Camera V4L2 架构分析

V4L2相关代码位于linux-3.4/drivers/media/video目录中。 v4l2-dev.c文件提供设备节点实现与用户层数据交流，设备节点在/dev/目录下以video0、video1等名字出现。注册字符设备的语句如下： /* Part 3: Initialize the character device */ vdev-cdev = cdev_

V4L2相关代码位于linux-3.4/drivers/media/video目录中。

v4l2-dev.c文件提供设备节点实现与用户层数据交流，设备节点在/dev/目录下以video0、video1等名字出现。注册字符设备的语句如下：

    /* Part 3: Initialize the character device */
    vdev->cdev = cdev_alloc();
    if (vdev->cdev == NULL) {
        ret = -ENOMEM;
        goto cleanup;
    }   
    vdev->cdev->ops = &v4l2_fops;
    vdev->cdev->owner = owner;
    ret = cdev_add(vdev->cdev, MKDEV(VIDEO_MAJOR, vdev->minor), 1); 
    if (ret cdev);
        vdev->cdev = NULL;
        goto cleanup;
    }   

    /* Part 4: register the device with sysfs */
    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);

只要调用函数video_register_device即可把一个video_device注册到V4L2架构中；但这只是一个总纲领，Camera驱动主要是实现V4L2的一个子系统的功能，子系统用一个v4l2_subdev结构体来描述，只有实现好了系统要求的相关函数操作，最后一步才是注册到V4L2中。

子系统实现的方式每个平台都有差别，这里分析的是全志A23平台的代码。

在sunxi-vfe/vfe.c文件中，以platform形式注册了前后摄像头的平台资源。匹配过程忽略，最终vfe_probe函数会被调用。在probe中，看到有函数v4l2_i2c_new_subdev_board：

    /* Create the i2c client */
    if (info->addr == 0 && probe_addrs)
        client = i2c_new_probed_device(adapter, info, probe_addrs,
                           NULL);
    else
        client = i2c_new_device(adapter, info);
...............................
    /* Register with the v4l2_device which increases the module's
       use count as well. */
    if (v4l2_device_register_subdev(v4l2_dev, sd))                                                                                                             
        sd = NULL;

这里的client获得在之前写的I2C驱动的文章已经分析过，可见驱动的知识都是环环相扣的；根据I2C驱动文章的分析可知获取client过程中，device_register(&client->dev)会被调用，而根据device和device_driver的模型关系可知，device所在的bus总线会进行匹配，device处于i2c总线下，根据I2C match函数的匹配，名字相同的device_driver将被匹配中；我们假设i2c_client的name为"ov5460"吧，这样ov5460.c中看看：

static struct i2c_driver sensor_driver = {
  .driver = {
    .owner = THIS_MODULE,
  .name = SENSOR_NAME,
  },
  .probe = sensor_probe,                                                                                                                                       
  .remove = sensor_remove,
  .id_table = sensor_id,
};

毫无疑问，sensor_probe函数会被调用了：

static int sensor_probe(struct i2c_client *client,                                                                                                             
      const struct i2c_device_id *id)
{
  struct v4l2_subdev *sd;
  struct sensor_info *info;
//  int ret;

  info = kzalloc(sizeof(struct sensor_info), GFP_KERNEL);
  if (info == NULL)
    return -ENOMEM;
  sd = &info->sd;
  glb_sd = sd;
  v4l2_i2c_subdev_init(sd, client, &sensor_ops);

  info->fmt = &sensor_formats[0];
  info->af_first_flag = 1;
  info->init_first_flag = 1;
  info->auto_focus = 0;

  return 0;
}

这里就看到了前面提到的v4l2_subdev结构体，v4l2_i2c_subdev_init函数会进入v4l2-subdev.c进行一系列初始化操作，并且用i2c_set_clientdata(client, sd);保存子系统指针，以便后续取出。这里sensor_ops结构体即为子系统支持的类型：

static const struct v4l2_subdev_ops sensor_ops = {                                                                                                             
  .core = &sensor_core_ops,
  .video = &sensor_video_ops,
};

