sensor的驱动
v4l2_i2c_new_subdev_board先用client = i2c_new_device(adapter, info);创建info对应的i2c_client对象（代表着一个i2c client），并进行驱动匹配。匹配就会触发i2c sensor驱动的probe调用。现在进入到目录drivers/media/i2c/soc_camera/，我们还是看OV2640驱动吧，毕竟前面的板级文件里只有在定义了CONFIG_SOC_CAMERA_OV2640宏才会编译进去。
static struct i2c_driver ov2640_i2c_driver = {
.driver = {
.name = "ov2640",
},
.probe = ov2640_probe,
.remove = ov2640_remove,
.id_table = ov2640_id,
};
module_i2c_driver(ov2640_i2c_driver);直接看ov2640_probe。这是一个i2c sensor驱动该做的事情。同样，直接将说明插入到代码中：
static int ov2640_probe(struct i2c_client *client,
const struct i2c_device_id *did)
{
struct ov2640_priv *priv;
struct soc_camera_subdev_desc *ssdd = soc_camera_i2c_to_desc(client);
struct i2c_adapter *adapter = to_i2c_adapter(client->dev.parent);
int ret;
if (!ssdd) {
dev_err(&adapter->dev,
"OV2640: Missing platform_data for driver\n");
return -EINVAL;
}
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA)) {
dev_err(&adapter->dev,
"OV2640: I2C-Adapter doesn't support SMBUS\n");
return -EIO;
}
priv = devm_kzalloc(&client->dev, sizeof(struct ov2640_priv), GFP_KERNEL);
if (!priv) {
dev_err(&adapter->dev,
"Failed to allocate memory for private data!\n");
return -ENOMEM;
}
v4l2_i2c_subdev_init(&priv->subdev, client, &ov2640_subdev_ops);//这里就是初始化v4l2框架提供的v4l2_subdev，前面说了i2c sensor在v4l2框架里是小弟，host才是老大，小弟用v4l2_subdev来描述，老大用v4l2_device来描述。这里仅仅是初始化，怎么和老大绑定起来的事情在前面还没讲完的soc_camera_probe里，后面会讲解
//这部分就是添加该sensor支持的ioctl了，采用v4l2提供的现有机制。我们会发现小弟sensor会有自己的ctrl handler，camera也有自己的ctrl handler，sensor和camera的关系属于包含关系，一般camera都包括了sensor。camera是整体，sensor是部分
v4l2_ctrl_handler_init(&priv->hdl, 2);
v4l2_ctrl_new_std(&priv->hdl, &ov2640_ctrl_ops,
V4L2_CID_VFLIP, 0, 1, 1, 0);
v4l2_ctrl_new_std(&priv->hdl, &ov2640_ctrl_ops,
V4L2_CID_HFLIP, 0, 1, 1, 0);
priv->subdev.ctrl_handler = &priv->hdl;
if (priv->hdl.error)
return priv->hdl.error;
priv->clk = v4l2_clk_get(&client->dev, "mclk");//这里就是获取之前在soc_camera_i2c_init里注册的时钟啦
if (IS_ERR(priv->clk)) {
ret = PTR_ERR(priv->clk);
goto eclkget;
}
ret = ov2640_video_probe(client);//这里就是初始化sensor了，通过i2c总线进行配置，不同的sensor配置方法不一样，这就不继续分析了。不过有一点要说明，就是前面不是注册了很多ioctl么，里面都会有默认值，但是分析到现在（也就是设备启动到现在），硬件还没有初始化到默认值，于是在ov2640_video_probe里调用v4l2_ctrl_handler_setup来将所有的ioctl执行一遍，进行初始化
if (ret) {
v4l2_clk_put(priv->clk);
eclkget:
v4l2_ctrl_handler_free(&priv->hdl);
} else {
dev_info(&adapter->dev, "OV2640 Probed\n");
}
return ret;
}从这里我们可以看到ov2640_probe主要就是分配了自己的数据结构，当然里面嵌入了嵌入v4l2框架的v4l2_subdev以及自己ioctl的支持v4l2_ctrl_handler等数据结构，并对这些数据结构进行相应的初始化以及将其绑定到i2c_client。ov2640_probe执行完后，我们前面的i2c_new_device就可以返回了。v4l2_i2c_new_subdev_board在执行完i2c_new_device后，用sd = i2c_get_clientdata(client);获取i2c sensor驱动里自己的数据结构里的v4l2_subdev。并调用v4l2_device_register_subdev将v4l2_subdev与老大v4l2_device进行绑定。绑定的过程中，会将sensor里的所有的ioctl操作添加到icd的ctrl里去。这样icd导出给应用的设备文件的某些ioctl的操作，会引起i2c sensor的ioctl也可以得到调用到。
分析到这里，基本就结束了。总结一下，camera通过通用的camera驱动将其先放到全局的链表中，然后camera host驱动会去探测属于它的camera驱动，然后将camera添加进来。添加的过程中，会触发camera里的sensor驱动进行匹配并初始化sensor，同时会将sensor驱动里的ioctl操作集合通过v4l2提供的ctrl机制添加到camera里来，最终通过video_device字符设备导出给应用层操作。下面通过几个情景来进一步了解内部的整个流程
导出给应用层的设备文件相关信息
