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kerneldriversmediav4l2-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)
",
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;
};
其中 list 域作为链表节点链接至 v4l2_dev 指向的 v4l2_device 结构中，这个结构中最重要的成员就是 struct v4l2_subdev_ops *ops，该域包含了 v4l2 设备支持的所有操作，定义如下：
struct v4l2_subdev_ops {
const struct v4l2_subdev_core_ops *core; /* 通用操作合集 */
const struct v4l2_subdev_tuner_ops *tuner; /* 调谐器操作合集 */
const struct v4l2_subdev_audio_ops *audio; /* 音频操作合集 */
const struct v4l2_subdev_video_ops *video; /* 视频操作合集 */
};
v4l2_subdev_core_ops 包含的操作合集是各种类型设备通用的：
struct v4l2_subdev_core_ops {
int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); /* 获取设备id */
int (*log_status)(struct v4l2_subdev *sd); /* 状态消息 */
int (*s_config)(struct v4l2_subdev *sd, int irq, void *platform_data); /* 设置配置信息 */
int (*init)(struct v4l2_subdev *sd, u32 val); /* 初始化设备 */
int (*load_fw)(struct v4l2_subdev *sd); /* 加载firmware */
int (*reset)(struct v4l2_subdev *sd, u32 val); /* 重置设备 */
int (*s_gpio)(struct v4l2_subdev *sd, u32 val); /* 设置gpio */
int (*queryctrl)(struct v4l2_subdev *sd, struct v4l2_queryctrl *qc); /* 查询设备支持的操作 */
int (*g_ctrl)(struct v4l2_subdev *sd, struct v4l2_control *ctrl); /* 获取当前命令值 */
int (*s_ctrl)(struct v4l2_subdev *sd, struct v4l2_control *ctrl); /* 设置当前命令值 */
int (*g_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls); /* 获取外置命令值 */
int (*s_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls); /* 设置外置命令值 */
int (*try_ext_ctrls)(struct v4l2_subdev *sd, struct v4l2_ext_controls *ctrls);
int (*querymenu)(struct v4l2_subdev *sd, struct v4l2_querymenu *qm); /* 查询操作菜单 */
int (*s_std)(struct v4l2_subdev *sd, v4l2_std_id norm); /* 设置数据标准 */
long (*ioctl)(struct v4l2_subdev *sd, unsigned int cmd, void *arg); /* 处理特殊命令 */
#ifdef CONFIG_VIDEO_ADV_DEBUG
int (*g_register)(struct v4l2_subdev *sd, struct v4l2_dbg_register *reg); /* 获取寄存器值 */
int (*s_register)(struct v4l2_subdev *sd, struct v4l2_dbg_register *reg); /* 设置寄存器值 */
#endif
};
v4l2_subdev_tuner_ops 包含的操作合集则是调谐器独有的：
struct v4l2_subdev_tuner_ops {
int (*s_mode)(struct v4l2_subdev *sd, enum v4l2_tuner_type); /* 设置调谐器模式 */
int (*s_radio)(struct v4l2_subdev *sd); /* 设置无线设备信息 */
int (*s_frequency)(struct v4l2_subdev *sd, struct v4l2_frequency *freq); /* 设置频率 */
int (*g_frequency)(struct v4l2_subdev *sd, struct v4l2_frequency *freq); /* 获取频率 */
int (*g_tuner)(struct v4l2_subdev *sd, struct v4l2_tuner *vt); /* 获取调谐器信息 */
int (*s_tuner)(struct v4l2_subdev *sd, struct v4l2_tuner *vt); /* 设置调谐器信息 */
int (*g_modulator)(struct v4l2_subdev *sd, struct v4l2_modulator *vm); /* 获取调幅器信息 */
int (*s_modulator)(struct v4l2_subdev *sd, struct v4l2_modulator *vm); /* 设置调幅器信息 */
int (*s_type_addr)(struct v4l2_subdev *sd, struct tuner_setup *type); /* 安装调谐器 */
int (*s_config)(struct v4l2_subdev *sd, const struct v4l2_priv_tun_config *config); /* 设置配置信息 */
int (*s_standby)(struct v4l2_subdev *sd); /* 设置标准 */
};
v4l2_subdev_audio_ops 包含的操作合集则是音频部分独有的：
struct v4l2_subdev_audio_ops {
int (*s_clock_freq)(struct v4l2_subdev *sd, u32 freq); /* 设置音频设备频率 */
int (*s_i2s_clock_freq)(struct v4l2_subdev *sd, u32 freq); /* 设置i2s总线频率 */
int (*s_routing)(struct v4l2_subdev *sd, u32 input, u32 output, u32 config); /* 设置音频路由 */
};
v4l2_subdev_video_ops 包含的操作合集则是视频部分独有的：
struct v4l2_subdev_video_ops {
int (*s_routing)(struct v4l2_subdev *sd, u32 input, u32 output, u32 config); /* 设置视频路由 */
int (*s_crystal_freq)(struct v4l2_subdev *sd, u32 freq, u32 flags); /* 设置设备频率 */
int (*decode_vbi_line)(struct v4l2_subdev *sd, struct v4l2_decode_vbi_line *vbi_line); /* 消隐区信息解码 */
int (*s_vbi_data)(struct v4l2_subdev *sd, const struct v4l2_sliced_vbi_data *vbi_data); /* 设置消隐区数据 */
int (*g_vbi_data)(struct v4l2_subdev *sd, struct v4l2_sliced_vbi_data *vbi_data); /* 获取消隐区数据 */
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); /* 设置标准输出 */
int (*querystd)(struct v4l2_subdev *sd, v4l2_std_id *std); /* 查询标准 */
int (*g_input_status)(struct v4l2_subdev *sd, u32 *status); /* 获取输入状态 */
int (*s_stream)(struct v4l2_subdev *sd, int enable); /* 设置数据流 */
int (*enum_fmt)(struct v4l2_subdev *sd, struct v4l2_fmtdesc *fmtdesc); /* 枚举视频格式 */
int (*g_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 获取视频格式 */
int (*try_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 尝试设置视频格式 */
int (*s_fmt)(struct v4l2_subdev *sd, struct v4l2_format *fmt); /* 设置视频格式 */
int (*cropcap)(struct v4l2_subdev *sd, struct v4l2_cropcap *cc); /* 视频剪辑功能 */
int (*g_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop); /* 获取剪辑功能 */
int (*s_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop); /* 设置剪辑功能 */
int (*g_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param); /* 获取参数 */
int (*s_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param); /* 设置参数 */
int (*enum_framesizes)(struct v4l2_subdev *sd, struct v4l2_frmsizeenum *fsize); /* 枚举帧大小 */
int (*enum_frameintervals)(struct v4l2_subdev *sd, struct v4l2_frmivalenum *fival); /* 枚举帧间隔 */
};
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：
