V4L2用户空间和kernel层driver的交互过程

V4L2用户空间和kernel层driver的交互过程

这篇文章详细分析了V4L2用户空间和kernel层driver的交互过程，目的只有一个：
更清晰的理解V4L2视频驱动程序的系统结构，驱动编程方法，为以后开发视频驱动打好基础

既然从用户层出发探究驱动层，这里先贴出应用层code：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <getopt.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <asm/types.h>
#include <linux/videodev2.h>
#include <linux/fb.h>
#define CLEAR(x) memset (&(x), 0, sizeof (x))

struct buffer {
    void * start;
    size_t length;
};

static char * dev_name = NULL;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0;
static int time_in_sec_capture=5;
static int fbfd = -1;
static struct fb_var_screeninfo vinfo;
static struct fb_fix_screeninfo finfo;
static char *fbp=NULL;
static long screensize=0;

static void errno_exit (const char * s)
{
    fprintf (stderr, "%s error %d, %s/n",s, errno, strerror (errno));
    exit (EXIT_FAILURE);
}

static int xioctl (int fd,int request,void * arg)
{
    int r;
    /* Here use this method to make sure cmd success*/
    do r = ioctl (fd, request, arg);
    while (-1 == r && EINTR == errno);
    return r;
}

inline int clip(int value, int min, int max) {
    return (value > max ? max : value < min ? min : value);
  }

static void process_image (const void * p){
    //ConvertYUVToRGB321;
    unsigned char* in=(char*)p;
    int width=640;
    int height=480;
    int istride=1280;
    int x,y,j;
    int y0,u,y1,v,r,g,b;
    long location=0;

    for ( y = 100; y < height + 100; ++y) {
        for (j = 0, x=100; j < width * 2 ; j += 4,x +=2) {
          location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
            (y+vinfo.yoffset) * finfo.line_length;

          y0 = in[j];
          u = in[j + 1] - 128;
          y1 = in[j + 2];
          v = in[j + 3] - 128;

          r = (298 * y0 + 409 * v + 128) >> 8;
          g = (298 * y0 - 100 * u - 208 * v + 128) >> 8;
          b = (298 * y0 + 516 * u + 128) >> 8;

          fbp[ location + 0] = clip(b, 0, 255);
          fbp[ location + 1] = clip(g, 0, 255);
          fbp[ location + 2] = clip(r, 0, 255);
          fbp[ location + 3] = 255;

          r = (298 * y1 + 409 * v + 128) >> 8;
          g = (298 * y1 - 100 * u - 208 * v + 128) >> 8;
          b = (298 * y1 + 516 * u + 128) >> 8;

          fbp[ location + 4] = clip(b, 0, 255);
          fbp[ location + 5] = clip(g, 0, 255);
          fbp[ location + 6] = clip(r, 0, 255);
          fbp[ location + 7] = 255;
          }
        in +=istride;
      }
}

static int read_frame (void)
{
    struct v4l2_buffer buf;
    unsigned int i;

    CLEAR (buf);
    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    buf.memory = V4L2_MEMORY_MMAP;
     /* 11. VIDIOC_DQBUF把数据放回缓存队列*/
    if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
        switch (errno) {
        case EAGAIN:
        return 0;
        case EIO:
        default:
            errno_exit ("VIDIOC_DQBUF");
        }
    }

    assert (buf.index < n_buffers);
    printf("v4l2_pix_format->field(%d)/n", buf.field);
    //assert (buf.field ==V4L2_FIELD_NONE);
    process_image (buffers[buf.index].start);

    /*12. VIDIOC_QBUF把数据从缓存中读取出来*/
    if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
        errno_exit ("VIDIOC_QBUF");

    return 1;
}

static void run (void)
{
    unsigned int count;
    int frames;
    frames = 30 * time_in_sec_capture;

    while (frames-- > 0) {
        for (;;) {
            fd_set fds;
            struct timeval tv;
            int r;
            FD_ZERO (&fds);
            FD_SET (fd, &fds);


            tv.tv_sec = 2;
            tv.tv_usec = 0;
             /* 10. poll method*/
            r = select (fd + 1, &fds, NULL, NULL, &tv);

            if (-1 == r) {
                if (EINTR == errno)
                    continue;
                errno_exit ("select");
            }

            if (0 == r) {
                fprintf (stderr, "select timeout/n");
                exit (EXIT_FAILURE);
            }

            if (read_frame())
                break;

            }
    }
}

static void stop_capturing (void)
{
    enum v4l2_buf_type type;

    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    /*13. VIDIOC_STREAMOFF结束视频显示函数*/
    if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
        errno_exit ("VIDIOC_STREAMOFF");
}

static void start_capturing (void)
{
    unsigned int i;
    enum v4l2_buf_type type;

    for (i = 0; i < n_buffers; ++i) {
        struct v4l2_buffer buf;
        CLEAR (buf);

        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = i;
         /* 8. VIDIOC_QBUF把数据从缓存中读取出来*/
        if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
            errno_exit ("VIDIOC_QBUF");
    }

    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
     /* 9. VIDIOC_STREAMON开始视频显示函数*/
    if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
        errno_exit ("VIDIOC_STREAMON");

}

static void uninit_device (void)
{
    unsigned int i;

    for (i = 0; i < n_buffers; ++i)
        if (-1 == munmap (buffers[i].start, buffers[i].length))
            errno_exit ("munmap");

    if (-1 == munmap(fbp, screensize)) {
          printf(" Error: framebuffer device munmap() failed./n");
          exit (EXIT_FAILURE) ;
        }
    free (buffers);
}


static void init_mmap (void)
{
    struct v4l2_requestbuffers req;

    //mmap framebuffer
    fbp = (char *)mmap(NULL,screensize,PROT_READ | PROT_WRITE,MAP_SHARED ,fbfd, 0);
    if ((int)fbp == -1) {
        printf("Error: failed to map framebuffer device to memory./n");
        exit (EXIT_FAILURE) ;
    }
    memset(fbp, 0, screensize);
    CLEAR (req);

    req.count = 4;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;
     /* 6. VIDIOC_REQBUFS分配内存*/
    if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
        if (EINVAL == errno) {
            fprintf (stderr, "%s does not support memory mapping/n", dev_name);
            exit (EXIT_FAILURE);
        } else {
            errno_exit ("VIDIOC_REQBUFS");
        }
    }

    if (req.count < 4) {
        fprintf (stderr, "Insufficient buffer memory on %s/n",dev_name);
        exit (EXIT_FAILURE);
    }

    buffers = calloc (req.count, sizeof (*buffers));

    if (!buffers) {
        fprintf (stderr, "Out of memory/n");
        exit (EXIT_FAILURE);
    }

    for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
        struct v4l2_buffer buf;

        CLEAR (buf);

        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = n_buffers;
         /* 7. VIDIOC_QUERYBUF把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址*/
        if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
            errno_exit ("VIDIOC_QUERYBUF");

        buffers[n_buffers].length = buf.length;
        buffers[n_buffers].start =mmap (NULL,buf.length,PROT_READ | PROT_WRITE ,MAP_SHARED,fd, buf.m.offset);

        if (MAP_FAILED == buffers[n_buffers].start)
            errno_exit ("mmap");
    }

}



static void init_device (void)
{
    struct v4l2_capability cap;
    struct v4l2_cropcap cropcap;
    struct v4l2_crop crop;
    struct v4l2_format fmt;
    unsigned int min;


    // Get fixed screen information
    if (-1==xioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
     printf("Error reading fixed information./n");
     exit (EXIT_FAILURE);
    }

    // Get variable screen information
    if (-1==xioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
     printf("Error reading variable information./n");
     exit (EXIT_FAILURE);
    }
    screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

     /* 2. VIDIOC_QUERYCAP查询驱动功能*/
    if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {
        if (EINVAL == errno) {
            fprintf (stderr, "%s is no V4L2 device/n",dev_name);
            exit (EXIT_FAILURE);
        } else {
            errno_exit ("VIDIOC_QUERYCAP");
        }
    }

    /* Check if it is a video capture device*/
    if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
        fprintf (stderr, "%s is no video capture device/n",dev_name);
        exit (EXIT_FAILURE);
    }

     /* Check if support streaming I/O ioctls*/
    if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
        fprintf (stderr, "%s does not support streaming i/o/n",dev_name);
        exit (EXIT_FAILURE);
    }

    CLEAR (cropcap);
    /* Set type*/
    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
     /* 3. VIDIOC_CROPCAP查询驱动的修剪能力*/
    /* 这里在vivi驱动中我们没有实现此方法，即不支持此操作*/
    if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
        crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        crop.c = cropcap.defrect;
        /* 4. VIDIOC_S_CROP设置视频信号的边框*/
        /* 同样不支持这个操作*/
        if (-1 == xioctl (fd, VIDIOC_S_CROP, &crop)) {
            switch (errno) {
            case EINVAL:
            break;
            default:
            break;
            }
        }
    }else { }

