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操作函數

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名稱

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平臺數據

};

依附在一個v4l2_device下面，并表示一個v4l2設備的子設備，v4l2_device下可以有多個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; //文件句柄的優先級

wait_queue_head_t wait;

struct mutex subscribe_lock;

struct list_head subscribed; //訂閱的事件列表

struct list_head available; //可用的事件

unsigned int navailable; //可用的事件數

u32 sequence;

struct v4l2_m2m_ctx *m2m_ctx;

};

追蹤的文件句柄用于

v4l2_device和v4l2_subdev的關系：

subdev 的設計目的是為了多路復用，就是控制使用一個v4l2_device可以掛接多個v4l2_subdev。這樣的多路復用就是一個攝像頭來多個攝像頭。手機和后置攝像頭。

在V4L2驅動中，使用v4l2_device來表示攝像頭攝像頭（ISP）。使用v4l2_subdev來表示具體的某個攝像頭（Sensor）。

v4l2_file_operations

//V4L2設備操作函數

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操作函數

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操作函數

struct v4l2_subdev_ops {

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

const struct v4l2_subdev_tuner_ops *tuner; //radio模式打開v4l設備時的操作回調

const struct v4l2_subdev_audio_ops *audio; //音頻相關設置回調

const struct v4l2_subdev_video_ops *video; //video模式打開v4l設備時的操作回調

const struct v4l2_subdev_vbi_ops *vbi; //通過vbi設備節點以video模式打開v4l設備時的操作回調

const struct v4l2_subdev_ir_ops *ir; //IR(紅外)設備操作函數

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

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

};

v4l22，所以多部分可以輕松地根據實際設備的需要實現的設備。

上面的_ioctl_ops中實現的部分ioctl最終到v4l2_dev_ops中的調用子函數。

API函數

//注冊/注銷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/釋放video_device

struct video_device * __must_check video_device_alloc(void);

void video_device_release(struct video_device *vdev)

//注冊/注銷v4l2_device

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

void v4l2_device_unregister(struct v4l2_device *v4l2_dev)

//注冊/注銷v4l2_subdev(關聯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, 該subdev是I2C設備

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, 該subdev是SPI設備

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

const struct v4l2_subdev_ops *ops)



審核編輯：劉清