前文已经说过，soc_camera_probe里面会调用video_dev_create创建并初始化了video_device并通过soc_camera_probe_finish间接(调用soc_camera_video_start->video_register_device)注册了video_device，通过__video_register_device分析可以知道，设备名及设备后缀的确立是根据类别以及注册的先后顺序来的(应用层通过设备节点/dev/videoX打开video4linux devices。/dev/videoX是一个字符设备，主设备号81，次设备号： (0～63)分配给capture设备，64~127分配给radio设备，223~255分配给VBI设备，128~191分配给其他类型的)。需要注意里面有一句vdev->dev_parent = vdev->v4l2_dev->dev;也就是说video_device的父是v4l2_device。还需要注意的是里面有一句vdev->ctrl_handler = vdev->v4l2_dev->ctrl_handler;它是不会执行的，因为在video_dev_create里已经将其初始化了（vdev->ctrl_handler = &icd->ctrl_handler;），前文也已经说明，v4l2_device的ctrl集合同时包含了i2c sensor的ctrl集合。还需要注意注册的时候操作集合设置为v4l2_fops，后面分析会用到它。
open操作流程
open的设备文件当然就是上面小节所说的设备文件。open操作最终会导向到字符设备注册时提供的ops，在这里也就是v4l2_fops。因此我们直接分析v4l2_fops里面的open吧！至于怎么导向到这里的过程，属于字符设备驱动范畴，这里不进行说明。先贴一下v4l2_open代码：
static int v4l2_open(struct inode *inode, struct file *filp)
{
struct video_device *vdev;
int ret = 0;
/* Check if the video device is available */
mutex_lock(&videodev_lock);
vdev = video_devdata(filp);
/* return ENODEV if the video device has already been removed. */
if (vdev == NULL || !video_is_registered(vdev)) {
mutex_unlock(&videodev_lock);
return -ENODEV;
}
/* and increase the device refcount */
video_get(vdev);
mutex_unlock(&videodev_lock);
if (vdev->fops->open) {
if (video_is_registered(vdev))
ret = vdev->fops->open(filp);
else
ret = -ENODEV;
}
if (vdev->debug)
printk(KERN_DEBUG "%s: open (%d)\n",
video_device_node_name(vdev), ret);
/* decrease the refcount in case of an error */
if (ret)
video_put(vdev);
return ret;
}
先通过video_devdata拿到video_device对象指针。这个内部实现是通过video_device注册的时候，会根据子设备号为索引将其放入到一个全局的video_device数组中，因此v4l2_open的时候，只需要根据当前的设备文件的子设备号为索引再到video_device里取出来即可。v4l2_open的核心操作还是通过：
if (vdev->fops->open) {
if (video_is_registered(vdev))
ret = vdev->fops->open(filp);
else
ret = -ENODEV;
}
将open的操作最终导向到video_device的回调函数集合里的open中去，其实就是回调到通用设备驱动实现的open。该回调操作集合在video_dev_create里面初始化为soc_camera_fops。因此最终调用到了soc_camera_fops里面的open，即soc_camera_open。它主要做了两件事情，第一，file->private_data = icd;将icd存放file->private_data中，这样在之后的read、write、ioctl时可以直接从private_data拿icd了，免去了video_get_drvdata获取的麻烦。第二，这件事只在第一次open该设备文件的时候会调用，主要代码如下：
/* Now we really have to activate the camera */
if (icd->use_count == 1) {
struct soc_camera_desc *sdesc = to_soc_camera_desc(icd);
/* Restore parameters before the last close() per V4L2 API */
struct v4l2_format f = {
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
.fmt.pix = {
.width = icd->user_width,
.height = icd->user_height,
.field = icd->field,
.colorspace = icd->colorspace,
.pixelformat =
icd->current_fmt->host_fmt->fourcc,
},
};
/* The camera could have been already on, try to reset */
if (sdesc->subdev_desc.reset)
sdesc->subdev_desc.reset(icd->pdev);
ret = soc_camera_add_device(icd);
if (ret < 0) {
dev_err(icd->pdev, "Couldn't activate the camera: %d\n", ret);
goto eiciadd;
}
ret = __soc_camera_power_on(icd);
if (ret < 0)
goto epower;
pm_runtime_enable(&icd->vdev->dev);
ret = pm_runtime_resume(&icd->vdev->dev);
if (ret < 0 && ret != -ENOSYS)
goto eresume;
/*
* Try to configure with default parameters. Notice: this is the
* very first open, so, we cannot race against other calls,
* apart from someone else calling open() simultaneously, but
* .host_lock is protecting us against it.
