Linux V4L2 UVC摄像头框架浅析

2021-04-01 3350 1

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。
V4L2 UVC摄像头框架浅析

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。

通过分析vivi.c结构如下
vivi_init

   -->vivi_create_instance

        -->v4l2_device_register   // 不是主要, 只是用于初始化一些东西，比如自旋锁、引用计数

                 vfd = video_device_alloc(); //分配video_device结构体

             1.  *vfd = vivi_template; // 设置

        　　    .fops           = &vivi_fops,
          　　  .ioctl_ops    = &vivi_ioctl_ops,
          　　  .release      = video_device_release,

　　　  2.  vfd->v4l2_dev = &dev->v4l2_dev;

　　　  3.  设置"ctrl属性"(用于APP的ioctl)：（设置ctrl的属性让ioctl调用或者使用例如亮度等在驱动程序中抽象出来一个结构体 v4l2_ctrl,每个Ctrl对应其中的一项（音量、亮度等等）v4l2_ctrl_handler来管理他们）
        　　     v4l2_ctrl_handler_init(hdl, 11);
        　　     dev->volume = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
          　　    V4L2_CID_AUDIO_VOLUME, 0, 255, 1, 200);
        　　     dev->brightness = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
         　　     V4L2_CID_BRIGHTNESS, 0, 255, 1, 127);
        　　     dev->contrast = v4l2_ctrl_new_std(hdl, &vivi_ctrl_ops,
         　　     V4L2_CID_CONTRAST, 0, 255, 1, 16);      

　　　　4. video_register_device(vfd, VFL_TYPE_GRABBER, video_nr);  //注册

　　　　　 　--> __video_register_device(vdev, type, nr, 1, vdev->fops->owner);

　　　　　　　　　-->vdev->cdev = cdev_alloc();  (v4l2.dev.c程序中)

　　　　　　　　　　 vdev->cdev->ops = &v4l2_fops;

　　　　　　　　　　 cdev_add(vdev->cdev, MKDEV(VIDEO_MAJOR, vdev->minor), 1);

1617250498978

通过V4L2驱动框架知道如何编写V4L2驱动

分配、设置、注册：v4l2_device —》 v4l2_device_register()（辅助作用，提供自旋锁、引用计数等功能）
分配一个video_device：video_device_alloc()
设置

1）vfd->v4l2_dev

2） .fops 设置vfd的fops 里的open、read、write 被上层调用
　　.ioctl_ops 设置属性被上层调用

3）注册：video_register_device()

接下来，应用层App可以通过ioctl来设置（获得）亮度等某些属性，在驱动程序里，谁来接收、存储、设置到硬件（提供这些信息）？

在驱动程序中抽象出来一个结构体v4l2_ctrl,每个Ctrl对应其中的一项（音量、亮度等等）；

v4l2_ctrl_handler来管理他们，在vivi.c的vivi_create_instance函数中：

1.初始化
v4l2_ctrl_handler_init
2.设置
v4l2_ctrl_new_std
v4l2_ctrl_new_custom
这些函数就是创建各个属性，并且放入v4l2_ctrl_handler的链表
3.跟vdev关联
dev->v4l2_dev.ctrl_handler = hdl;

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_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;  
};

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_RANGES  
    switch (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;  
    }  
#endif  
    vdev->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);  
    }   
}

Linux V4L2 UVC摄像头框架浅析

通过V4L2驱动框架知道如何编写V4L2驱动

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Linux V4L2 UVC摄像头框架浅析

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