V4L2简介_v4l2是什么

3.1 关系绑定

提到关系绑定，就必须介绍下V4L2几个重要结构体。

• struct video_device：主要的任务就是负责向内核注册字符设备
• struct v4l2_device：一个硬件设备可能包含多个子设备，比如一个电视卡除了有capture设备，可能还有VBI设备或者FM tunner。而v4l2_device就是所有这些设备的根节点，负责管理所有的子设备。
• struct v4l2_subdev：子设备，负责实现具体的功能。

v4l2_device,v4l2_subdev可以看作所有设备和子设备的基类。我们在编写自己的驱动时，往往需要继承这些设备基类，添加一些自己的数据成员。例如第三章要讲到的soc_camera_host结构体，就是继承v4l2_device，并添加了互斥锁、子设备列表等成员变量。

v4l2 framework 简略版

绑定的基本流程

• 根据需要”重载”v4l2_device或v4l2_subdev结构体，添加需要的结构体成员。例如 ：

  • linux/include/media/soc_camera.h文件中soc_camera_host重载了v4l2_device：

    struct soc_camera_host {
    struct v4l2_device v4l2_dev;
    struct list_head list;
    struct mutex host_lock;         /* Protect during probing */
    unsigned char nr;               /* Host number */
    void *priv;
    const char *drv_name;
    struct soc_camera_host_ops *ops;
    };
    

  • linux/drivers/media/video/Ml86v7667.c中ml86v7667_priv结构体”重载”了v4l2_subdev：

      struct ml86v7667_priv {
          struct v4l2_subdev                sd;
          struct v4l2_ctrl_handler  hdl;
          v4l2_std_id                       std;
      };
    

• v4l2_device与V4L2框架的绑定：通过调用v4l2_device_register函数实现。例如，上面提到的soc_camera_host的绑定：

  int soc_camera_host_register(struct soc_camera_host *ici)
  {
        struct soc_camera_host *ix;
        int ret;
  
        if (!ici || !ici->ops ||
          !ici->ops->try_fmt ||
          !ici->ops->set_fmt ||
          !ici->ops->set_bus_param ||
          !ici->ops->querycap ||
          ((!ici->ops->init_videobuf ||
          !ici->ops->reqbufs) &&
          !ici->ops->init_videobuf2) ||
          !ici->ops->add ||
          !ici->ops->remove ||
          !ici->ops->poll ||
          !ici->v4l2_dev.dev)
            return -EINVAL;
  
        if (!ici->ops->set_crop)
            ici->ops->set_crop = default_s_crop;
        if (!ici->ops->get_crop)
            ici->ops->get_crop = default_g_crop;
        if (!ici->ops->cropcap)
            ici->ops->cropcap = default_cropcap;
        if (!ici->ops->set_parm)
            ici->ops->set_parm = default_s_parm;
        if (!ici->ops->get_parm)
            ici->ops->get_parm = default_g_parm;
        if (!ici->ops->enum_fsizes)
            ici->ops->enum_fsizes = default_enum_fsizes;
  
        mutex_lock(&list_lock);
        list_for_each_entry(ix, &hosts, list) {
            if (ix->nr == ici->nr) {
                ret = -EBUSY;
                goto edevreg;
            }
        }
  
        ret = v4l2_device_register(ici->v4l2_dev.dev, &ici->v4l2_dev);
        if (ret < 0)
            goto edevreg;
  
        list_add_tail(&ici->list, &hosts);
        mutex_unlock(&list_lock);
  
        mutex_init(&ici->host_lock);
        scan_add_host(ici);
  
        return 0;
  
        edevreg:
        mutex_unlock(&list_lock);
        return ret;
    }
  

• v4l2_subdev与v4l2_device的绑定：通过v4l2_device_register_subdev函数，将subdev注册到根节点上。例如：

  static int soc_camera_platform_probe(struct platform_device *pdev)
  {
      struct soc_camera_host *ici;
      struct soc_camera_platform_priv *priv;
      struct soc_camera_platform_info *p = pdev->dev.platform_data;
      struct soc_camera_device *icd;
      int ret;
  
      if (!p)
  	return -EINVAL;
  
      if (!p->icd) {
  	    dev_err(&pdev->dev,
  		    "Platform has not set soc_camera_device pointer!\n");
  	    return -EINVAL;
      }
  
      priv = kzalloc(sizeof(*priv), GFP_KERNEL);
      if (!priv)
  	    return -ENOMEM;
  
      icd = p->icd;
  
      /* soc-camera convention: control's drvdata points to the subdev */
      platform_set_drvdata(pdev, &priv->subdev);
      /* Set the control device reference */
      icd->control = &pdev->dev;
  
      ici = to_soc_camera_host(icd->parent);
  
      v4l2_subdev_init(&priv->subdev, &platform_subdev_ops);
      v4l2_set_subdevdata(&priv->subdev, p);
      strncpy(priv->subdev.name, dev_name(&pdev->dev), V4L2_SUBDEV_NAME_SIZE);
  
      ret = v4l2_device_register_subdev(&ici->v4l2_dev, &priv->subdev);
      if (ret)
  	    goto evdrs;
  
      return ret;
  
    evdrs:
      platform_set_drvdata(pdev, NULL);
      kfree(priv);
      return ret;
  }
  