*/
ret = soc_camera_set_fmt(icd, &f);
if (ret < 0)
goto esfmt;
if (ici->ops->init_videobuf) {
ici->ops->init_videobuf(&icd->vb_vidq, icd);
} else {
ret = ici->ops->init_videobuf2(&icd->vb2_vidq, icd);
if (ret < 0)
goto einitvb;
}
v4l2_ctrl_handler_setup(&icd->ctrl_handler);
}看第一行注释其实就知道它是要做什么了，主要是激活设备，所谓的设备就是camera啦。首先复位设备，这个在板级相关文件里通过camera_link来实现reset的回调，可能会为NULL。其次，调用soc_camera_add_device来开启clk及host的add回调。然后调用__soc_camera_power_on来让i2c sensor控制power on，然后就是最重要的，调用ici->ops->init_videobuf或者ici->ops->init_videobuf2，即host实现的init_videobuf来初始化video buffer queue，最后调用v4l2_ctrl_handler_setup来将所有的ioctl执行一遍，进行初始化，这里用icd的ctrl集合。总的来说，open操作所做的主要是各种初始化准备工作，过程中会回调host及i2c sensor驱动实现的一些api。
close操作
close操作和open操作类似，最终会回调到soc_camera_close，它主要是做一些去初始化操作，不过因此可能多次打开了设备文件，所以它只在使用技术为0的时候彻底去初始化open所做的工作。
ioctl操作
这部分是应用操作的关键。应用层透过ioctl并基于v4l2提供的支持，来配置设备。对应到这里，就是配置sensor和camera host以及v4l2相关的东西。先从网上引入一张v4l2 ioctl相关的图片，好有个大体的了解：
下面从正常使用v4l2获取视频的流程来分析ioctl
一般在open设备文件后，我们会先获取该设备的能力集合，通过如下代码（以下所有代码仅仅是为了展示，正式代码应该要做出错检查什么的哈）
Struct v4l2_capability cap;
ioctl(fd, VIDIOC_QUERYCAP, &cap); 这里使用v4l2头文件里定义的命令宏VIDIOC_QUERYCAP来获取设备的能力集合。我们看看获取的过程吧！和open操作类似，ioctl操作最终会导向到字符设备注册时提供的v4l2_fops。因此我们直接分析v4l2_fops里面的v4l2_ioctl吧！同样，采用代码行内注释方式：
static long v4l2_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
struct video_device *vdev = video_devdata(filp);
int ret = -ENODEV;
if (vdev->fops->unlocked_ioctl) {//如果video_device的回调函数集合里有实现unlocked_ioctl，那么进入这个里面，通用驱动
//soc_camera实现了unlocked_ioctl，即video_ioctl2。因此，我们会进入到这个里面
struct mutex *lock = v4l2_ioctl_get_lock(vdev, cmd);
if (lock && mutex_lock_interruptible(lock))
return -ERESTARTSYS;
if (video_is_registered(vdev))
ret = vdev->fops->unlocked_ioctl(filp, cmd, arg);//这里就是调用通用驱动soc_camera实现的video_ioctl2了，后面会详细分析这个函数
if (lock)
mutex_unlock(lock);
} else if (vdev->fops->ioctl) {
/* This code path is a replacement for the BKL. It is a major
* hack but it will have to do for those drivers that are not
* yet converted to use unlocked_ioctl.
*
* There are two options: if the driver implements struct
* v4l2_device, then the lock defined there is used to
* serialize the ioctls. Otherwise the v4l2 core lock defined
* below is used. This lock is really bad since it serializes
* completely independent devices.
*
* Both variants suffer from the same problem: if the driver
* sleeps, then it blocks all ioctls since the lock is still
* held. This is very common for VIDIOC_DQBUF since that
* normally waits for a frame to arrive. As a result any other
* ioctl calls will proceed very, very slowly since each call
* will have to wait for the VIDIOC_QBUF to finish. Things that
* should take 0.01s may now take 10-20 seconds.
*
* The workaround is to *not* take the lock for VIDIOC_DQBUF.
* This actually works OK for videobuf-based drivers, since
* videobuf will take its own internal lock.
*/
static DEFINE_MUTEX(v4l2_ioctl_mutex);
struct mutex *m = vdev->v4l2_dev ?
&vdev->v4l2_dev->ioctl_lock : &v4l2_ioctl_mutex;
if (cmd != VIDIOC_DQBUF && mutex_lock_interruptible(m))
return -ERESTARTSYS;
if (video_is_registered(vdev))
ret = vdev->fops->ioctl(filp, cmd, arg);
if (cmd != VIDIOC_DQBUF)
mutex_unlock(m);
} else
ret = -ENOTTY;
return ret;
}
现在继续看video_ioctl2：
long video_ioctl2(struct file *file,
unsigned int cmd, unsigned long arg)
{
return video_usercopy(file, cmd, arg, __video_do_ioctl);
}这个函数简单的不能再简单了，不过之所以简单，是因为video_usercopy帮忙做了很多事情。它实现了用户空间和内核空间传输传递的实现，因此在__video_do_ioctl不用再考虑这些了。下面继续看__video_do_ioctl，同样，采用代码行内注释方式：
static long __video_do_ioctl(struct file *file,