    CLEAR (fmt);

    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;
    fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
     /* 5. VIDIOC_S_FMT设置当前驱动的频捕获格式*/
    if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
        errno_exit ("VIDIOC_S_FMT");

    init_mmap ();

}

static void close_device (void)
{
    if (-1 == close (fd))
    errno_exit ("close");
    fd = -1;
    /*14. close method*/
    close(fbfd);
}

static void open_device (void)
{
    struct stat st;

    if (-1 == stat (dev_name, &st)) {
     fprintf (stderr, "Cannot identify '%s': %d, %s/n",dev_name, errno, strerror (errno));
     exit (EXIT_FAILURE);
    }

    if (!S_ISCHR (st.st_mode)) {
     fprintf (stderr, "%s is no device/n", dev_name);
     exit (EXIT_FAILURE);
    }

    fbfd = open("/dev/fb0", O_RDWR);
    if (fbfd==-1) {
        printf("Error: cannot open framebuffer device./n");
        exit (EXIT_FAILURE);
    }

    /* 1. open the char device */
    fd = open (dev_name, O_RDWR| O_NONBLOCK, 0);
    if (-1 == fd) {
     fprintf (stderr, "Cannot open '%s': %d, %s/n",dev_name, errno, strerror (errno));
     exit (EXIT_FAILURE);
    }
}

static void usage (FILE * fp,int argc,char ** argv)
{
    fprintf (fp,
    "Usage: %s [options]/n/n"
    "Options:/n"
    "-d | --device name Video device name [/dev/video]/n"
    "-h | --help Print this message/n"
    "-t | --how long will display in seconds/n"
    "",
    argv[0]);
}

static const char short_options [] = "d:ht:";
static const struct option long_options [] = {
    { "device", required_argument, NULL, 'd' },
    { "help", no_argument, NULL, 'h' },
    { "time", no_argument, NULL, 't' },
    { 0, 0, 0, 0 }
};

int main (int argc,char ** argv)
{
    dev_name = "/dev/video0";
    for (;;)
    {
        int index;
        int c;

        c = getopt_long (argc, argv,short_options, long_options,&index);
        if (-1 == c)
        break;

        switch (c) {
        case 0:
        break;

        case 'd':
        dev_name = optarg;
        break;

        case 'h':
        usage (stdout, argc, argv);
        exit (EXIT_SUCCESS);
        case 't':
        time_in_sec_capture = atoi(optarg);
        break;

        default:
        usage (stderr, argc, argv);
        exit (EXIT_FAILURE);
        }
    }

    open_device();
    init_device();
    start_capturing();
    run();
    stop_capturing();
    uninit_device();
    close_device();
    exit(EXIT_SUCCESS);
    return 0;
}

上面code中我已经标注出程序顺序指向的步骤1--14步，下面将一一说明应用从做这14步时驱动层是怎样响应，变化过程，驱动加载初始化部分上一篇文章已经说过了
正式开始取经之路哇。。。。。。。

STEP 1：
fd = open (dev_name, O_RDWR| O_NONBLOCK, 0);
打开字符设备，这个字符设备是video_device_register时创建的，code在v4l2_dev.c中，具体：

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);

    /*
    * Here using the API you get the method you get the open() method write
    * The other methods in fops use the same method to use you own code
    */
    if (vdev->fops->open) {
        if (vdev->lock && mutex_lock_interruptible(vdev->lock)) {
            ret = -ERESTARTSYS;
            goto err;
        }
        if (video_is_registered(vdev))
            ret = vdev->fops->open(filp);
        else
            ret = -ENODEV;
        if (vdev->lock)
            mutex_unlock(vdev->lock);
    }

err:
    /* decrease the refcount in case of an error */
    if (ret)
        video_put(vdev);
    return ret;
}

重点在标注部分，通过这个V4L2的API调用我们自己驱动程序中定义的open方法，我们自己的open方法所属的fops是在vivi.c驱动程序的vivi_create_instance方法中video_device_register之前关联进来的

int v4l2_fh_open(struct file *filp)
{
    struct video_device *vdev = video_devdata(filp);
    struct v4l2_fh *fh = kzalloc(sizeof(*fh), GFP_KERNEL);

    /*
    * IN the open method, do only one job
    * set v4l2_fh into filp->private_data for later use, and initial v4l2_fh
    */
    filp->private_data = fh;
    if (fh == NULL)
        return -ENOMEM;
    v4l2_fh_init(fh, vdev);
    v4l2_fh_add(fh);
    return 0;
}
EXPORT_SYMBOL_GPL(v4l2_fh_open);

这个open方法只是初始化了一个v4l2_fh，并关联到filp->private中，方便以后使用
这里设置V4L2_FL_USES_V4L2_FH这个标志位，设置优先级为UNSET，如果我们的自己驱动程序实现了，支持
VIDIOC_SUBSCRIBE_EVENT,那么v4l2_event_init，在events初始化中初始化链表，并设置sequence为-1，如果不支持，则设置fh->events为NULL
最后add list

STEP 2：
if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap))
这么调用完成下面过程，不行的从驱动层获取cap。直到成功拿到我们想要的数据

static int xioctl (int fd,int request,void * arg)
{
    int r;
    /* Here use this method to make sure cmd success*/
    do r = ioctl (fd, request, arg);
    while (-1 == r && EINTR == errno);
    return r;
}

也就是调用驱动层的ioctl方法，从v4l2 api中的ictol 调用我们自己定义的ioctl ，这中间的过程不在多做说明，我们自己的驱动的控制过程由v4l2_ioctl.c这个文件中的方法实现，一个很庞大的switch
值得一提的是，慢慢后面你会明白的，这里v4l2_ioctl.c这个文件中的方法实现其实只是会中转站，它接着就回调了我们自己驱动程序中定义的控制接口，后面再说吧

long video_ioctl2(struct file *file,
     unsigned int cmd, unsigned long arg)
{
    return video_usercopy(file, cmd, arg, __video_do_ioctl);
}

这里这个__video_do_ioctl方法其实完全做了我们所有的控制过程，又为什么又要经过video_usercopy这个方法呢，不妨看一看这个方法

long
video_usercopy(struct file *file, unsigned int cmd, unsigned long arg,
     v4l2_kioctl func)
{
    char    sbuf[128];
    void *mbuf = NULL;
    void    *parg = (void *)arg;
    long    err = -EINVAL;
    bool    has_array_args;
    size_t array_size = 0;
    void __user *user_ptr = NULL;
    void    **kernel_ptr = NULL;

    /* Copy arguments into temp kernel buffer */
    if (_IOC_DIR(cmd) != _IOC_NONE) {
........这里检查128个字节的大小是否够存放用户端发送来的数据，不够则需要重新申请一个新的内存用来存放，指向parg这个地址
        if (_IOC_SIZE(cmd) <= sizeof(sbuf)) {
            parg = sbuf;
        } else {
            /* too big to allocate from stack */
            mbuf = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
            if (NULL == mbuf)
                return -ENOMEM;
            parg = mbuf;
        }

        err = -EFAULT;
        if (_IOC_DIR(cmd) & _IOC_WRITE) {
            unsigned long n = cmd_input_size(cmd);

            if (copy_from_user(parg, (void __user *)arg, n))
                goto out;

            /* zero out anything we don't copy from userspace */
            if (n < _IOC_SIZE(cmd))
                memset((u8 *)parg + n, 0, _IOC_SIZE(cmd) - n);
        } else {
            /* read-only ioctl */
            memset(parg, 0, _IOC_SIZE(cmd));
        }
    }
....check
    err = check_array_args(cmd, parg, &array_size, &user_ptr, &kernel_ptr);
    if (err < 0)
        goto out;
    has_array_args = err;
....这里这块如果用户端有数据写到kernel，这里负责数据拷贝
    if (has_array_args) {
        /*
         * When adding new types of array args, make sure that the
         * parent argument to ioctl (which contains the pointer to the
         * array) fits into sbuf (so that mbuf will still remain
         * unused up to here).
         */
        mbuf = kmalloc(array_size, GFP_KERNEL);
        err = -ENOMEM;
        if (NULL == mbuf)
            goto out_array_args;
        err = -EFAULT;
        if (copy_from_user(mbuf, user_ptr, array_size))
            goto out_array_args;
        *kernel_ptr = mbuf;
    }

    /* Handles IOCTL */
    err = func(file, cmd, parg);
    if (err == -ENOIOCTLCMD)
        err = -EINVAL;

    if (has_array_args) {
        *kernel_ptr = user_ptr;
        if (copy_to_user(user_ptr, mbuf, array_size))
            err = -EFAULT;
        goto out_array_args;
    }
    if (err < 0)
        goto out;

out_array_args:
    /* Copy results into user buffer */
    switch (_IOC_DIR(cmd)) {
    case _IOC_READ:
    case (_IOC_WRITE | _IOC_READ):
        if (copy_to_user((void __user *)arg, parg, _IOC_SIZE(cmd)))
            err = -EFAULT;
        break;
    }

out:
    kfree(mbuf);
    return err;
}
EXPORT_SYMBOL(video_usercopy);

