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 < 0) {
printk(KERN_ERR "%s: cdev_add failed\n", __func__);
kfree(vdev->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))
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