V4L2用户空间和kernel层driver的交互过程

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mb60ffdbe016b5d 2021-07-27 21:56:08 博主文章分类：Linux

文章标签 V4L2 文章分类 C/C++ 后端开发

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
V4L2用户空间和kernel层driver的交互过程_V4L2

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。。。。 V4L2用户空间和kernel层driver的交互过程_V4L2_02

。。。

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);