unsigned int cmd, void *arg)
{
struct video_device *vfd = video_devdata(file);
const struct v4l2_ioctl_ops *ops = vfd->ioctl_ops;
bool write_only = false;
struct v4l2_ioctl_info default_info;
const struct v4l2_ioctl_info *info;
void *fh = file->private_data;
struct v4l2_fh *vfh = NULL;
int use_fh_prio = 0;
int debug = vfd->debug;
long ret = -ENOTTY;
if (ops == NULL) {
pr_warn("%s: has no ioctl_ops.\n",
video_device_node_name(vfd));
return ret;
}
if (test_bit(V4L2_FL_USES_V4L2_FH, &vfd->flags)) {//这个不会进入，因为soc_camera根本就没设置它，所以直接忽略吧
vfh = file->private_data;
use_fh_prio = test_bit(V4L2_FL_USE_FH_PRIO, &vfd->flags);
}
if (v4l2_is_known_ioctl(cmd)) {//这里是判断是不是已知的ioctl命令调用，我们后面会看看，都有哪些已知调用
info = &v4l2_ioctls[_IOC_NR(cmd)];
if (!test_bit(_IOC_NR(cmd), vfd->valid_ioctls) &&//检测ioctl是否有效
!((info->flags & INFO_FL_CTRL) && vfh && vfh->ctrl_handler))
goto done;
if (use_fh_prio && (info->flags & INFO_FL_PRIO)) {//优先级检测，暂时也忽略吧
ret = v4l2_prio_check(vfd->prio, vfh->prio);
if (ret)
goto done;
}
} else {//如果不是已知ioctl调用
default_info.ioctl = cmd;
default_info.flags = 0;
default_info.debug = v4l_print_default;
info = &default_info;
}
write_only = _IOC_DIR(cmd) == _IOC_WRITE;
if (info->flags & INFO_FL_STD) {//如果设置了INFO_FL_STD（后面分析v4l2_is_known_ioctl的时候，会知道哪些ioctl会设置它，哪些不会设置）
typedef int (*vidioc_op)(struct file *file, void *fh, void *p);
const void *p = vfd->ioctl_ops;
const vidioc_op *vidioc = p + info->u.offset;
ret = (*vidioc)(file, fh, arg);
} else if (info->flags & INFO_FL_FUNC) {//如果设置了INFO_FL_FUNC（后面分析v4l2_is_known_ioctl的时候，会知道哪些ioctl会设置它，哪些不会设置）
ret = info->u.func(ops, file, fh, arg);
} else if (!ops->vidioc_default) {
ret = -ENOTTY;
} else {//如果不是已知ioctl且ops->vidioc_default不为NULL
ret = ops->vidioc_default(file, fh,
use_fh_prio ? v4l2_prio_check(vfd->prio, vfh->prio) >= 0 : 0,
cmd, arg);
}
done:
if (debug) {
v4l_printk_ioctl(video_device_node_name(vfd), cmd);
if (ret < 0)
pr_cont(": error %ld", ret);
if (debug == V4L2_DEBUG_IOCTL)
pr_cont("\n");
else if (_IOC_DIR(cmd) == _IOC_NONE)
info->debug(arg, write_only);
else {
pr_cont(": ");
info->debug(arg, write_only);
}
}
return ret;
}下面看看v4l2_is_known_ioctl吧！继续代码内注释，你懂得！
#define IOCTL_INFO_STD(_ioctl, _vidioc, _debug, _flags) \
[_IOC_NR(_ioctl)] = { \
.ioctl = _ioctl, \
.flags = _flags | INFO_FL_STD, \
.name = #_ioctl, \
.u.offset = offsetof(struct v4l2_ioctl_ops, _vidioc), \
.debug = _debug, \
}
#define IOCTL_INFO_FNC(_ioctl, _func, _debug, _flags) \
[_IOC_NR(_ioctl)] = { \
.ioctl = _ioctl, \
.flags = _flags | INFO_FL_FUNC, \
.name = #_ioctl, \
.u.func = _func, \
.debug = _debug, \
}
static struct v4l2_ioctl_info v4l2_ioctls[] = {
IOCTL_INFO_FNC(VIDIOC_QUERYCAP, v4l_querycap, v4l_print_querycap, 0),
IOCTL_INFO_FNC(VIDIOC_ENUM_FMT, v4l_enum_fmt, v4l_print_fmtdesc, INFO_FL_CLEAR(v4l2_fmtdesc, type)),
IOCTL_INFO_FNC(VIDIOC_G_FMT, v4l_g_fmt, v4l_print_format, INFO_FL_CLEAR(v4l2_format, type)),
IOCTL_INFO_FNC(VIDIOC_S_FMT, v4l_s_fmt, v4l_print_format, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_REQBUFS, v4l_reqbufs, v4l_print_requestbuffers, INFO_FL_PRIO | INFO_FL_QUEUE),
IOCTL_INFO_FNC(VIDIOC_QUERYBUF, v4l_querybuf, v4l_print_buffer, INFO_FL_QUEUE | INFO_FL_CLEAR(v4l2_buffer, length)),
IOCTL_INFO_STD(VIDIOC_G_FBUF, vidioc_g_fbuf, v4l_print_framebuffer, 0),
IOCTL_INFO_STD(VIDIOC_S_FBUF, vidioc_s_fbuf, v4l_print_framebuffer, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_OVERLAY, v4l_overlay, v4l_print_u32, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_QBUF, v4l_qbuf, v4l_print_buffer, INFO_FL_QUEUE),
