我的uvc开源地址：gitee-uvc
- 字符设备驱动程序核心：V4L2本身就是一个字符设备，具有字符设备所有的特性，暴露接口给用户空间。
- V4L2 驱动核心：主要是构建一个内核中标准视频设备驱动的框架，为视频操作提供统一的接口函数。
- 平台V4L2设备驱动：在V4L2框架下，根据平台自身的特性实现与平台相关的V4L2驱动部分，包括注册video_device和v4l2_dev。
- 具体的sensor驱动：主要上电、提供工作时钟、视频图像裁剪、流IO开启等，实现各种设备控制方法供上层调用并注册v4l2_subdev。
1 从字符设备开始：
熟悉v4l2用户空间编程的都知道, v4l2编程主要是调用一系列的ioctl函数去对v4l2设备进行打开, 关闭, 查询, 设置等操作. v4l2设备是一个字符设备, 而且其驱动的主要工作就是实现各种各样的ioctl.
v4l2的整体框架如下图所示：
V4L2 ：video for linux version 2 ，是 linux 里一套标准的视频驱动。本文来分析一下它的核心框架。
在v4l2的核心中对这个file_operations的实现如下:
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,
};这个v4l2_fops函数最终绑定在一个cdev上, 并注册到系统中。
以v4l2_open为例（代码在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))
//这里就是调用了file_operations的open函数
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结构体：
video_device结构体用于在/dev目录下生成设备节点文件，把操作设备的接口暴露给用户空间。
我们是使用video_device来操作的，看看video_device这个结构体：
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结构体：
这个结构体中包含了一个非常重要的结构体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的注册和注销：
int v4l2_device_register(struct device*dev, struct v4l2_device *v4l2_dev)
static void v4l2_device_release(struct kref *ref)4. v4l2_subdev结构体
V4l2_subdev代表子设备，包含了子设备的相关属性和操作。先来看下结构体原型：
struct v4l2_subdev {
struct v4l2_device *v4l2_dev; //指向父设备
//提供一些控制v4l2设备的接口
const struct v4l2_subdev_ops *ops;
//向V4L2框架提供的接口函数
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;
};4.1 subdev的注册和注销
当我们把v4l2_subdev需要实现的成员都已经实现，就可以调用以下函数把子设备注册到V4L2核心层：
int v4l2_device_register_subdev(struct v4l2_device*v4l2_dev, struct v4l2_subdev *sd)当卸载子设备时，可以调用以下函数进行注销：
void v4l2_device_unregister_subdev(struct v4l2_subdev*sd)5. 应用层具体流程框架：
我们使用一张图来体现吧：
