我的uvc開源地址：gitee-uvc
- 字符設備驅動程序核心：V4L2本身就是一個字符設備，具有字符設備所有的特性，暴露接口給用戶空間。
- V4L2 驅動核心：主要是構建一個內核中標准視頻設備驅動的框架，為視頻操作提供統一的接口函數。
- 平台V4L2設備驅動：在V4L2框架下，根據平台自身的特性實現與平台相關的V4L2驅動部分，包括注冊video_device和v4l2_dev。
- 具體的sensor驅動：主要上電、提供工作時鍾、視頻圖像裁剪、流IO開啟等，實現各種設備控制方法供上層調用並注冊v4l2_subdev。
1 從字符設備開始：
熟悉v4l2用戶空間編程的都知道, v4l2編程主要是調用一系列的ioctl函數去對v4l2設備進行打開, 關閉, 查詢, 設置等操作. v4l2設備是一個字符設備, 而且其驅動的主要工作就是實現各種各樣的ioctl.
v4l2的整體框架如下圖所示：
V4L2 ：video for linux version 2 ，是 linux 里一套標准的視頻驅動。本文來分析一下它的核心框架。
在v4l2的核心中對這個file_operations的實現如下:
static const struct file_operations v4l2_fops = {
.owner = THIS_MODULE,
.read = v4l2_read,
.write = v4l2_write,
.open = v4l2_open,
.get_unmapped_area = v4l2_get_unmapped_area,
.mmap = v4l2_mmap,
.unlocked_ioctl = v4l2_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl = v4l2_compat_ioctl32,
#endif
.release = v4l2_release,
.poll = v4l2_poll,
.llseek = no_llseek,
};
這個v4l2_fops函數最終綁定在一個cdev上, 並注冊到系統中。
以v4l2_open為例（代碼在kernel\drivers\media\v4l2-core中）：
/* Override for the open function */
static int v4l2_open(struct inode *inode, struct file *filp)
{
struct video_device *vdev;
int ret = 0;
/* Check if the video device is available */
mutex_lock(&videodev_lock);
vdev = video_devdata(filp);
/* return ENODEV if the video device has already been removed. */
if (vdev == NULL || !video_is_registered(vdev)) {
mutex_unlock(&videodev_lock);
return -ENODEV;
}
/* and increase the device refcount */
video_get(vdev);
mutex_unlock(&videodev_lock);
if (vdev->fops->open) {
if (video_is_registered(vdev))
//這里就是調用了file_operations的open函數
ret = vdev->fops->open(filp);
else
ret = -ENODEV;
}
if (vdev->debug)
printk(KERN_DEBUG "%s: open (%d)\n",
video_device_node_name(vdev), ret);
/* decrease the refcount in case of an error */
if (ret)
video_put(vdev);
return ret;
}
2. video_device結構體：
video_device結構體用於在/dev目錄下生成設備節點文件，把操作設備的接口暴露給用戶空間。
我們是使用video_device來操作的，看看video_device這個結構體：
struct video_device
{
#if defined(CONFIG_MEDIA_CONTROLLER)
struct media_entity entity;
#endif
/* device ops */
const struct v4l2_file_operations *fops;
/* sysfs */
struct device dev; /* v4l device */
struct cdev *cdev; /* character device */
/* Set either parent or v4l2_dev if your driver uses v4l2_device */
struct device *parent; /* device parent */
struct v4l2_device *v4l2_dev; /* v4l2_device parent */
/* Control handler associated with this device node. May be NULL. */
struct v4l2_ctrl_handler *ctrl_handler;
/* vb2_queue associated with this device node. May be NULL. */
struct vb2_queue *queue;
/* Priority state. If NULL, then v4l2_dev->prio will be used. */
struct v4l2_prio_state *prio;
/* device info */
char name[32];
int vfl_type; /* device type */
int vfl_dir; /* receiver, transmitter or m2m */
/* 'minor' is set to -1 if the registration failed */
int minor;
u16 num;
/* use bitops to set/clear/test flags */
unsigned long flags;
/* attribute to differentiate multiple indices on one physical device */
int index;
/* V4L2 file handles */
spinlock_t fh_lock; /* Lock for all v4l2_fhs */
struct list_head fh_list; /* List of struct v4l2_fh */
int debug; /* Activates debug level*/
/* Video standard vars */
v4l2_std_id tvnorms; /* Supported tv norms */
v4l2_std_id current_norm; /* Current tvnorm */
/* callbacks */
void (*release)(struct video_device *vdev);
/* ioctl callbacks */
const struct v4l2_ioctl_ops *ioctl_ops;
DECLARE_BITMAP(valid_ioctls, BASE_VIDIOC_PRIVATE);
/* serialization lock */
DECLARE_BITMAP(disable_locking, BASE_VIDIOC_PRIVATE);
struct mutex *lock;
};
3. V4L2_device結構體：
這個結構體中包含了一個非常重要的結構體v4l2_device：
struct v4l2_device {
/* dev->driver_data points to this struct.