IOCTL_INFO_STD(VIDIOC_EXPBUF, vidioc_expbuf, v4l_print_exportbuffer, INFO_FL_QUEUE | INFO_FL_CLEAR(v4l2_exportbuffer, flags)),
IOCTL_INFO_FNC(VIDIOC_DQBUF, v4l_dqbuf, v4l_print_buffer, INFO_FL_QUEUE),
IOCTL_INFO_FNC(VIDIOC_STREAMON, v4l_streamon, v4l_print_buftype, INFO_FL_PRIO | INFO_FL_QUEUE),
IOCTL_INFO_FNC(VIDIOC_STREAMOFF, v4l_streamoff, v4l_print_buftype, INFO_FL_PRIO | INFO_FL_QUEUE),
IOCTL_INFO_FNC(VIDIOC_G_PARM, v4l_g_parm, v4l_print_streamparm, INFO_FL_CLEAR(v4l2_streamparm, type)),
IOCTL_INFO_FNC(VIDIOC_S_PARM, v4l_s_parm, v4l_print_streamparm, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_STD, vidioc_g_std, v4l_print_std, 0),
IOCTL_INFO_FNC(VIDIOC_S_STD, v4l_s_std, v4l_print_std, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_ENUMSTD, v4l_enumstd, v4l_print_standard, INFO_FL_CLEAR(v4l2_standard, index)),
IOCTL_INFO_FNC(VIDIOC_ENUMINPUT, v4l_enuminput, v4l_print_enuminput, INFO_FL_CLEAR(v4l2_input, index)),
IOCTL_INFO_FNC(VIDIOC_G_CTRL, v4l_g_ctrl, v4l_print_control, INFO_FL_CTRL | INFO_FL_CLEAR(v4l2_control, id)),
IOCTL_INFO_FNC(VIDIOC_S_CTRL, v4l_s_ctrl, v4l_print_control, INFO_FL_PRIO | INFO_FL_CTRL),
IOCTL_INFO_FNC(VIDIOC_G_TUNER, v4l_g_tuner, v4l_print_tuner, INFO_FL_CLEAR(v4l2_tuner, index)),
IOCTL_INFO_FNC(VIDIOC_S_TUNER, v4l_s_tuner, v4l_print_tuner, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_AUDIO, vidioc_g_audio, v4l_print_audio, 0),
IOCTL_INFO_STD(VIDIOC_S_AUDIO, vidioc_s_audio, v4l_print_audio, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_QUERYCTRL, v4l_queryctrl, v4l_print_queryctrl, INFO_FL_CTRL | INFO_FL_CLEAR(v4l2_queryctrl, id)),
IOCTL_INFO_FNC(VIDIOC_QUERYMENU, v4l_querymenu, v4l_print_querymenu, INFO_FL_CTRL | INFO_FL_CLEAR(v4l2_querymenu, index)),
IOCTL_INFO_STD(VIDIOC_G_INPUT, vidioc_g_input, v4l_print_u32, 0),
IOCTL_INFO_FNC(VIDIOC_S_INPUT, v4l_s_input, v4l_print_u32, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_EDID, vidioc_g_edid, v4l_print_edid, INFO_FL_CLEAR(v4l2_edid, edid)),
IOCTL_INFO_STD(VIDIOC_S_EDID, vidioc_s_edid, v4l_print_edid, INFO_FL_PRIO | INFO_FL_CLEAR(v4l2_edid, edid)),
IOCTL_INFO_STD(VIDIOC_G_OUTPUT, vidioc_g_output, v4l_print_u32, 0),
IOCTL_INFO_FNC(VIDIOC_S_OUTPUT, v4l_s_output, v4l_print_u32, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_ENUMOUTPUT, v4l_enumoutput, v4l_print_enumoutput, INFO_FL_CLEAR(v4l2_output, index)),
IOCTL_INFO_STD(VIDIOC_G_AUDOUT, vidioc_g_audout, v4l_print_audioout, 0),
IOCTL_INFO_STD(VIDIOC_S_AUDOUT, vidioc_s_audout, v4l_print_audioout, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_G_MODULATOR, v4l_g_modulator, v4l_print_modulator, INFO_FL_CLEAR(v4l2_modulator, index)),
IOCTL_INFO_STD(VIDIOC_S_MODULATOR, vidioc_s_modulator, v4l_print_modulator, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_G_FREQUENCY, v4l_g_frequency, v4l_print_frequency, INFO_FL_CLEAR(v4l2_frequency, tuner)),
IOCTL_INFO_FNC(VIDIOC_S_FREQUENCY, v4l_s_frequency, v4l_print_frequency, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_CROPCAP, v4l_cropcap, v4l_print_cropcap, INFO_FL_CLEAR(v4l2_cropcap, type)),
IOCTL_INFO_FNC(VIDIOC_G_CROP, v4l_g_crop, v4l_print_crop, INFO_FL_CLEAR(v4l2_crop, type)),
IOCTL_INFO_FNC(VIDIOC_S_CROP, v4l_s_crop, v4l_print_crop, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_SELECTION, vidioc_g_selection, v4l_print_selection, 0),
IOCTL_INFO_STD(VIDIOC_S_SELECTION, vidioc_s_selection, v4l_print_selection, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_JPEGCOMP, vidioc_g_jpegcomp, v4l_print_jpegcompression, 0),