• video_device与v4l2_device的绑定：将v4l2_device的地址赋值给video_device的v4l2_dev即可。

3.2 函数绑定

在v4l2 framework 简略版图中，绿色的方框都是需要我们绑定并实现的。

其中v4l2_file_operations和v4l2_ioctl_ops是必须实现的。而v4l2_subdev_ops下的八类ops中，v4l2_subdev_core_ops是必须实现的，其余需要根据设备类型选择实现的。比如video capture类设备需要实现v4l2_subdev_core_ops, v4l2_subdev_video_ops。

• v4l2_file_operations：实现文件类操作，比如open,close,read,write,mmap等。但是ioctl是不需要实现的，一般都是用video_ioctl2代替。例如linux/drivers/media/video/soc_camera.c文件中soc_camera_fops的实现：

static struct v4l2_file_operations soc_camera_fops = {
    .owner          = THIS_MODULE,
    .open           = soc_camera_open,
    .release        = soc_camera_close,
    .unlocked_ioctl = video_ioctl2,
    .read           = soc_camera_read,
    .mmap           = soc_camera_mmap,
    .poll           = soc_camera_poll,
};

• v4l2_ioctl_ops：V4L2导出给应用层使用的所有ioctl都是在这个地方实现的。但不必全部实现，只实现自己相关的ioctl即可。例如linux/drivers/media/video/soc_camera.c中soc_camera_ioctl_ops的实现：

static const struct v4l2_ioctl_ops soc_camera_ioctl_ops = {
    .vidioc_querycap         = soc_camera_querycap,
    .vidioc_try_fmt_vid_cap  = soc_camera_try_fmt_vid_cap,
    .vidioc_g_fmt_vid_cap    = soc_camera_g_fmt_vid_cap,
    .vidioc_s_fmt_vid_cap    = soc_camera_s_fmt_vid_cap,
    .vidioc_enum_fmt_vid_cap = soc_camera_enum_fmt_vid_cap,
    .vidioc_enum_input       = soc_camera_enum_input,
    .vidioc_g_input          = soc_camera_g_input,
    .vidioc_s_input          = soc_camera_s_input,
    .vidioc_s_std            = soc_camera_s_std,
    .vidioc_g_std            = soc_camera_g_std,
    .vidioc_enum_framesizes  = soc_camera_enum_fsizes,
    .vidioc_reqbufs          = soc_camera_reqbufs,
    .vidioc_querybuf         = soc_camera_querybuf,
    .vidioc_qbuf             = soc_camera_qbuf,
    .vidioc_dqbuf            = soc_camera_dqbuf,
    .vidioc_create_bufs      = soc_camera_create_bufs,
    .vidioc_prepare_buf      = soc_camera_prepare_buf,
    .vidioc_streamon         = soc_camera_streamon,
    .vidioc_streamoff        = soc_camera_streamoff,
    .vidioc_cropcap          = soc_camera_cropcap,
    .vidioc_g_crop           = soc_camera_g_crop,
    .vidioc_s_crop           = soc_camera_s_crop,
    .vidioc_g_parm           = soc_camera_g_parm,
    .vidioc_s_parm           = soc_camera_s_parm,
    .vidioc_g_chip_ident     = soc_camera_g_chip_ident,
#ifdef CONFIG_VIDEO_ADV_DEBUG
    .vidioc_g_register       = soc_camera_g_register,
    .vidioc_s_register       = soc_camera_s_register,
#endif
};

• v4l2_subdev_ops：v4l2_subdev有可能需要实现的ops的总合。分为8类，core,audio,video,vbi,tuner......等。例如，

  linuxdriversmediavideosoc_camera_platform.c中platform_subdev_ops的实现

static struct v4l2_subdev_video_ops platform_subdev_video_ops = {
    .s_stream       = soc_camera_platform_s_stream,
    .enum_mbus_fmt  = soc_camera_platform_enum_fmt,
    .cropcap        = soc_camera_platform_cropcap,
    .g_crop         = soc_camera_platform_g_crop,
    .try_mbus_fmt   = soc_camera_platform_fill_fmt,
    .g_mbus_fmt     = soc_camera_platform_fill_fmt,
    .s_mbus_fmt     = soc_camera_platform_fill_fmt,
    .g_mbus_config  = soc_camera_platform_g_mbus_config,
};

static struct v4l2_subdev_ops platform_subdev_ops = {
    .core   = &platform_subdev_core_ops,
    .video  = &platform_subdev_video_ops,
};

函数绑定只是将驱动所实现的函数赋值给相关的变量即可。