Note: dev might be NULL if there is no parent device
as is the case with e.g. ISA devices. */
struct device *dev;
#if defined(CONFIG_MEDIA_CONTROLLER)
struct media_device *mdev;
#endif
/* used to keep track of the registered subdevs */
struct list_head subdevs;
/* lock this struct; can be used by the driver as well if this
struct is embedded into a larger struct. */
spinlock_t lock;
/* unique device name, by default the driver name + bus ID */
char name[V4L2_DEVICE_NAME_SIZE];
/* notify callback called by some sub-devices. */
void (*notify)(struct v4l2_subdev *sd,
unsigned int notification, void *arg);
/* The control handler. May be NULL. */
struct v4l2_ctrl_handler *ctrl_handler;
/* Device's priority state */
struct v4l2_prio_state prio;
/* BKL replacement mutex. Temporary solution only. */
struct mutex ioctl_lock;
/* Keep track of the references to this struct. */
struct kref ref;
/* Release function that is called when the ref count goes to 0. */
void (*release)(struct v4l2_device *v4l2_dev);
};
3.1 v4l2_device的注冊和注銷：
int v4l2_device_register(struct device*dev, struct v4l2_device *v4l2_dev)
static void v4l2_device_release(struct kref *ref)
4. v4l2_subdev結構體
V4l2_subdev代表子設備，包含了子設備的相關屬性和操作。先來看下結構體原型：
struct v4l2_subdev {
struct v4l2_device *v4l2_dev; //指向父設備
//提供一些控制v4l2設備的接口
const struct v4l2_subdev_ops *ops;
//向V4L2框架提供的接口函數
const struct v4l2_subdev_internal_ops *internal_ops;
//subdev控制接口
struct v4l2_ctrl_handler *ctrl_handler;
/* name must be unique */
charname[V4L2_SUBDEV_NAME_SIZE];
/*subdev device node */
struct video_device *devnode;
};
其中 list 域作為鏈表節點鏈接至 v4l2_dev 指向的 v4l2_device 結構中，這個結構中最重要的成員就是 struct v4l2_subdev_ops *ops，該域包含了 v4l2 設備支持的所有操作，定義如下：
struct v4l2_subdev_ops {
const struct v4l2_subdev_core_ops *core; /* 通用操作合集 */
const struct v4l2_subdev_tuner_ops *tuner; /* 調諧器操作合集 */
const struct v4l2_subdev_audio_ops *audio; /* 音頻操作合集 */
const struct v4l2_subdev_video_ops *video; /* 視頻操作合集 */
};
v4l2_subdev_core_ops 包含的操作合集是各種類型設備通用的：
struct v4l2_subdev_core_ops {
int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); /* 獲取設備id */
int (*log_status)(struct v4l2_subdev *sd); /* 狀態消息 */
int (*s_config)(struct v4l2_subdev *sd, int irq, void *platform_data); /* 設置配置信息 */
int (*init)(struct v4l2_subdev *sd, u32 val); /* 初始化設備 */
int (*load_fw)(struct v4l2_subdev *sd); /* 加載firmware */
int (*reset)(struct v4l2_subdev *sd, u32 val); /* 重置設備 */
int (*s_gpio)(struct v4l2_subdev *sd, u32 val); /* 設置gpio */
int (*queryctrl)(struct v4l2_subdev *sd, struct v4l2_queryctrl *qc); /* 查詢設備支持的操作 */
int (*g_ctrl)(struct v4l2_subdev *sd, struct v4l2_control *ctrl); /* 獲取當前命令值 */
int (*s_ctrl)(struct v4l2_subdev *sd, struct v4l2_control *ctrl); /* 設置當前命令值 */
int (*g_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls); /* 獲取外置命令值 */
int (*s_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls); /* 設置外置命令值 */
int (*try_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls);
int (*querymenu)(struct v4l2_subdev *sd, struct v4l2_querymenu *qm); /* 查詢操作菜單 */
int (*s_std)(struct v4l2_subdev *sd, v4l2_std_id norm); /* 設置數據標准 */
long (*ioctl)(struct v4l2_subdev *sd, unsigned int cmd, void *arg); /* 處理特殊命令 */
#ifdef CONFIG_VIDEO_ADV_DEBUG
int (*g_register)(struct v4l2_subdev *sd, struct v4l2_dbg_register *reg); /* 獲取寄存器值 */
int (*s_register)(struct v4l2_subdev *sd, struct v4l2_dbg_register *reg); /* 設置寄存器值 */