IOCTL_INFO_STD(VIDIOC_S_JPEGCOMP, vidioc_s_jpegcomp, v4l_print_jpegcompression, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_QUERYSTD, v4l_querystd, v4l_print_std, 0),
IOCTL_INFO_FNC(VIDIOC_TRY_FMT, v4l_try_fmt, v4l_print_format, 0),
IOCTL_INFO_STD(VIDIOC_ENUMAUDIO, vidioc_enumaudio, v4l_print_audio, INFO_FL_CLEAR(v4l2_audio, index)),
IOCTL_INFO_STD(VIDIOC_ENUMAUDOUT, vidioc_enumaudout, v4l_print_audioout, INFO_FL_CLEAR(v4l2_audioout, index)),
IOCTL_INFO_FNC(VIDIOC_G_PRIORITY, v4l_g_priority, v4l_print_u32, 0),
IOCTL_INFO_FNC(VIDIOC_S_PRIORITY, v4l_s_priority, v4l_print_u32, INFO_FL_PRIO),
IOCTL_INFO_FNC(VIDIOC_G_SLICED_VBI_CAP, v4l_g_sliced_vbi_cap, v4l_print_sliced_vbi_cap, INFO_FL_CLEAR(v4l2_sliced_vbi_cap, type)),
IOCTL_INFO_FNC(VIDIOC_LOG_STATUS, v4l_log_status, v4l_print_newline, 0),
IOCTL_INFO_FNC(VIDIOC_G_EXT_CTRLS, v4l_g_ext_ctrls, v4l_print_ext_controls, INFO_FL_CTRL),
IOCTL_INFO_FNC(VIDIOC_S_EXT_CTRLS, v4l_s_ext_ctrls, v4l_print_ext_controls, INFO_FL_PRIO | INFO_FL_CTRL),
IOCTL_INFO_FNC(VIDIOC_TRY_EXT_CTRLS, v4l_try_ext_ctrls, v4l_print_ext_controls, INFO_FL_CTRL),
IOCTL_INFO_STD(VIDIOC_ENUM_FRAMESIZES, vidioc_enum_framesizes, v4l_print_frmsizeenum, INFO_FL_CLEAR(v4l2_frmsizeenum, pixel_format)),
IOCTL_INFO_STD(VIDIOC_ENUM_FRAMEINTERVALS, vidioc_enum_frameintervals, v4l_print_frmivalenum, INFO_FL_CLEAR(v4l2_frmivalenum, height)),
IOCTL_INFO_STD(VIDIOC_G_ENC_INDEX, vidioc_g_enc_index, v4l_print_enc_idx, 0),
IOCTL_INFO_STD(VIDIOC_ENCODER_CMD, vidioc_encoder_cmd, v4l_print_encoder_cmd, INFO_FL_PRIO | INFO_FL_CLEAR(v4l2_encoder_cmd, flags)),
IOCTL_INFO_STD(VIDIOC_TRY_ENCODER_CMD, vidioc_try_encoder_cmd, v4l_print_encoder_cmd, INFO_FL_CLEAR(v4l2_encoder_cmd, flags)),
IOCTL_INFO_STD(VIDIOC_DECODER_CMD, vidioc_decoder_cmd, v4l_print_decoder_cmd, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_TRY_DECODER_CMD, vidioc_try_decoder_cmd, v4l_print_decoder_cmd, 0),
IOCTL_INFO_FNC(VIDIOC_DBG_S_REGISTER, v4l_dbg_s_register, v4l_print_dbg_register, 0),
IOCTL_INFO_FNC(VIDIOC_DBG_G_REGISTER, v4l_dbg_g_register, v4l_print_dbg_register, 0),
IOCTL_INFO_FNC(VIDIOC_S_HW_FREQ_SEEK, v4l_s_hw_freq_seek, v4l_print_hw_freq_seek, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_S_DV_TIMINGS, vidioc_s_dv_timings, v4l_print_dv_timings, INFO_FL_PRIO),
IOCTL_INFO_STD(VIDIOC_G_DV_TIMINGS, vidioc_g_dv_timings, v4l_print_dv_timings, 0),
IOCTL_INFO_FNC(VIDIOC_DQEVENT, v4l_dqevent, v4l_print_event, 0),
IOCTL_INFO_FNC(VIDIOC_SUBSCRIBE_EVENT, v4l_subscribe_event, v4l_print_event_subscription, 0),
IOCTL_INFO_FNC(VIDIOC_UNSUBSCRIBE_EVENT, v4l_unsubscribe_event, v4l_print_event_subscription, 0),
IOCTL_INFO_FNC(VIDIOC_CREATE_BUFS, v4l_create_bufs, v4l_print_create_buffers, INFO_FL_PRIO | INFO_FL_QUEUE),
IOCTL_INFO_FNC(VIDIOC_PREPARE_BUF, v4l_prepare_buf, v4l_print_buffer, INFO_FL_QUEUE),
IOCTL_INFO_STD(VIDIOC_ENUM_DV_TIMINGS, vidioc_enum_dv_timings, v4l_print_enum_dv_timings, 0),
IOCTL_INFO_STD(VIDIOC_QUERY_DV_TIMINGS, vidioc_query_dv_timings, v4l_print_dv_timings, 0),
IOCTL_INFO_STD(VIDIOC_DV_TIMINGS_CAP, vidioc_dv_timings_cap, v4l_print_dv_timings_cap, INFO_FL_CLEAR(v4l2_dv_timings_cap, type)),
IOCTL_INFO_FNC(VIDIOC_ENUM_FREQ_BANDS, v4l_enum_freq_bands, v4l_print_freq_band, 0),
IOCTL_INFO_FNC(VIDIOC_DBG_G_CHIP_INFO, v4l_dbg_g_chip_info, v4l_print_dbg_chip_info, INFO_FL_CLEAR(v4l2_dbg_chip_info, match)),
};
#define V4L2_IOCTLS ARRAY_SIZE(v4l2_ioctls)
bool v4l2_is_known_ioctl(unsigned int cmd)
{
if (_IOC_NR(cmd) >= V4L2_IOCTLS)//如果该命令对应的数量超过了v4l2_ioctls的大小（linux ioctl