Camera当然是是video的了，core是核心，应该是不可少的操作吧。其实v4l2_subdev所支持的类型很多，其全部类型定义如下：

struct v4l2_subdev_ops {                                                                                                                                       
    const struct v4l2_subdev_core_ops   *core;
    const struct v4l2_subdev_tuner_ops  *tuner;
    const struct v4l2_subdev_audio_ops  *audio;
    const struct v4l2_subdev_video_ops  *video;
    const struct v4l2_subdev_vbi_ops    *vbi;
    const struct v4l2_subdev_ir_ops     *ir;
    const struct v4l2_subdev_sensor_ops *sensor;
    const struct v4l2_subdev_pad_ops    *pad;
};

tuner应该属于电视一类的吧，还有ir，红外；audio，音频等。看看video要实现的操作：

struct v4l2_subdev_video_ops {
    int (*s_routing)(struct v4l2_subdev *sd, u32 input, u32 output, u32 config);
    int (*s_crystal_freq)(struct v4l2_subdev *sd, u32 freq, u32 flags);
    int (*s_std_output)(struct v4l2_subdev *sd, v4l2_std_id std);
    int (*g_std_output)(struct v4l2_subdev *sd, v4l2_std_id *std);
    int (*querystd)(struct v4l2_subdev *sd, v4l2_std_id *std);
    int (*g_tvnorms_output)(struct v4l2_subdev *sd, v4l2_std_id *std);
    int (*g_input_status)(struct v4l2_subdev *sd, u32 *status);
    int (*s_stream)(struct v4l2_subdev *sd, int enable);
    int (*cropcap)(struct v4l2_subdev *sd, struct v4l2_cropcap *cc);
    int (*g_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop);
    int (*s_crop)(struct v4l2_subdev *sd, struct v4l2_crop *crop);                                                                                             
    int (*g_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param);
    int (*s_parm)(struct v4l2_subdev *sd, struct v4l2_streamparm *param);
    int (*g_frame_interval)(struct v4l2_subdev *sd,
                struct v4l2_subdev_frame_interval *interval);
    int (*s_frame_interval)(struct v4l2_subdev *sd,
                struct v4l2_subdev_frame_interval *interval);
    int (*enum_framesizes)(struct v4l2_subdev *sd, struct v4l2_frmsizeenum *fsize);
    int (*enum_frameintervals)(struct v4l2_subdev *sd, struct v4l2_frmivalenum *fival);
    int (*enum_dv_presets) (struct v4l2_subdev *sd,
            struct v4l2_dv_enum_preset *preset);
    int (*s_dv_preset)(struct v4l2_subdev *sd,
            struct v4l2_dv_preset *preset);
    int (*g_dv_preset)(struct v4l2_subdev *sd,
            struct v4l2_dv_preset *preset);
    int (*query_dv_preset)(struct v4l2_subdev *sd,
            struct v4l2_dv_preset *preset);
    int (*s_dv_timings)(struct v4l2_subdev *sd,
            struct v4l2_dv_timings *timings);
    int (*g_dv_timings)(struct v4l2_subdev *sd,
            struct v4l2_dv_timings *timings);
    int (*enum_mbus_fmt)(struct v4l2_subdev *sd, unsigned int index,
                 enum v4l2_mbus_pixelcode *code);
    int (*enum_mbus_fsizes)(struct v4l2_subdev *sd,
                 struct v4l2_frmsizeenum *fsize);
    int (*g_mbus_fmt)(struct v4l2_subdev *sd,
              struct v4l2_mbus_framefmt *fmt);
    int (*try_mbus_fmt)(struct v4l2_subdev *sd,
                struct v4l2_mbus_framefmt *fmt);
    int (*s_mbus_fmt)(struct v4l2_subdev *sd,
              struct v4l2_mbus_framefmt *fmt);
    int (*g_mbus_config)(struct v4l2_subdev *sd,
                 struct v4l2_mbus_config *cfg);
    int (*s_mbus_config)(struct v4l2_subdev *sd,
                 const struct v4l2_mbus_config *cfg);
};