#endif
};
v4l2_subdev_tuner_ops 包含的操作合集則是調諧器獨有的：
struct v4l2_subdev_tuner_ops {
int (*s_mode)(struct v4l2_subdev *sd, enum v4l2_tuner_type); /* 設置調諧器模式 */
int (*s_radio)(struct v4l2_subdev *sd); /* 設置無線設備信息 */
int (*s_frequency)(struct v4l2_subdev *sd, struct v4l2_frequency *freq); /* 設置頻率 */
int (*g_frequency)(struct v4l2_subdev *sd, struct v4l2_frequency *freq); /* 獲取頻率 */
int (*g_tuner)(struct v4l2_subdev *sd, struct v4l2_tuner *vt); /* 獲取調諧器信息 */
int (*s_tuner)(struct v4l2_subdev *sd, struct v4l2_tuner *vt); /* 設置調諧器信息 */
int (*g_modulator)(struct v4l2_subdev *sd, struct v4l2_modulator *vm); /* 獲取調幅器信息 */
int (*s_modulator)(struct v4l2_subdev *sd, struct v4l2_modulator *vm); /* 設置調幅器信息 */
int (*s_type_addr)(struct v4l2_subdev *sd, struct tuner_setup *type); /* 安裝調諧器 */
int (*s_config)(struct v4l2_subdev *sd, const struct v4l2_priv_tun_config *config); /* 設置配置信息 */
int (*s_standby)(struct v4l2_subdev *sd); /* 設置標准 */
};
v4l2_subdev_audio_ops 包含的操作合集則是音頻部分獨有的：
struct v4l2_subdev_audio_ops {
int (*s_clock_freq)(struct v4l2_subdev *sd, u32 freq); /* 設置音頻設備頻率 */
int (*s_i2s_clock_freq)(struct v4l2_subdev *sd, u32 freq); /* 設置i2s總線頻率 */
int (*s_routing)(struct v4l2_subdev *sd, u32 input, u32 output, u32 config); /* 設置音頻路由 */
};
v4l2_subdev_video_ops 包含的操作合集則是視頻部分獨有的：
struct v4l2_subdev_video_ops {
int (*s_routing)(struct v4l2_subdev *sd, u32 input, u32 output, u32 config); /* 設置視頻路由 */
int (*s_crystal_freq)(struct v4l2_subdev *sd, u32 freq, u32 flags); /* 設置設備頻率 */
int (*decode_vbi_line)(struct v4l2_subdev *sd, struct v4l2_decode_vbi_line *vbi_line); /* 消隱區信息解碼 */
int (*s_vbi_data)(struct v4l2_subdev *sd, const struct v4l2_sliced_vbi_data *vbi_data); /* 設置消隱區數據 */
int (*g_vbi_data)(struct v4l2_subdev *sd, struct v4l2_sliced_vbi_data *vbi_data); /* 獲取消隱區數據 */
int (*g_sliced_vbi_cap)(struct v4l2_subdev *sd, struct v4l2_sliced_vbi_cap *cap);
int (*s_std_output)(struct v4l2_subdev *sd, v4l2_std_id std); /* 設置標准輸出 */
int (*querystd)(struct v4l2_subdev *sd, v4l2_std_id *std); /* 查詢標准 */
int (*g_input_status)(struct v4l2_subdev *sd, u32 *status); /* 獲取輸入狀態 */
int (*s_stream)(struct v4l2_subdev *sd, int enable); /* 設置數據流 */
int (*enum_fmt)(struct v4l2_subdev *sd, struct v4l2_fmtdesc *fmtdesc); /* 枚舉視頻格式 */
int (*g_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 獲取視頻格式 */
int (*try_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 嘗試設置視頻格式 */
int (*s_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 設置視頻格式 */
int (*cropcap)(struct v4l2_subdev *sd, struct v4l2_cropcap *cc); /* 視頻剪輯功能 */
int (*g_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop); /* 獲取剪輯功能 */
int (*s_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop); /* 設置剪輯功能 */
int (*g_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param); /* 獲取參數 */
int (*s_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param); /* 設置參數 */
int (*enum_framesizes)(struct v4l2_subdev *sd, struct v4l2_frmsizeenum *fsize); /* 枚舉幀大小 */
int (*enum_frameintervals)(struct v4l2_subdev *sd, struct v4l2_frmivalenum *fival); /* 枚舉幀間隔 */
};
4.1 subdev的注冊和注銷
當我們把v4l2_subdev需要實現的成員都已經實現，就可以調用以下函數把子設備注冊到V4L2核心層：
int v4l2_device_register_subdev(struct v4l2_device*v4l2_dev, struct v4l2_subdev *sd)
當卸載子設備時，可以調用以下函數進行注銷：
void v4l2_device_unregister_subdev(struct v4l2_subdev*sd)
5. 應用層具體流程框架：
我們使用一張圖來體現吧：