//的cmd有一套机制的，它由方向、类型、序号、负载数据大小组成，所以可以通过cmd提取出该cmd对应的序号），
//那么就不是已知命令咯。v4l2根据视频这类设备的实际情况，默认定义了一些命令
return false;
return v4l2_ioctls[_IOC_NR(cmd)].ioctl == cmd;//进一步判断从cmd里提取出的序号上的成员的cmd与它是否完全相等，相等，才表示确实是已知命令啦
}需要补充下，IOCTL_INFO_FNC定义了INFO_FL_FUNC flag，而IOCTL_INFO_STD定义了INFO_FL_STD flag，不同的命令又会再附加一些flag，比如INFO_FL_PRIO、INFO_FL_CTRL、INFO_FL_QUEUE等等。用IOCTL_INFO_FNC定义的命令条目，存放的是函数指针，而INFO_FL_STD定义的命令条目，存放的是相对于结构体v4l2_ioctl_ops的偏移。v4l2_ioctl_ops相信都布魔人啦，就是video_device设备的ioctl_ops啦，对应到我们这里，就是soc_camera_ioctl_ops。总结一下，有些ioctl，会直接回调v4l2_ioctls数组里面对应条目初始化时填写的函数，有些则会回调video_device设备的ioctl_ops里面的函数，具体哪个函数，则是通过v4l2_ioctls数组里面对应条目初始化时填写的函数偏移决定。
经过上面的层层分析，我们知道：
IOCTL_INFO_FNC(VIDIOC_QUERYCAP, v4l_querycap, v4l_print_querycap, 0),最终会被使用，看到它是用IOCTL_INFO_FNC定义的，于是我们就知道了v4l_querycap函数会被回调。下面看v4l_querycap：
static int v4l_querycap(const struct v4l2_ioctl_ops *ops,
struct file *file, void *fh, void *arg)
{
struct v4l2_capability *cap = (struct v4l2_capability *)arg;
cap->version = LINUX_VERSION_CODE;
return ops->vidioc_querycap(file, fh, cap);
}代码很清晰，其中，ops就是__video_do_ioctl通过ops = vfd->ioctl_ops;从video_device拿到的，对应到我们这里，就是soc_camera_ioctl_ops啦！函数将应用层的ioctl转移到soc_camera_ioctl_ops里面的vidioc_querycap了。对应代码：
static int soc_camera_querycap(struct file *file, void *priv,
struct v4l2_capability *cap)
{
struct soc_camera_device *icd = file->private_data;
struct soc_camera_host *ici = to_soc_camera_host(icd->parent);
WARN_ON(priv != file->private_data);
strlcpy(cap->driver, ici->drv_name, sizeof(cap->driver));
return ici->ops->querycap(ici, cap);
}
看到这里，估计要骂人了。或许都在想，为什么不直接调用啊，这样一层一层回调多麻烦，对吧！这里将ioctl又转移到ici->ops->querycap了。我认为这就是高手与菜鸟之间的差别了。菜鸟写的代码没有层次感，扩展性差，模块之间紧耦合等等。我们看到v4l_querycap属于v4l2架构层，它负责填cap->version，因为这个是它可以完成而别的模块不应该填写的，不然就重复了，对吧。而soc_camera_querycap负责填充了cap->driver，soc_camera_querycap是属于通用soc_camera驱动层，它当然可以填写cap->driver咯，不然每个host驱动都要自己填写cap->driver，重复，对吧！v4l2框架是只与soc_camera打交道的(因此是它使用v4l2机制注册了video_device，v4l2就认video_device)，它不用知道camera host的任何信息，camera host是由soc_camera来负责的。层次是很清晰的。我们继续看ici->ops->querycap，即isi_soc_camera_host_ops里的isi_camera_querycap：
static int isi_camera_querycap(struct soc_camera_host *ici,
struct v4l2_capability *cap)
{
strcpy(cap->driver, "atmel-isi");
strcpy(cap->card, "Atmel Image Sensor Interface");
cap->capabilities = (V4L2_CAP_VIDEO_CAPTURE |
V4L2_CAP_STREAMING);
return 0;
}
设备的能力集，当然只有host最清楚了，所以才由host填capabilities。不过我们看到第一行代码貌似是多余的，也许写该驱动的人没像我这样分析过吧！^_^
分析到这里，总算把VIDIOC_QUERYCAP分析完了。后面分析其他命令的时候，就不会再向上面一样一步一步分析啦，因为都是相同的路线，最多就是这个进if，那个进else，这个直接callback，那个通过偏移callback啦！
在获取VIDIOC_QUERYCAP后，一般会设置视频捕获格式，通过如下代码（以下所有代码仅仅是为了展示，正式代码应该要做出错检查什么的哈）
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = 640;
fmt.fmt.pix.height = 480;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; //像素格式
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
ioctl(fd, VIDIOC_S_FMT, &fmt);直接看IOCTL_INFO_FNC(VIDIOC_S_FMT, v4l_s_fmt, v4l_print_format, INFO_FL_PRIO)，再看v4l_s_fmt：
static int v4l_s_fmt(const struct v4l2_ioctl_ops *ops,
struct file *file, void *fh, void *arg)
{
struct v4l2_format *p = arg;
struct video_device *vfd = video_devdata(file);
bool is_vid = vfd->vfl_type == VFL_TYPE_GRABBER;
bool is_sdr = vfd->vfl_type == VFL_TYPE_SDR;
bool is_rx = vfd->vfl_dir != VFL_DIR_TX;
bool is_tx = vfd->vfl_dir != VFL_DIR_RX;
switch (p->type) {
case V4L2_BUF_TYPE_VIDEO_CAPTURE:
if (unlikely(!is_rx || !is_vid || !ops->vidioc_s_fmt_vid_cap))
break;
CLEAR_AFTER_FIELD(p, fmt.pix);
return ops->vidioc_s_fmt_vid_cap(file, fh, arg);
......
......