自我感觉这个方法还是有很多精妙之处的，主要的控制过程是在我标注的地方调用完成的，这个调用之前做check动作，检查用户端发来的命令是否合法，
最重要的是把用户端的数据copy到kernel 端；而这个调用之后，则是我们处理完我们的动作之后，我们在这里吧用户端请求的数据从kernel 端copy到用户端
这样做的好处是显而易见的，任务明确，控制只做控制，用户空间和kernel空间数据的copy在所有控制之前，控制之后进行
以上动作做完之后，进入庞大的控制中枢，这来开始至贴出具体到某一个控制的代码，否则code过大，不易分析：

case VIDIOC_QUERYCAP://查询视频设备的功能
    {
        struct v4l2_capability *cap = (struct v4l2_capability *)arg;

        if (!ops->vidioc_querycap)
            break;

        ret = ops->vidioc_querycap(file, fh, cap);
        if (!ret)/* i don't think here need to check */
            dbgarg(cmd, "driver=%s, card=%s, bus=%s, "
                    "version=0x%08x, "
                    "capabilities=0x%08x\n",
                    cap->driver, cap->card, cap->bus_info,
                    cap->version,
                    cap->capabilities);
        break;
    }

这来调用了我们自己驱动中填充的v4l2_ioctl_ops结构体，从这里开始，我上面说到的话得到了验证，这就是linux 中API 的强大之处
作为中间层的这个控制中枢又回调驱动自己定义编写的控制

/* ------------------------------------------------------------------
    IOCTL vidioc handling
   ------------------------------------------------------------------*/
static int vidioc_querycap(struct file *file, void *priv,
                    struct v4l2_capability *cap)
{
    struct vivi_dev *dev = video_drvdata(file);

    strcpy(cap->driver, "vivi");
    strcpy(cap->card, "vivi");
    strlcpy(cap->bus_info, dev->v4l2_dev.name, sizeof(cap->bus_info));
    cap->version = VIVI_VERSION;
    cap->capabilities = V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING | \
             V4L2_CAP_READWRITE;
    return 0;
}

这来做的事情很简单，只是将配置信息保存到cap这个变量中，之后上传给用户空间

STEP 3：
/* 3. VIDIOC_CROPCAP查询驱动的修剪能力*/
/* 这里在vivi 驱动中我们没有实现此方法，即不支持此操作*/
if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap))
这个判断在中间层控制中枢中进行的，check到我们自己的驱动中没有这个控制功能的支持
所以这里的STEP 4同样不会进行

STEP 5：
/* 5. VIDIOC_S_FMT设置当前驱动的频捕获格式*/
if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
对应到控制中心是这样的

case VIDIOC_S_FMT:
    {
        struct v4l2_format *f = (struct v4l2_format *)arg;

        /* FIXME: Should be one dump per type */
        dbgarg(cmd, "type=%s\n", prt_names(f->type, v4l2_type_names));

        switch (f->type) {
        case V4L2_BUF_TYPE_VIDEO_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.pix);
            v4l_print_pix_fmt(vfd, &f->fmt.pix);
            if (ops->vidioc_s_fmt_vid_cap) {
                ret = ops->vidioc_s_fmt_vid_cap(file, fh, f);
            } else if (ops->vidioc_s_fmt_vid_cap_mplane) {
                if (fmt_sp_to_mp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_cap_mplane(file, fh,
                                    &f_copy);
                if (ret)
                    break;

                if (f_copy.fmt.pix_mp.num_planes > 1) {
                    /* Drivers shouldn't adjust from 1-plane
                     * to more than 1-plane formats */
                    ret = -EBUSY;
                    WARN_ON(1);
                    break;
                }

                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE:
            CLEAR_AFTER_FIELD(f, fmt.pix_mp);
            v4l_print_pix_fmt_mplane(vfd, &f->fmt.pix_mp);
            if (ops->vidioc_s_fmt_vid_cap_mplane) {
                ret = ops->vidioc_s_fmt_vid_cap_mplane(file,
                                    fh, f);
            } else if (ops->vidioc_s_fmt_vid_cap &&
                    f->fmt.pix_mp.num_planes == 1) {
                if (fmt_mp_to_sp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_cap(file,
                                fh, &f_copy);
                if (ret)
                    break;

                ret = fmt_sp_to_mp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OVERLAY:
            CLEAR_AFTER_FIELD(f, fmt.win);
            if (ops->vidioc_s_fmt_vid_overlay)
                ret = ops->vidioc_s_fmt_vid_overlay(file,
                                 fh, f);
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.pix);
            v4l_print_pix_fmt(vfd, &f->fmt.pix);
            if (ops->vidioc_s_fmt_vid_out) {
                ret = ops->vidioc_s_fmt_vid_out(file, fh, f);
            } else if (ops->vidioc_s_fmt_vid_out_mplane) {
                if (fmt_sp_to_mp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_out_mplane(file, fh,
                                    &f_copy);
                if (ret)
                    break;

                if (f_copy.fmt.pix_mp.num_planes > 1) {
                    /* Drivers shouldn't adjust from 1-plane
                     * to more than 1-plane formats */
                    ret = -EBUSY;
                    WARN_ON(1);
                    break;
                }

                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE:
            CLEAR_AFTER_FIELD(f, fmt.pix_mp);
            v4l_print_pix_fmt_mplane(vfd, &f->fmt.pix_mp);
            if (ops->vidioc_s_fmt_vid_out_mplane) {
                ret = ops->vidioc_s_fmt_vid_out_mplane(file,
                                    fh, f);
            } else if (ops->vidioc_s_fmt_vid_out &&
                    f->fmt.pix_mp.num_planes == 1) {
                if (fmt_mp_to_sp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_out(file,
                                fh, &f_copy);
                if (ret)
                    break;

                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY:
            CLEAR_AFTER_FIELD(f, fmt.win);
            if (ops->vidioc_s_fmt_vid_out_overlay)
                ret = ops->vidioc_s_fmt_vid_out_overlay(file,
                    fh, f);
            break;
        case V4L2_BUF_TYPE_VBI_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.vbi);
            if (ops->vidioc_s_fmt_vbi_cap)
                ret = ops->vidioc_s_fmt_vbi_cap(file, fh, f);
            break;
        case V4L2_BUF_TYPE_VBI_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.vbi);
            if (ops->vidioc_s_fmt_vbi_out)
                ret = ops->vidioc_s_fmt_vbi_out(file, fh, f);
            break;
        case V4L2_BUF_TYPE_SLICED_VBI_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.sliced);
            if (ops->vidioc_s_fmt_sliced_vbi_cap)
                ret = ops->vidioc_s_fmt_sliced_vbi_cap(file,
                                    fh, f);
            break;
        case V4L2_BUF_TYPE_SLICED_VBI_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.sliced);
            if (ops->vidioc_s_fmt_sliced_vbi_out)
                ret = ops->vidioc_s_fmt_sliced_vbi_out(file,
                                    fh, f);
            break;
        case V4L2_BUF_TYPE_PRIVATE:
            /* CLEAR_AFTER_FIELD(f, fmt.raw_data); <- does nothing */
            if (ops->vidioc_s_fmt_type_private)
                ret = ops->vidioc_s_fmt_type_private(file,
                                fh, f);
            break;
        }
        break;
    }

以后根据不同的type 决定了我们自己驱动程序中不同的控制实现，这个type是根据用户空间的设置而定的，还包括其他几个参数，如下：

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;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;

这里根据设定的type，所以驱动程序的处理过程如下：

static int vidioc_s_fmt_vid_cap(struct file *file, void *priv,
                    struct v4l2_format *f)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;
....在下面这个函数中，做了一些试探性的动作，如果试探失败则下面不会赋值，试探通过则后续正常设置即可，在这个试探函数中同时做了一些设置动作
    int ret = vidioc_try_fmt_vid_cap(file, priv, f);
    if (ret < 0)
        return ret;

    if (vb2_is_streaming(q)) {
        dprintk(dev, 1, "%s device busy\n", __func__);
        return -EBUSY;
    }
....按用户空间需求设置
    dev->fmt = get_format(f);
    dev->width = f->fmt.pix.width;
    dev->height = f->fmt.pix.height;
    dev->field = f->fmt.pix.field;

    return 0;
}

STEP6 ：
/* 6. VIDIOC_REQBUFS分配内存*/
if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req))
中间层控制中枢：

case VIDIOC_REQBUFS:
    {
        struct v4l2_requestbuffers *p = arg;

        if (!ops->vidioc_reqbufs)
            break;
........这个方法check 驱动必须实现了fmt方法，看具体看代码
        ret = check_fmt(ops, p->type);
        if (ret)
            break;

        if (p->type < V4L2_BUF_TYPE_PRIVATE)
            CLEAR_AFTER_FIELD(p, memory);

        ret = ops->vidioc_reqbufs(file, fh, p);
        dbgarg(cmd, "count=%d, type=%s, memory=%s\n",
                p->count,
                prt_names(p->type, v4l2_type_names),
                prt_names(p->memory, v4l2_memory_names));
        break;
    }