太多了，这就是一个具体摄像头主要实现的操作，而ov5640只是支持其中一部分操作而已：

static const struct v4l2_subdev_core_ops sensor_core_ops = {
  .g_chip_ident = sensor_g_chip_ident,
  .g_ctrl = sensor_g_ctrl,
  .s_ctrl = sensor_s_ctrl,
  .queryctrl = sensor_queryctrl,
  .reset = sensor_reset,
  .init = sensor_init,
  .s_power = sensor_power,
  .ioctl = sensor_ioctl,
};

static const struct v4l2_subdev_video_ops sensor_video_ops = {
  .enum_mbus_fmt = sensor_enum_fmt,                                                                                                                            
  .enum_framesizes = sensor_enum_size,
  .try_mbus_fmt = sensor_try_fmt,
  .s_mbus_fmt = sensor_s_fmt,
  .s_parm = sensor_s_parm,
  .g_parm = sensor_g_parm,
  .g_mbus_config = sensor_g_mbus_config,
};

这样回到v4l2_i2c_new_subdev_board函数中，调用v4l2_device_register_subdev把sd注册到了子系统中去。

回到vfe.c的probe函数中。常用到的操作是v4l2_subdev_call，其实这是一个宏：

#define v4l2_subdev_call(sd, o, f, args...)             \                                                                                                      
    (!(sd) ? -ENODEV : (((sd)->ops->o && (sd)->ops->o->f) ? \
        (sd)->ops->o->f((sd) , ##args) : -ENOIOCTLCMD))

好处在于检查子系统是否支持某个操作，比如上述ov5640的，只支持sensor_core_ops和sensor_video_ops所定义的操作。其他的返回error。

最后：

vfd = video_device_alloc();
*vfd = vfe_template;
ret = video_register_device(vfd, VFL_TYPE_GRABBER, dev->id);

分配video_device空间并赋值，赋值内容如下：

static struct video_device vfe_template = {                                                                                                                    
  .name       = "vfe",
  .fops       = &vfe_fops,
  .ioctl_ops  = &vfe_ioctl_ops,
  .release    = video_device_release,
};

最后注册到了V4L2系统中。

如果从上层ioctl调用开始分析，vdev->fops->ioctl(filp, cmd, arg);调用的就是这里的vfe_fops：

static const struct v4l2_file_operations vfe_fops = {
  .owner          = THIS_MODULE,
  .open           = vfe_open,
  .release        = vfe_close,
  .read           = vfe_read,
  .poll           = vfe_poll,
  .ioctl          = video_ioctl2,                                                                                                                              
  //.unlocked_ioctl = 
  .mmap           = vfe_mmap,
};  

video_ioctl2是要调用v4l2-ioctl.c通用的ioctl来处理，它会检查用户空间参数时候合法等操作。在该文件中调用__video_do_ioctl函数，包含了所有V4L2支持的ioctl操作命令，函数很长，代码就不贴出来了；在该ioctl中最终才调用vfe_ioctl_ops的操作：

static const struct v4l2_ioctl_ops vfe_ioctl_ops = {                                                                                                           
  .vidioc_querycap          = vidioc_querycap,
  .vidioc_enum_fmt_vid_cap  = vidioc_enum_fmt_vid_cap,
  .vidioc_enum_framesizes   = vidioc_enum_framesizes,
  .vidioc_g_fmt_vid_cap     = vidioc_g_fmt_vid_cap,
  .vidioc_try_fmt_vid_cap   = vidioc_try_fmt_vid_cap,
  .vidioc_s_fmt_vid_cap     = vidioc_s_fmt_vid_cap,
  .vidioc_reqbufs           = vidioc_reqbufs,
  .vidioc_querybuf          = vidioc_querybuf,
  .vidioc_qbuf              = vidioc_qbuf,
  .vidioc_dqbuf             = vidioc_dqbuf,
  .vidioc_enum_input        = vidioc_enum_input,
  .vidioc_g_input           = vidioc_g_input,
  .vidioc_s_input           = vidioc_s_input,
  .vidioc_streamon          = vidioc_streamon,
  .vidioc_streamoff         = vidioc_streamoff,
  .vidioc_queryctrl         = vidioc_queryctrl,
  .vidioc_g_ctrl            = vidioc_g_ctrl,
  .vidioc_s_ctrl            = vidioc_s_ctrl,
  .vidioc_g_parm            = vidioc_g_parm,
  .vidioc_s_parm            = vidioc_s_parm,
#ifdef CONFIG_VIDEO_V4L1_COMPAT
  .vidiocgmbuf              = vidiocgmbuf,
#endif
};

而在这些通用的函数操作用，最终用v4l2_subdev_call来调用具体的Camera操作如ov5640