V4L2（一）框架浅析

  V4L2 ：video for linux version 2 ，是 linux 里一套标准的视频驱动，它支持 uvc 标准的摄像头。本文来分析一下它的核心框架。

  整个v4l2的框架分为三层：

    在应用层，我们可以在 /dev 目录发现 video0 类似的设备节点，上层的摄像头程序打开设备节点进行数据捕获，显示视频画面。设备节点的名字很统一，video0 video1 video2...这些设备节点在是核心层注册。

    核心层 v4l2-dev.c，承上启下，对于每一个硬件相关层注册进来的设备，设置一个统一的接口 v4l2_fops ，既然是统一的接口必然不是具体的视频设备的操作函数，应用层调用 v4l2_fops 中的函数最终将调用到硬件相关层的 video_device 的 fops 。

    硬件相关层，与具体的视频硬件打交道，分配、设置、注册 video_device 结构体。

static int __init videodev_init(void){/* 申请设备号，留给 video 设备使用 */dev_t dev = MKDEV(VIDEO_MAJOR, 0);ret = register_chrdev_region(dev, VIDEO_NUM_DEVICES, VIDEO_NAME);/* 创建 video 类 */ret = class_register(&video_class);return 0;}

struct video_device{/* 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;/* Priority state. If NULL, then v4l2_dev->prio will be used. */struct v4l2_prio_state *prio;/* device info */char name[32];int vfl_type;/* '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_tfh_lock; /* Lock for all v4l2_fhs */struct list_headfh_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;};

struct v4l2_device {struct device *dev;/* used to keep track of the registered subdevs */struct list_head subdevs;spinlock_t lock;char name[V4L2_DEVICE_NAME_SIZE];void (*notify)(struct v4l2_subdev *sd, unsigned int notification, void *arg);struct v4l2_ctrl_handler *ctrl_handler;struct v4l2_prio_state prio;struct mutex ioctl_lock;struct kref ref;void (*release)(struct v4l2_device *v4l2_dev);};

static inline int __must_check video_register_device(struct video_device *vdev,int type, int nr){return __video_register_device(vdev, type, nr, 1, vdev->fops->owner);}int __video_register_device(struct video_device *vdev, int type, int nr,int warn_if_nr_in_use, struct module *owner){int i = 0;int ret;int minor_offset = 0;int minor_cnt = VIDEO_NUM_DEVICES;const char *name_base;/* A minor value of -1 marks this video device as never having been registered */vdev->minor = -1;/* 视频设备的设备节点一般为 video0 ..video 就是由此而来 */switch (type) {case VFL_TYPE_GRABBER:name_base = "video";break;case VFL_TYPE_VBI:name_base = "vbi";break;...}vdev->vfl_type = type;vdev->cdev = NULL;/* Part 2: find a free minor, device node number and device index. */#ifdef CONFIG_VIDEO_FIXED_MINOR_RANGESswitch (type) {case VFL_TYPE_GRABBER:minor_offset = 0;minor_cnt = 64;break;...#endif/* 寻找一个空的项，这个好像并不重要 */mutex_lock(&videodev_lock);nr = devnode_find(vdev, nr == -1 ? 0 : nr, minor_cnt);if (nr == minor_cnt)nr = devnode_find(vdev, 0, minor_cnt);if (nr == minor_cnt) {printk(KERN_ERR "could not get a free device node number\n");mutex_unlock(&videodev_lock);return -ENFILE;}#ifdef CONFIG_VIDEO_FIXED_MINOR_RANGES/* 1-on-1 mapping of device node number to minor number */i = nr;#else/* 在全局 video_deivce 数组中寻找一个空的项,下标+minor_offset作为设备的次设备号 */for (i = 0; i < VIDEO_NUM_DEVICES; i++)if (video_device[i] == NULL)break;if (i == VIDEO_NUM_DEVICES) {mutex_unlock(&videodev_lock);printk(KERN_ERR "could not get a free minor\n");return -ENFILE;}#endifvdev->minor = i + minor_offset;vdev->num = nr;devnode_set(vdev);if (vdev->ioctl_ops)determine_valid_ioctls(vdev);/* 注册字符设备 */vdev->cdev = cdev_alloc();vdev->cdev->ops = &v4l2_fops;vdev->cdev->owner = owner;ret = cdev_add(vdev->cdev, MKDEV(VIDEO_MAJOR, vdev->minor), 1);/* 得把device注册进内核，mdev才能自动创建设备节点，/dev 目录下的video0 等就是来自这里 */vdev->dev.class = &video_class;vdev->dev.devt = MKDEV(VIDEO_MAJOR, vdev->minor);if (vdev->parent)vdev->dev.parent = vdev->parent;dev_set_name(&vdev->dev, "%s%d", name_base, vdev->num);ret = device_register(&vdev->dev);vdev->dev.release = v4l2_device_release;/* Part 6: Activate this minor. The char device can now be used. */set_bit(V4L2_FL_REGISTERED, &vdev->flags);mutex_lock(&videodev_lock);video_device[vdev->minor] = vdev;mutex_unlock(&videodev_lock);return 0;}EXPORT_SYMBOL(__video_register_device);

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,};

static int v4l2_open(struct inode *inode, struct file *filp){struct video_device *vdev = video_devdata(filp);if (vdev->fops->open) {if (video_is_registered(vdev))ret = vdev->fops->open(filp);}}static ssize_t v4l2_read(struct file *filp, char __user *buf,size_t sz, loff_t *off){struct video_device *vdev = video_devdata(filp);if (!vdev->fops->read)return -EINVAL;if (video_is_registered(vdev))ret = vdev->fops->read(filp, buf, sz, off);}static int v4l2_mmap(struct file *filp, struct vm_area_struct *vm){struct video_device *vdev = video_devdata(filp);if (!vdev->fops->mmap)return ret;ret = vdev->fops->mmap(filp, vm);}static long v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg){struct video_device *vdev = video_devdata(filp);if (vdev->fops->unlocked_ioctl) {if (video_is_registered(vdev))ret = vdev->fops->unlocked_ioctl(filp, cmd, arg);} else if (vdev->fops->ioctl) {if (video_is_registered(vdev))ret = vdev->fops->ioctl(filp, cmd, arg);} }