驱动中实现：

static int vidioc_reqbufs(struct file *file, void *priv,
             struct v4l2_requestbuffers *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_reqbufs(&dev->vb_vidq, p);
}

到了这里来到了这个全新的话题，实现
vb2_reqbufs(&dev->vb_vidq, p);
这里暂且不讨论这个方法，相对较复杂，待日后研究，先把注释部分放到这里，包括其他内存操作，之后深入研究补充，专门作为一篇整理
/**
 * Should be called from vidioc_reqbufs ioctl handler of a driver.
 * This function:
 * 1) verifies streaming parameters passed from the userspace,
 * 2) sets up the queue,
 * 3) negotiates number of buffers and planes per buffer with the driver to be used during streaming,
 * 4) allocates internal buffer structures (struct vb2_buffer), according to the agreed parameters,
 * 5) for MMAP memory type, allocates actual video memory, using the memory handling/allocation routines provided during queue initialization
 * If req->count is 0, all the memory will be freed instead.
 * If the queue has been allocated previously (by a previous vb2_reqbufs) call
 * and the queue is not busy, memory will be reallocated.
 * The return values from this function are intended to be directly returned from vidioc_reqbufs handler in driver.
 */

STEP 7：
/* 7. VIDIOC_QUERYBUF把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址*/
if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
中间层控制中枢：

case VIDIOC_QUERYBUF:
    {
        struct v4l2_buffer *p = arg;

        if (!ops->vidioc_querybuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;

        ret = ops->vidioc_querybuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }

驱动中控制实现：

static int vidioc_querybuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_querybuf(&dev->vb_vidq, p);
}

/**
 * Should be called from vidioc_querybuf ioctl handler in driver.
 * This function will verify the passed v4l2_buffer structure and fill the
 * relevant information for the userspace.
 * The return values from this function are intended to be directly returned from vidioc_querybuf handler in driver.
 */

STEP 8：
/* 8. VIDIOC_QBUF把数据从缓存中读取出来*/
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
中间层控制中枢：

case VIDIOC_QBUF:
    {
        struct v4l2_buffer *p = arg;

        if (!ops->vidioc_qbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;

        ret = ops->vidioc_qbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }

驱动中控制实现：

static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_qbuf(&dev->vb_vidq, p);
}

/**
 * Should be called from vidioc_qbuf ioctl handler of a driver.
 * This function:
 * 1) verifies the passed buffer,
 * 2) calls buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed,
 * 3) if streaming is on, queues the buffer in driver by the means of buf_queue callback for processing.
 * The return values from this function are intended to be directly returned from vidioc_qbuf handler in driver.
 */

STEP 9：
/* 9. VIDIOC_STREAMON开始视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
中间层控制中枢：

case VIDIOC_STREAMON:
    {
        enum v4l2_buf_type i = *(int *)arg;

        if (!ops->vidioc_streamon)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamon(file, fh, i);
        break;
    }

驱动控制实现;

static int vidioc_streamon(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamon(&dev->vb_vidq, i);
}

/**
 * Should be called from vidioc_streamon handler of a driver.
 * This function:
 * 1) verifies current state
 * 2) starts streaming and passes any previously queued buffers to the driver
 * The return values from this function are intended to be directly returned from vidioc_streamon handler in the driver.
 */

STEP 10：
/* 10. poll method*/
select (fd + 1, &fds, NULL, NULL, &tv);
从V4L2驱动API开始：

static unsigned int v4l2_poll(struct file *filp, struct poll_table_struct *poll)
{
    struct video_device *vdev = video_devdata(filp);
    int ret = POLLERR | POLLHUP;

    if (!vdev->fops->poll)
        return DEFAULT_POLLMASK;
    if (vdev->lock)
        mutex_lock(vdev->lock);
    if (video_is_registered(vdev))
        ret = vdev->fops->poll(filp, poll);
    if (vdev->lock)
        mutex_unlock(vdev->lock);
    return ret;
}

驱动实现：

static unsigned int
vivi_poll(struct file *file, struct poll_table_struct *wait)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;

    dprintk(dev, 1, "%s\n", __func__);
    return vb2_poll(q, file, wait);
}

/**
 * This function implements poll file operation handler for a driver.
 * For CAPTURE queues, if a buffer is ready to be dequeued, the userspace will be informed that the file descriptor of a video device is available for reading.
 * For OUTPUT queues, if a buffer is ready to be dequeued, the file descriptor will be reported as available for writing.
 * The return values from this function are intended to be directly returned from poll handler in driver.
 */

STEP 11：
/* 11. VIDIOC_DQBUF把数据放回缓存队列*/
if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf))
中间层控制中枢：

case VIDIOC_DQBUF:
    {
        struct v4l2_buffer *p = arg;

        if (!ops->vidioc_dqbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;

        ret = ops->vidioc_dqbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }

驱动控制实现：

static int vidioc_dqbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_dqbuf(&dev->vb_vidq, p, file->f_flags & O_NONBLOCK);
}

/**
 * Should be called from vidioc_dqbuf ioctl handler of a driver.
 * This function:
 * 1) verifies the passed buffer,
 * 2) calls buf_finish callback in the driver (if provided), in which driver can perform any additional operations that may be required before returning the buffer to userspace, such as cache sync,
 * 3) the buffer struct members are filled with relevant information for the userspace.
 * The return values from this function are intended to be directly returned from vidioc_dqbuf handler in driver.
 */

STEP 12：
/*12. VIDIOC_QBUF把数据从缓存中读取出来*/
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
中间层控制中枢：

case VIDIOC_QBUF:
    {
        struct v4l2_buffer *p = arg;

        if (!ops->vidioc_qbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;

        ret = ops->vidioc_qbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }

驱动控制实现：

static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_qbuf(&dev->vb_vidq, p);
}

STEP 13：
/*13. VIDIOC_STREAMOFF结束视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
中间层控制中枢：

case VIDIOC_STREAMOFF:
    {
        enum v4l2_buf_type i = *(int *)arg;

        if (!ops->vidioc_streamoff)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamoff(file, fh, i);
        break;
    }

驱动控制实现：

static int vidioc_streamoff(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamoff(&dev->vb_vidq, i);
}

STEP 13：
/*13. VIDIOC_STREAMOFF结束视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
中间层控制中枢：

case VIDIOC_STREAMOFF:
    {
        enum v4l2_buf_type i = *(int *)arg;

        if (!ops->vidioc_streamoff)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamoff(file, fh, i);
        break;
    }

驱动控制实现：

static int vidioc_streamoff(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamoff(&dev->vb_vidq, i);
}

STEP 14：
/*14. close method*/
close(fbfd);

static int v4l2_release(struct inode *inode, struct file *filp)
{
    struct video_device *vdev = video_devdata(filp);
    int ret = 0;

    if (vdev->fops->release) {
        if (vdev->lock)
            mutex_lock(vdev->lock);
        vdev->fops->release(filp);
        if (vdev->lock)
            mutex_unlock(vdev->lock);
    }
    /* decrease the refcount unconditionally since the release()
     return value is ignored. */
    video_put(vdev);
    return ret;
}

static int vivi_close(struct file *file)
{
    struct video_device *vdev = video_devdata(file);
    struct vivi_dev *dev = video_drvdata(file);

    dprintk(dev, 1, "close called (dev=%s), file %p\n",
        video_device_node_name(vdev), file);

    if (v4l2_fh_is_singular_file(file))
        vb2_queue_release(&dev->vb_vidq);
    return v4l2_fh_release(file);
}

到此为止，整个过程算是基本完结了，不过其中videobuf2_core.c 在我看来自己必须专门钻研一下了
videobuf2_core.c 是视频数据传输的核心
也可以说是视频驱动的重中之重
待续。。。。。。V4L2用户空间和kernel层driver的交互过程


这篇文章详细分析了V4L2用户空间和kernel层driver的交互过程，目的只有一个：
更清晰的理解V4L2视频驱动程序的系统结构，驱动编程方法，为以后开发视频驱动打好基础