}
return -EINVAL;
}最终又调用到soc_camera_ioctl_ops里的vidioc_s_fmt_vid_cap了，即soc_camera_s_fmt_vid_cap。我想应该能想到最终会调用到host的callback吧。soc_camera_set_fmt->(ici->ops->set_fmt(icd, f));
在设置了视频捕获格式后，就是向驱动申请缓冲区了，通过如下代码（以下所有代码仅仅是为了展示，正式代码应该要做出错检查什么的哈）
Struct v4l2_requestbuffers req;
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
ioctl(fd, VIDIOC_REQBUFS, &req);直接看IOCTL_INFO_FNC(VIDIOC_REQBUFS, v4l_reqbufs, v4l_print_requestbuffers, INFO_FL_PRIO | INFO_FL_QUEUE),再看v4l_reqbufs:
static int v4l_reqbufs(const struct v4l2_ioctl_ops *ops,
struct file *file, void *fh, void *arg)
{
struct v4l2_requestbuffers *p = arg;
int ret = check_fmt(file, p->type);
if (ret)
return ret;
CLEAR_AFTER_FIELD(p, memory);
return ops->vidioc_reqbufs(file, fh, p);
}还是一样的，先回调到soc_camera层的vidioc_reqbufs，即soc_camera_reqbufs，最终调用到host初始化buffer相关callback了:
static int soc_camera_reqbufs(struct file *file, void *priv,
struct v4l2_requestbuffers *p)
{
int ret;
struct soc_camera_device *icd = file->private_data;
struct soc_camera_host *ici = to_soc_camera_host(icd->parent);
WARN_ON(priv != file->private_data);
if (icd->streamer && icd->streamer != file)
return -EBUSY;
if (ici->ops->init_videobuf) {
ret = videobuf_reqbufs(&icd->vb_vidq, p);
if (ret < 0)
return ret;
ret = ici->ops->reqbufs(icd, p);
} else {
ret = vb2_reqbufs(&icd->vb2_vidq, p);
}
if (!ret && !icd->streamer)
icd->streamer = file;
return ret;
}注意，v4l2使用的buffer我们可以用v4l2提供的现有的videobuf2机制就可以了。
请求完buffer，应用层一般会获取它请求的buffer，然后对其做mmap:
获取每个缓冲区的信息，映射到用户空间
structbuffer {
void *start;
size_t length;
} *buffers;
buffers = calloc(req.count, sizeof(*buffers));
for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
struct v4l2_buffer buf;
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
if (-1 == ioctl(fd, VIDIOC_QUERYBUF, & buf))
errno_exit("VIDIOC_QUERYBUF");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start=
mmap(NULL /* start anywhere */,
buf.length,
PROT_READ | PROT_WRITE /* required */,
MAP_SHARED /* recommended */,
fd, buf.m.offset);
} VIDIOC_QUERYBUF对应的回调函数是v4l_querybuf，还是回调soc_camera的vidioc_querybuf，即soc_camera_querybuf，它内部的实现就部分析了，和请求buffer的思想是一样的。
下面看mmap的实现，对应的video_device的mmap是soc_camera_mmap，内部的实现还是和上面的一样，如果使用自己的buffer实现，就走自己的路，如果是用v4l2提供的现有的机制实现，那么就继续调用相应的api即可。
缓冲区mmap好后，我们剩下的就是将发命令将buffer放入到视频采集的buffer队列中去，并开启流，好让底层去填充它：
for (i =0; i < n_buffers; ++i) {
struct v4l2_buffer buf;
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = i;
if (-1 == ioctl(fd, VIDIOC_QBUF, &buf))
errno_exit("VIDIOC_QBUF");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fd, VIDIOC_STREAMON, & type);VIDIOC_QBUF对应的回调函数是v4l_qbuf，还是回调soc_camera的vidioc_qbuf，即soc_camera_qbuf，它内部的实现就部分析了，和上面类似。
VIDIOC_STREAMON对应的回调函数是v4l_streamon，还是回调soc_camera的vidioc_streamon，即soc_camera_streamon，它内部的实现就部分析了，和上面类似，不过需要额外做一些事情，比如让sensor开始工作，对吧!
static int soc_camera_streamon(struct file *file, void *priv,
enum v4l2_buf_type i)
{
struct soc_camera_device *icd = file->private_data;
struct soc_camera_host *ici = to_soc_camera_host(icd->parent);
struct v4l2_subdev *sd = soc_camera_to_subdev(icd);
int ret;
WARN_ON(priv != file->private_data);
if (i != V4L2_BUF_TYPE_VIDEO_CAPTURE)
return -EINVAL;
if (icd->streamer != file)
return -EBUSY;
/* This calls buf_queue from host driver's videobuf_queue_ops */
if (ici->ops->init_videobuf)
ret = videobuf_streamon(&icd->vb_vidq);
else
ret = vb2_streamon(&icd->vb2_vidq, i);
if (!ret)
v4l2_subdev_call(sd, video, s_stream, 1);
return ret;
}
这里会调用v4l2_subdev的video类ops的s_stream。如果看懂了前文的相关内容，应该知道v4l2_subdev对应的ops就是i2c sensor驱动实现的函数集！看文件ov2640.c
static struct v4l2_subdev_video_ops ov2640_subdev_video_ops = {
.s_stream = ov2640_s_stream,
.g_mbus_fmt = ov2640_g_fmt,
.s_mbus_fmt = ov2640_s_fmt,
.try_mbus_fmt = ov2640_try_fmt,
.cropcap = ov2640_cropcap,
.g_crop = ov2640_g_crop,
.enum_mbus_fmt = ov2640_enum_fmt,
.g_mbus_config = ov2640_g_mbus_config,
};
static struct v4l2_subdev_ops ov2640_subdev_ops = {
.core = &ov2640_subdev_core_ops,
.video = &ov2640_subdev_video_ops,
};
于是，这里就是调用ov2640_s_stream了。
总结
基本上都分析完了。我突然想起另外一个问题，那就是v4l2提供的ctrl机制怎么调用的呢！i2c sensor里都添加了这些ctrl啊，对吧！这个其实是通过命令VIDIOC_G_CTRL和VIDIOC_S_CTRL来调用的。我们可以通过它们来配置sensor的一些参数，比如曝光、白平衡等。我认为这篇博文应该可以帮助要写sensor驱动或者host驱动的人吧^_^
基本分析完了吧！！！！！！ 有什么漏掉的，欢迎大家指出！
2015年6月