既然从用户层出发探究驱动层，这里先贴出应用层code：
#include
#include
#include
#include
#include  
#include  
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define CLEAR(x) memset (&(x), 0, sizeof (x))
 
struct buffer {
    void * start;
    size_t length;
};
 
static char * dev_name = NULL;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0;
static int time_in_sec_capture=5;
static int fbfd = -1;
static struct fb_var_screeninfo vinfo;
static struct fb_fix_screeninfo finfo;
static char *fbp=NULL;
static long screensize=0;
 
static void errno_exit (const char * s)
{
    fprintf (stderr, "%s error %d, %s/n",s, errno, strerror (errno));
    exit (EXIT_FAILURE);
}
 
static int xioctl (int fd,int request,void * arg)
{
    int r;
    /* Here use this method to make sure cmd success*/
    do r = ioctl (fd, request, arg);
    while (-1 == r && EINTR == errno);
    return r;
}
 
inline int clip(int value, int min, int max) {
    return (value > max ? max : value < min ? min : value);
  }
 
static void process_image (const void * p){
    //ConvertYUVToRGB321;
    unsigned char* in=(char*)p;
    int width=640;
    int height=480;
    int istride=1280;
    int x,y,j;
    int y0,u,y1,v,r,g,b;
    long location=0;
 
    for ( y = 100; y < height + 100; ++y) {
        for (j = 0, x=100; j < width * 2 ; j += 4,x +=2) {
          location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
            (y+vinfo.yoffset) * finfo.line_length;
            
          y0 = in[j];
          u = in[j + 1] - 128; 
          y1 = in[j + 2]; 
          v = in[j + 3] - 128; 
 
          r = (298 * y0 + 409 * v + 128) >> 8;
          g = (298 * y0 - 100 * u - 208 * v + 128) >> 8;
          b = (298 * y0 + 516 * u + 128) >> 8;
        
          fbp[ location + 0] = clip(b, 0, 255);
          fbp[ location + 1] = clip(g, 0, 255);
          fbp[ location + 2] = clip(r, 0, 255); 
          fbp[ location + 3] = 255; 
 
          r = (298 * y1 + 409 * v + 128) >> 8;
          g = (298 * y1 - 100 * u - 208 * v + 128) >> 8;
          b = (298 * y1 + 516 * u + 128) >> 8;
 
          fbp[ location + 4] = clip(b, 0, 255);
          fbp[ location + 5] = clip(g, 0, 255);
          fbp[ location + 6] = clip(r, 0, 255); 
          fbp[ location + 7] = 255; 
          }
        in +=istride;
      }
}
 
static int read_frame (void)
{
    struct v4l2_buffer buf;
    unsigned int i;
 
    CLEAR (buf);
    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    buf.memory = V4L2_MEMORY_MMAP;
     /* 11. VIDIOC_DQBUF把数据放回缓存队列*/
    if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
        switch (errno) {
        case EAGAIN:
        return 0;
        case EIO: 
        default:
            errno_exit ("VIDIOC_DQBUF");
        }
    }
 
    assert (buf.index < n_buffers);
    printf("v4l2_pix_format->field(%d)/n", buf.field);
    //assert (buf.field ==V4L2_FIELD_NONE);
    process_image (buffers[buf.index].start);


    /*12. VIDIOC_QBUF把数据从缓存中读取出来*/
    if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
        errno_exit ("VIDIOC_QBUF");
 
    return 1;
}
 
static void run (void)
{
    unsigned int count;
    int frames;
    frames = 30 * time_in_sec_capture;
 
    while (frames-- > 0) {
        for (;;) {
            fd_set fds;
            struct timeval tv;
            int r;
            FD_ZERO (&fds);
            FD_SET (fd, &fds);
 
            
            tv.tv_sec = 2;
            tv.tv_usec = 0;
             /* 10. poll method*/
            r = select (fd + 1, &fds, NULL, NULL, &tv);
 
            if (-1 == r) {
                if (EINTR == errno)
                    continue;
                errno_exit ("select");
            }
 
            if (0 == r) {
                fprintf (stderr, "select timeout/n");
                exit (EXIT_FAILURE);
            }
 
            if (read_frame())
                break;
            
            }
    }
}
 
static void stop_capturing (void)
{
    enum v4l2_buf_type type;
 
    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    /*13. VIDIOC_STREAMOFF结束视频显示函数*/
    if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
        errno_exit ("VIDIOC_STREAMOFF");
}
 
static void start_capturing (void)
{
    unsigned int i;
    enum v4l2_buf_type type;
 
    for (i = 0; i < n_buffers; ++i) {
        struct v4l2_buffer buf;
        CLEAR (buf);
 
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = i;
         /* 8. VIDIOC_QBUF把数据从缓存中读取出来*/
        if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
            errno_exit ("VIDIOC_QBUF");
    }
 
    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
     /* 9. VIDIOC_STREAMON开始视频显示函数*/
    if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
        errno_exit ("VIDIOC_STREAMON");
    
}
 
static void uninit_device (void)
{
    unsigned int i;
 
    for (i = 0; i < n_buffers; ++i)
        if (-1 == munmap (buffers[i].start, buffers[i].length))
            errno_exit ("munmap");
    
    if (-1 == munmap(fbp, screensize)) {
          printf(" Error: framebuffer device munmap() failed./n");
          exit (EXIT_FAILURE) ;
        } 
    free (buffers);
}
 
 
static void init_mmap (void)
{
    struct v4l2_requestbuffers req;
 
    //mmap framebuffer
    fbp = (char *)mmap(NULL,screensize,PROT_READ | PROT_WRITE,MAP_SHARED ,fbfd, 0);
    if ((int)fbp == -1) {
        printf("Error: failed to map framebuffer device to memory./n");
        exit (EXIT_FAILURE) ;
    }
    memset(fbp, 0, screensize);
    CLEAR (req);
 
    req.count = 4;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;
     /* 6. VIDIOC_REQBUFS分配内存*/
    if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
        if (EINVAL == errno) {
            fprintf (stderr, "%s does not support memory mapping/n", dev_name);
            exit (EXIT_FAILURE);
        } else {
            errno_exit ("VIDIOC_REQBUFS");
        }
    }
 
    if (req.count < 4) {
        fprintf (stderr, "Insufficient buffer memory on %s/n",dev_name);
        exit (EXIT_FAILURE);
    }
 
    buffers = calloc (req.count, sizeof (*buffers));
 
    if (!buffers) {
        fprintf (stderr, "Out of memory/n");
        exit (EXIT_FAILURE);
    }
 
    for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
        struct v4l2_buffer buf;
 
        CLEAR (buf);
 
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = n_buffers;
         /* 7. VIDIOC_QUERYBUF把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址*/
        if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
            errno_exit ("VIDIOC_QUERYBUF");
 
        buffers[n_buffers].length = buf.length;
        buffers[n_buffers].start =mmap (NULL,buf.length,PROT_READ | PROT_WRITE ,MAP_SHARED,fd, buf.m.offset);
 
        if (MAP_FAILED == buffers[n_buffers].start)
            errno_exit ("mmap");
    }
 
}
 
 
 
static void init_device (void)
{
    struct v4l2_capability cap;
    struct v4l2_cropcap cropcap;
    struct v4l2_crop crop;
    struct v4l2_format fmt;
    unsigned int min;
 
 
    // Get fixed screen information
    if (-1==xioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
     printf("Error reading fixed information./n");
     exit (EXIT_FAILURE);
    }
 
    // Get variable screen information
    if (-1==xioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
     printf("Error reading variable information./n");
     exit (EXIT_FAILURE);
    }
    screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;


     /* 2. VIDIOC_QUERYCAP查询驱动功能*/
    if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {
        if (EINVAL == errno) {
            fprintf (stderr, "%s is no V4L2 device/n",dev_name);
            exit (EXIT_FAILURE);
        } else {
            errno_exit ("VIDIOC_QUERYCAP");
        }
    }


    /* Check if it is a video capture device*/
    if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
        fprintf (stderr, "%s is no video capture device/n",dev_name);
        exit (EXIT_FAILURE);
    }


     /* Check if support streaming I/O ioctls*/
    if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
        fprintf (stderr, "%s does not support streaming i/o/n",dev_name);
        exit (EXIT_FAILURE);
    }
 
    CLEAR (cropcap);
    /* Set type*/
    cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
     /* 3. VIDIOC_CROPCAP查询驱动的修剪能力*/
    /* 这里在vivi驱动中我们没有实现此方法，即不支持此操作*/
    if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
        crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        crop.c = cropcap.defrect;
        /* 4. VIDIOC_S_CROP设置视频信号的边框*/
        /* 同样不支持这个操作*/
        if (-1 == xioctl (fd, VIDIOC_S_CROP, &crop)) {
            switch (errno) {
            case EINVAL: 
            break;
            default:
            break;
            }
        }
    }else { }
 
    CLEAR (fmt);
 
    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;
    fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
     /* 5. VIDIOC_S_FMT设置当前驱动的频捕获格式*/
    if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
        errno_exit ("VIDIOC_S_FMT");
 
    init_mmap ();
 
}
 
static void close_device (void)
{
    if (-1 == close (fd))
    errno_exit ("close");
    fd = -1;
    /*14. close method*/
    close(fbfd);
}
 
static void open_device (void)
{
    struct stat st; 
 
    if (-1 == stat (dev_name, &st)) {
     fprintf (stderr, "Cannot identify '%s': %d, %s/n",dev_name, errno, strerror (errno));
     exit (EXIT_FAILURE);
    }
 
    if (!S_ISCHR (st.st_mode)) {
     fprintf (stderr, "%s is no device/n", dev_name);
     exit (EXIT_FAILURE);
    }


    fbfd = open("/dev/fb0", O_RDWR);
    if (fbfd==-1) {
        printf("Error: cannot open framebuffer device./n");
        exit (EXIT_FAILURE);
    }
 
    /* 1. open the char device */
    fd = open (dev_name, O_RDWR| O_NONBLOCK, 0);
    if (-1 == fd) {
     fprintf (stderr, "Cannot open '%s': %d, %s/n",dev_name, errno, strerror (errno));
     exit (EXIT_FAILURE);
    }
}
 
static void usage (FILE * fp,int argc,char ** argv)
{
    fprintf (fp,
    "Usage: %s [options]/n/n"
    "Options:/n"
    "-d | --device name Video device name [/dev/video]/n"
    "-h | --help Print this message/n"
    "-t | --how long will display in seconds/n"
    "",
    argv[0]);
}
 
static const char short_options [] = "d:ht:";
static const struct option long_options [] = {
    { "device", required_argument, NULL, 'd' },
    { "help", no_argument, NULL, 'h' },
    { "time", no_argument, NULL, 't' },
    { 0, 0, 0, 0 }
};
 
int main (int argc,char ** argv)
{
    dev_name = "/dev/video0";
    for (;;) 
    {
        int index;
        int c;
 
        c = getopt_long (argc, argv,short_options, long_options,&index);
        if (-1 == c)
        break;
 
        switch (c) {
        case 0:
        break;
 
        case 'd':
        dev_name = optarg;
        break;
 
        case 'h':
        usage (stdout, argc, argv);
        exit (EXIT_SUCCESS);
        case 't':
        time_in_sec_capture = atoi(optarg);
        break;
 
        default:
        usage (stderr, argc, argv);
        exit (EXIT_FAILURE);
        }
    }
 
    open_device();
    init_device();
    start_capturing();
    run();
    stop_capturing();
    uninit_device();
    close_device();
    exit(EXIT_SUCCESS);
    return 0;
}
上面code中我已经标注出程序顺序指向的步骤1--14步，下面将一一说明应用从做这14步时驱动层是怎样响应，变化过程，驱动加载初始化部分上一篇文章已经说过了
正式开始取经之路哇。。。。。。。


STEP 1：
fd = open (dev_name, O_RDWR| O_NONBLOCK, 0);
打开字符设备，这个字符设备是video_device_register时创建的，code在v4l2_dev.c中，具体：
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);


    /* 
    * Here using the API you get the method you get the open() method write
    * The other methods in fops use the same method to use you own code 
    */
    if (vdev->fops->open) {
        if (vdev->lock && mutex_lock_interruptible(vdev->lock)) {
            ret = -ERESTARTSYS;
            goto err;
        }
        if (video_is_registered(vdev))
            ret = vdev->fops->open(filp);
        else
            ret = -ENODEV;
        if (vdev->lock)
            mutex_unlock(vdev->lock);
    }


err:
    /* decrease the refcount in case of an error */
    if (ret)
        video_put(vdev);
    return ret;
}
重点在标注部分，通过这个V4L2的API调用我们自己驱动程序中定义的open方法，我们自己的open方法所属的fops是在vivi.c驱动程序的vivi_create_instance方法中video_device_register之前关联进来的


int v4l2_fh_open(struct file *filp)
{
    struct video_device *vdev = video_devdata(filp);
    struct v4l2_fh *fh = kzalloc(sizeof(*fh), GFP_KERNEL);


    /*
    * IN the open method, do only one job
    * set v4l2_fh into filp->private_data for later use, and initial v4l2_fh
    */
    filp->private_data = fh;
    if (fh == NULL)
        return -ENOMEM;
    v4l2_fh_init(fh, vdev);
    v4l2_fh_add(fh);
    return 0;
}
EXPORT_SYMBOL_GPL(v4l2_fh_open);
这个open方法只是初始化了一个v4l2_fh，并关联到filp->private中，方便以后使用
这里设置V4L2_FL_USES_V4L2_FH这个标志位，设置优先级为UNSET，如果我们的自己驱动程序实现了，支持
VIDIOC_SUBSCRIBE_EVENT,那么v4l2_event_init，在events初始化中初始化链表，并设置sequence为-1，如果不支持，则设置fh->events为NULL
最后add list


STEP 2：
if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap))
这么调用完成下面过程，不行的从驱动层获取cap。直到成功拿到我们想要的数据
static int xioctl (int fd,int request,void * arg)
{
    int r;
    /* Here use this method to make sure cmd success*/
    do r = ioctl (fd, request, arg);
    while (-1 == r && EINTR == errno);
    return r;
}
也就是调用驱动层的ioctl方法，从v4l2 api中的ictol 调用我们自己定义的ioctl ，这中间的过程不在多做说明，我们自己的驱动的控制过程由v4l2_ioctl.c这个文件中的方法实现，一个很庞大的switch
值得一提的是，慢慢后面你会明白的，这里v4l2_ioctl.c这个文件中的方法实现其实只是会中转站，它接着就回调了我们自己驱动程序中定义的控制接口，后面再说吧
long video_ioctl2(struct file *file,
     unsigned int cmd, unsigned long arg)
{
    return video_usercopy(file, cmd, arg, __video_do_ioctl);
}
这里这个__video_do_ioctl方法其实完全做了我们所有的控制过程，又为什么又要经过video_usercopy这个方法呢，不妨看一看这个方法
long
video_usercopy(struct file *file, unsigned int cmd, unsigned long arg,
     v4l2_kioctl func)
{
    char    sbuf[128];
    void *mbuf = NULL;
    void    *parg = (void *)arg;
    long    err = -EINVAL;
    bool    has_array_args;
    size_t array_size = 0;
    void __user *user_ptr = NULL;
    void    **kernel_ptr = NULL;


    /* Copy arguments into temp kernel buffer */
    if (_IOC_DIR(cmd) != _IOC_NONE) {
........这里检查128个字节的大小是否够存放用户端发送来的数据，不够则需要重新申请一个新的内存用来存放，指向parg这个地址
        if (_IOC_SIZE(cmd) <= sizeof(sbuf)) {
            parg = sbuf;
        } else {
            /* too big to allocate from stack */
            mbuf = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
            if (NULL == mbuf)
                return -ENOMEM;
            parg = mbuf;
        }


        err = -EFAULT;
        if (_IOC_DIR(cmd) & _IOC_WRITE) {
            unsigned long n = cmd_input_size(cmd);


            if (copy_from_user(parg, (void __user *)arg, n))
                goto out;


            /* zero out anything we don't copy from userspace */
            if (n < _IOC_SIZE(cmd))
                memset((u8 *)parg + n, 0, _IOC_SIZE(cmd) - n);
        } else {
            /* read-only ioctl */
            memset(parg, 0, _IOC_SIZE(cmd));
        }
    }
....check
    err = check_array_args(cmd, parg, &array_size, &user_ptr, &kernel_ptr);
    if (err < 0)
        goto out;
    has_array_args = err;
....这里这块如果用户端有数据写到kernel，这里负责数据拷贝
    if (has_array_args) {
        /*
         * When adding new types of array args, make sure that the
         * parent argument to ioctl (which contains the pointer to the
         * array) fits into sbuf (so that mbuf will still remain
         * unused up to here).
         */
        mbuf = kmalloc(array_size, GFP_KERNEL);
        err = -ENOMEM;
        if (NULL == mbuf)
            goto out_array_args;
        err = -EFAULT;
        if (copy_from_user(mbuf, user_ptr, array_size))
            goto out_array_args;
        *kernel_ptr = mbuf;
    }


    /* Handles IOCTL */
    err = func(file, cmd, parg);
    if (err == -ENOIOCTLCMD)
        err = -EINVAL;
    if (has_array_args) {
        *kernel_ptr = user_ptr;
        if (copy_to_user(user_ptr, mbuf, array_size))
            err = -EFAULT;
        goto out_array_args;
    }
    if (err < 0)
        goto out;


out_array_args:
    /* Copy results into user buffer */
    switch (_IOC_DIR(cmd)) {
    case _IOC_READ:
    case (_IOC_WRITE | _IOC_READ):
        if (copy_to_user((void __user *)arg, parg, _IOC_SIZE(cmd)))
            err = -EFAULT;
        break;
    }


out:
    kfree(mbuf);
    return err;
}
EXPORT_SYMBOL(video_usercopy);
自我感觉这个方法还是有很多精妙之处的，主要的控制过程是在我标注的地方调用完成的，这个调用之前做check动作，检查用户端发来的命令是否合法，
最重要的是把用户端的数据copy到kernel 端；而这个调用之后，则是我们处理完我们的动作之后，我们在这里吧用户端请求的数据从kernel 端copy到用户端
这样做的好处是显而易见的，任务明确，控制只做控制，用户空间和kernel空间数据的copy在所有控制之前，控制之后进行
以上动作做完之后，进入庞大的控制中枢，这来开始至贴出具体到某一个控制的代码，否则code过大，不易分析：


case VIDIOC_QUERYCAP://查询视频设备的功能
    {
        struct v4l2_capability *cap = (struct v4l2_capability *)arg;


        if (!ops->vidioc_querycap)
            break;


        ret = ops->vidioc_querycap(file, fh, cap);
        if (!ret)/* i don't think here need to check */
            dbgarg(cmd, "driver=%s, card=%s, bus=%s, "
                    "version=0x%08x, "
                    "capabilities=0x%08x\n",
                    cap->driver, cap->card, cap->bus_info,
                    cap->version,
                    cap->capabilities);
        break;
    }
这来调用了我们自己驱动中填充的v4l2_ioctl_ops结构体，从这里开始，我上面说到的话得到了验证，这就是linux 中API 的强大之处
作为中间层的这个控制中枢又回调驱动自己定义编写的控制
/* ------------------------------------------------------------------
    IOCTL vidioc handling
   ------------------------------------------------------------------*/
static int vidioc_querycap(struct file *file, void *priv,
                    struct v4l2_capability *cap)
{
    struct vivi_dev *dev = video_drvdata(file);


    strcpy(cap->driver, "vivi");
    strcpy(cap->card, "vivi");
    strlcpy(cap->bus_info, dev->v4l2_dev.name, sizeof(cap->bus_info));
    cap->version = VIVI_VERSION;
    cap->capabilities = V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING | \
             V4L2_CAP_READWRITE;
    return 0;
}
这来做的事情很简单，只是将配置信息保存到cap这个变量中，之后上传给用户空间


STEP 3：
/* 3. VIDIOC_CROPCAP查询驱动的修剪能力*/
/* 这里在vivi 驱动中我们没有实现此方法，即不支持此操作*/
if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap))
这个判断在中间层控制中枢中进行的，check到我们自己的驱动中没有这个控制功能的支持
所以这里的STEP 4同样不会进行


STEP 5：
/* 5. VIDIOC_S_FMT设置当前驱动的频捕获格式*/
if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
对应到控制中心是这样的


case VIDIOC_S_FMT:
    {
        struct v4l2_format *f = (struct v4l2_format *)arg;


        /* FIXME: Should be one dump per type */
        dbgarg(cmd, "type=%s\n", prt_names(f->type, v4l2_type_names));


        switch (f->type) {
        case V4L2_BUF_TYPE_VIDEO_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.pix);
            v4l_print_pix_fmt(vfd, &f->fmt.pix);
            if (ops->vidioc_s_fmt_vid_cap) {
                ret = ops->vidioc_s_fmt_vid_cap(file, fh, f);
            } else if (ops->vidioc_s_fmt_vid_cap_mplane) {
                if (fmt_sp_to_mp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_cap_mplane(file, fh,
                                    &f_copy);
                if (ret)
                    break;


                if (f_copy.fmt.pix_mp.num_planes > 1) {
                    /* Drivers shouldn't adjust from 1-plane
                     * to more than 1-plane formats */
                    ret = -EBUSY;
                    WARN_ON(1);
                    break;
                }


                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE:
            CLEAR_AFTER_FIELD(f, fmt.pix_mp);
            v4l_print_pix_fmt_mplane(vfd, &f->fmt.pix_mp);
            if (ops->vidioc_s_fmt_vid_cap_mplane) {
                ret = ops->vidioc_s_fmt_vid_cap_mplane(file,
                                    fh, f);
            } else if (ops->vidioc_s_fmt_vid_cap &&
                    f->fmt.pix_mp.num_planes == 1) {
                if (fmt_mp_to_sp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_cap(file,
                                fh, &f_copy);
                if (ret)
                    break;


                ret = fmt_sp_to_mp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OVERLAY:
            CLEAR_AFTER_FIELD(f, fmt.win);
            if (ops->vidioc_s_fmt_vid_overlay)
                ret = ops->vidioc_s_fmt_vid_overlay(file,
                                 fh, f);
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.pix);
            v4l_print_pix_fmt(vfd, &f->fmt.pix);
            if (ops->vidioc_s_fmt_vid_out) {
                ret = ops->vidioc_s_fmt_vid_out(file, fh, f);
            } else if (ops->vidioc_s_fmt_vid_out_mplane) {
                if (fmt_sp_to_mp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_out_mplane(file, fh,
                                    &f_copy);
                if (ret)
                    break;


                if (f_copy.fmt.pix_mp.num_planes > 1) {
                    /* Drivers shouldn't adjust from 1-plane
                     * to more than 1-plane formats */
                    ret = -EBUSY;
                    WARN_ON(1);
                    break;
                }


                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE:
            CLEAR_AFTER_FIELD(f, fmt.pix_mp);
            v4l_print_pix_fmt_mplane(vfd, &f->fmt.pix_mp);
            if (ops->vidioc_s_fmt_vid_out_mplane) {
                ret = ops->vidioc_s_fmt_vid_out_mplane(file,
                                    fh, f);
            } else if (ops->vidioc_s_fmt_vid_out &&
                    f->fmt.pix_mp.num_planes == 1) {
                if (fmt_mp_to_sp(f, &f_copy))
                    break;
                ret = ops->vidioc_s_fmt_vid_out(file,
                                fh, &f_copy);
                if (ret)
                    break;


                ret = fmt_mp_to_sp(&f_copy, f);
            }
            break;
        case V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY:
            CLEAR_AFTER_FIELD(f, fmt.win);
            if (ops->vidioc_s_fmt_vid_out_overlay)
                ret = ops->vidioc_s_fmt_vid_out_overlay(file,
                    fh, f);
            break;
        case V4L2_BUF_TYPE_VBI_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.vbi);
            if (ops->vidioc_s_fmt_vbi_cap)
                ret = ops->vidioc_s_fmt_vbi_cap(file, fh, f);
            break;
        case V4L2_BUF_TYPE_VBI_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.vbi);
            if (ops->vidioc_s_fmt_vbi_out)
                ret = ops->vidioc_s_fmt_vbi_out(file, fh, f);
            break;
        case V4L2_BUF_TYPE_SLICED_VBI_CAPTURE:
            CLEAR_AFTER_FIELD(f, fmt.sliced);
            if (ops->vidioc_s_fmt_sliced_vbi_cap)
                ret = ops->vidioc_s_fmt_sliced_vbi_cap(file,
                                    fh, f);
            break;
        case V4L2_BUF_TYPE_SLICED_VBI_OUTPUT:
            CLEAR_AFTER_FIELD(f, fmt.sliced);
            if (ops->vidioc_s_fmt_sliced_vbi_out)
                ret = ops->vidioc_s_fmt_sliced_vbi_out(file,
                                    fh, f);
            break;
        case V4L2_BUF_TYPE_PRIVATE:
            /* CLEAR_AFTER_FIELD(f, fmt.raw_data); <- does nothing */
            if (ops->vidioc_s_fmt_type_private)
                ret = ops->vidioc_s_fmt_type_private(file,
                                fh, f);
            break;
        }
        break;
    }
以后根据不同的type 决定了我们自己驱动程序中不同的控制实现，这个type是根据用户空间的设置而定的，还包括其他几个参数，如下：
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;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
这里根据设定的type，所以驱动程序的处理过程如下：


static int vidioc_s_fmt_vid_cap(struct file *file, void *priv,
                    struct v4l2_format *f)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;
....在下面这个函数中，做了一些试探性的动作，如果试探失败则下面不会赋值，试探通过则后续正常设置即可，在这个试探函数中同时做了一些设置动作
    int ret = vidioc_try_fmt_vid_cap(file, priv, f);
    if (ret < 0)
        return ret;


    if (vb2_is_streaming(q)) {
        dprintk(dev, 1, "%s device busy\n", __func__);
        return -EBUSY;
    }
....按用户空间需求设置
    dev->fmt = get_format(f);
    dev->width = f->fmt.pix.width;
    dev->height = f->fmt.pix.height;
    dev->field = f->fmt.pix.field;


    return 0;
}


STEP6 ：
/* 6. VIDIOC_REQBUFS分配内存*/
if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req))
中间层控制中枢：
case VIDIOC_REQBUFS:
    {
        struct v4l2_requestbuffers *p = arg;


        if (!ops->vidioc_reqbufs)
            break;
........这个方法check 驱动必须实现了fmt方法，看具体看代码
        ret = check_fmt(ops, p->type);
        if (ret)
            break;


        if (p->type < V4L2_BUF_TYPE_PRIVATE)
            CLEAR_AFTER_FIELD(p, memory);


        ret = ops->vidioc_reqbufs(file, fh, p);
        dbgarg(cmd, "count=%d, type=%s, memory=%s\n",
                p->count,
                prt_names(p->type, v4l2_type_names),
                prt_names(p->memory, v4l2_memory_names));
        break;
    }
驱动中实现：
static int vidioc_reqbufs(struct file *file, void *priv,
             struct v4l2_requestbuffers *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_reqbufs(&dev->vb_vidq, p);
}
到了这里来到了这个全新的话题，实现
vb2_reqbufs(&dev->vb_vidq, p);
这里暂且不讨论这个方法，相对较复杂，待日后研究，先把注释部分放到这里，包括其他内存操作，之后深入研究补充，专门作为一篇整理
/**
 * Should be called from vidioc_reqbufs ioctl handler of a driver.
 * This function:
 * 1) verifies streaming parameters passed from the userspace,
 * 2) sets up the queue,
 * 3) negotiates number of buffers and planes per buffer with the driver to be used during streaming,
 * 4) allocates internal buffer structures (struct vb2_buffer), according to the agreed parameters,
 * 5) for MMAP memory type, allocates actual video memory, using the memory handling/allocation routines provided during queue initialization
 * If req->count is 0, all the memory will be freed instead.
 * If the queue has been allocated previously (by a previous vb2_reqbufs) call
 * and the queue is not busy, memory will be reallocated.
 * The return values from this function are intended to be directly returned from vidioc_reqbufs handler in driver.
 */


STEP 7：
/* 7. VIDIOC_QUERYBUF把VIDIOC_REQBUFS中分配的数据缓存转换成物理地址*/
if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
中间层控制中枢：
case VIDIOC_QUERYBUF:
    {
        struct v4l2_buffer *p = arg;


        if (!ops->vidioc_querybuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;


        ret = ops->vidioc_querybuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }
驱动中控制实现：
static int vidioc_querybuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_querybuf(&dev->vb_vidq, p);
}
/**
 * Should be called from vidioc_querybuf ioctl handler in driver.
 * This function will verify the passed v4l2_buffer structure and fill the
 * relevant information for the userspace.
 * The return values from this function are intended to be directly returned from vidioc_querybuf handler in driver.
 */


STEP 8：
/* 8. VIDIOC_QBUF把数据从缓存中读取出来*/
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
中间层控制中枢：
case VIDIOC_QBUF:
    {
        struct v4l2_buffer *p = arg;


        if (!ops->vidioc_qbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;


        ret = ops->vidioc_qbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }
驱动中控制实现：


static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_qbuf(&dev->vb_vidq, p);
}
/**
 * Should be called from vidioc_qbuf ioctl handler of a driver.
 * This function:
 * 1) verifies the passed buffer,
 * 2) calls buf_prepare callback in the driver (if provided), in which driver-specific buffer initialization can be performed,
 * 3) if streaming is on, queues the buffer in driver by the means of buf_queue callback for processing.
 * The return values from this function are intended to be directly returned from vidioc_qbuf handler in driver.
 */


STEP 9：
/* 9. VIDIOC_STREAMON开始视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
中间层控制中枢：
case VIDIOC_STREAMON:
    {
        enum v4l2_buf_type i = *(int *)arg;


        if (!ops->vidioc_streamon)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamon(file, fh, i);
        break;
    }
驱动控制实现;


static int vidioc_streamon(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamon(&dev->vb_vidq, i);
}
/**
 * Should be called from vidioc_streamon handler of a driver.
 * This function:
 * 1) verifies current state
 * 2) starts streaming and passes any previously queued buffers to the driver
 * The return values from this function are intended to be directly returned from vidioc_streamon handler in the driver.
 */


STEP 10：
/* 10. poll method*/
select (fd + 1, &fds, NULL, NULL, &tv);
从V4L2驱动API开始：
static unsigned int v4l2_poll(struct file *filp, struct poll_table_struct *poll)
{
    struct video_device *vdev = video_devdata(filp);
    int ret = POLLERR | POLLHUP;


    if (!vdev->fops->poll)
        return DEFAULT_POLLMASK;
    if (vdev->lock)
        mutex_lock(vdev->lock);
    if (video_is_registered(vdev))
        ret = vdev->fops->poll(filp, poll);
    if (vdev->lock)
        mutex_unlock(vdev->lock);
    return ret;
}
驱动实现：
static unsigned int
vivi_poll(struct file *file, struct poll_table_struct *wait)
{
    struct vivi_dev *dev = video_drvdata(file);
    struct vb2_queue *q = &dev->vb_vidq;


    dprintk(dev, 1, "%s\n", __func__);
    return vb2_poll(q, file, wait);
}
/**
 * This function implements poll file operation handler for a driver.
 * For CAPTURE queues, if a buffer is ready to be dequeued, the userspace will be informed that the file descriptor of a video device is available for reading.
 * For OUTPUT queues, if a buffer is ready to be dequeued, the file descriptor will be reported as available for writing.
 * The return values from this function are intended to be directly returned from poll handler in driver.
 */


STEP 11：
/* 11. VIDIOC_DQBUF把数据放回缓存队列*/
if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf))
中间层控制中枢：
case VIDIOC_DQBUF:
    {
        struct v4l2_buffer *p = arg;


        if (!ops->vidioc_dqbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;


        ret = ops->vidioc_dqbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }
驱动控制实现：


static int vidioc_dqbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_dqbuf(&dev->vb_vidq, p, file->f_flags & O_NONBLOCK);
}
/**
 * Should be called from vidioc_dqbuf ioctl handler of a driver.
 * This function:
 * 1) verifies the passed buffer,
 * 2) calls buf_finish callback in the driver (if provided), in which driver can perform any additional operations that may be required before returning the buffer to userspace, such as cache sync,
 * 3) the buffer struct members are filled with relevant information for the userspace.
 * The return values from this function are intended to be directly returned from vidioc_dqbuf handler in driver.
 */


STEP 12：
/*12. VIDIOC_QBUF把数据从缓存中读取出来*/
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
中间层控制中枢：
case VIDIOC_QBUF:
    {
        struct v4l2_buffer *p = arg;


        if (!ops->vidioc_qbuf)
            break;
        ret = check_fmt(ops, p->type);
        if (ret)
            break;


        ret = ops->vidioc_qbuf(file, fh, p);
        if (!ret)
            dbgbuf(cmd, vfd, p);
        break;
    }
驱动控制实现：


static int vidioc_qbuf(struct file *file, void *priv, struct v4l2_buffer *p)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_qbuf(&dev->vb_vidq, p);
}


STEP 13：
/*13. VIDIOC_STREAMOFF结束视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
中间层控制中枢：
case VIDIOC_STREAMOFF:
    {
        enum v4l2_buf_type i = *(int *)arg;


        if (!ops->vidioc_streamoff)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamoff(file, fh, i);
        break;
    }
驱动控制实现：


static int vidioc_streamoff(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamoff(&dev->vb_vidq, i);
}


STEP 13：
/*13. VIDIOC_STREAMOFF结束视频显示函数*/
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
中间层控制中枢：
case VIDIOC_STREAMOFF:
    {
        enum v4l2_buf_type i = *(int *)arg;


        if (!ops->vidioc_streamoff)
            break;
        dbgarg(cmd, "type=%s\n", prt_names(i, v4l2_type_names));
        ret = ops->vidioc_streamoff(file, fh, i);
        break;
    }
驱动控制实现：


static int vidioc_streamoff(struct file *file, void *priv, enum v4l2_buf_type i)
{
    struct vivi_dev *dev = video_drvdata(file);
    return vb2_streamoff(&dev->vb_vidq, i);
}


STEP 14：
/*14. close method*/
close(fbfd);


static int v4l2_release(struct inode *inode, struct file *filp)
{
    struct video_device *vdev = video_devdata(filp);
    int ret = 0;


    if (vdev->fops->release) {
        if (vdev->lock)
            mutex_lock(vdev->lock);
        vdev->fops->release(filp);
        if (vdev->lock)
            mutex_unlock(vdev->lock);
    }
    /* decrease the refcount unconditionally since the release()
     return value is ignored. */
    video_put(vdev);
    return ret;
}


static int vivi_close(struct file *file)
{
    struct video_device *vdev = video_devdata(file);
    struct vivi_dev *dev = video_drvdata(file);


    dprintk(dev, 1, "close called (dev=%s), file %p\n",
        video_device_node_name(vdev), file);


    if (v4l2_fh_is_singular_file(file))
        vb2_queue_release(&dev->vb_vidq);
    return v4l2_fh_release(file);
}
到此为止，整个过程算是基本完结了，不过其中videobuf2_core.c 在我看来自己必须专门钻研一下了
videobuf2_core.c 是视频数据传输的核心
也可以说是视频驱动的重中之重
待续。。。。。。