v4l2虚拟驱动的应用测试程序讲解

简介

　　在前面我们已经完成了myvivi这个虚拟的v4l2摄像头驱动程序的编写。这里继续编写一个该驱动的应用测试程序来加深一下该驱动的工作原理。

具体代码

#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))

#define SET_WIDTH 320
#define SET_HEIGHT 240

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;
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){
unsigned char* in=(char*)p;
int width=SET_WIDTH;
int height=SET_HEIGHT;
int istride=SET_WIDTH * 2;
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;

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);
process_image(buffers[buf.index].start);
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;

r = select (fd + 1, &fds, NULL, NULL, &tv);

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

if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
}

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

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;

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) {    //if (req.count < 2)
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;

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;

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");
}
}

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

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

CLEAR (fmt);

fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = SET_WIDTH;
fmt.fmt.pix.height = SET_HEIGHT;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;

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;
close(fbfd);
}

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

//open camera
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);
}
}

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

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

return 0;
}

效果演示

　　用gcc编译好这个应用程序之后，使用ctrl+Alt+F1切换控制台，然后在运行这个应用程序，才能看到效果。。当然，运行这个程序之前，需要保证myvivi.ko
这个驱动正常加载成功了。最后演示效果如下：

代码分析

open_device

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

//open camera
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);
}
}

　　这里打开了一个用来在电脑上显示的framebuffer，和我们对应的myvivi.ko生成的/dev/video0设备节点。
这里的open /dev/video0操作，就是对应调用到myvivi驱动程序中的myvivi_fops-->myvivi_open。

init_devices

static void init_device (void)
{
// Get fixed screen information
if (-1==xioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
}

// Get variable screen information
if (-1==xioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
}
screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;

if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {
}

if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
}

if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
}

CLEAR (fmt);

fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = SET_WIDTH;
fmt.fmt.pix.height = SET_HEIGHT;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;

if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
errno_exit ("VIDIOC_S_FMT");

init_mmap();
}

　　在init_device中，最开始是获得fb0相关信息，这个我们暂时不关系，然后ioctl-->VIDIOC_QUERYCAP来判断/dev/video0是否是一个video设备，对应到myvivi
驱动中的函数：myvivi_ioctl_ops-->myvivi_ioctl_ops。在这个函数中我们还设置了它的属性为V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING;这两个属性，也在
这里检查了一遍。接着该函数使用ioctl-->VIDIOC_S_FMT来设置video格式为V4L2_PIX_FMT_YUYV。对应到myvivi驱动中函数为：
myvivi_ioctl_ops-->myvivi_vidioc_s_fmt_vid_cap,我们还记得这个函数会首先利用函数myvivi_vidioc_try_fmt_vid_cap来检查一遍应用程序这里传入的video格式
是否被myvivi驱动支持，如果支持就保存到驱动的一个结构体：myvivi_format中去。显然fmt.fmt.pix.width=320，fmt.fmt.pix.height=240,以及格式为
V4L2_PIX_FMT_YUYV，正好是被myvivi这个设备驱动程序支持的。

init_mmap

　　同时在init_device函数的最后还调用了init_mmap来做buffer队列的初始化。

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;

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) {    //if (req.count < 2)
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;

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");
}

}

　　最开始也是fb0相关的内存操作，我们这里不关心。然后接着是ioctl--->VIDIOC_REQBUFS,对应到驱动中被调用到的函数为：
myvivi_ioctl_ops-->myvivi_vidioc_reqbufs,然后之前我们说过在函数myvivi_vidioc_reqbufs中最终会调用到myvivi_video_qops-->myvivi_buffer_setup。
在函数myvivi_buffer_setup中，因为传入的块数不为0，所以继续保持为4块，然后将myvivi_format.fmt.pix.sizeimage这个图像大小保存到myvivi_vb_vidqueue
队列中。

　　接着init_mmap中，循环req.count次，也就是4次，来分配和映射数据块内存。
　　具体方式为，首先利用ioctl-->VIDIOC_QUERYBUF,对应调用到驱动中函数：myvivi_ioctl_ops-->myvivi_vidioc_querybuf-->videobuf_querybuf.
然后会返回buffer数据块长度和对应的offset位置。接着通过mmap函数来分配这块数据内存并映射到buffers[n_buffers].start，如此循环req.count次之后，
也就设置好了对应的数据块。mmap函数就是对应到驱动中的：myvivi_fops-->myvivi_mmap-->videobuf_mmap_mapper-->__videobuf_mmap_mapper。然后在
__videobuf_mmap_mapper函数中根据传入的参数，进行内存分配和映射。

start_capturing

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;

if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
}

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
errno_exit ("VIDIOC_STREAMON");
}

　　循环了块的次数，这里也就是4次，然后在里面进行四次ioctl-->VIDIOC_QBUF。对应的也就是驱动函数:myvivi_ioctl_ops--->myvivi_vidioc_qbuf。
这个函数最终调用的是v4l2内部实现的函数videobuf_qbuf。这个函数中，依次调用了函数：myvivi_video_qops-->myvivi_buffer_prepare 和
myvivi_video_qops-->myvivi_buffer_queue，在myvivi_buffer_prepare 函数中往videobuf_queue队列中填入了imagesize、field之列的关键信息。接着
用myvivi_buffer_queue函数将当前的myvivi_vb_vidqueue->queue放入到了驱动的本地队列myvivi_vb_local_queue。这样循环4次之后，就将myvivi_vb_vidqueue
中所有的queue都放入了本地队列myvivi_vb_local_queue中。

　　接着应用程序ioctl-->VIDIOC_STREAMON,对应到驱动函数myvivi_vidioc_streamon中，表示开始启动使用。

run

static void run (void)
{
unsigned int count;
int frames;
frames = 30 * 5;

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;

r = select (fd + 1, &fds, NULL, NULL, &tv);
...........
if(read_frame())
break;
}
}
}

　　这里相当于是循环了5秒，一秒30fps。然后循环中用select来检控。前面提到过select对应到驱动的poll中，如果queue中没有数据，就会睡眠等待。
直到定时器函数填充好数据，然后唤醒在poll中等待的进程。然后用read_frame来读取数据。

read_frame

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;

if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
.................
}
assert (buf.index < n_buffers);
process_image(buffers[buf.index].start);
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");

return 1;
}

　　首先ioctl-->VIDIOC_DQBUF,对应到驱动中:myvivi_vidioc_dqbuf,取出已经比定时器函数填充好数据了的myvivi_vb_vidqueue->queue到buf中，然后将该数据
交给process_image进行转换和在fb0中进行显示出来。处理完数据之后接着又一次用ioctl-->VIDIOC_QBUF，对应到驱动中将当前myvivi_vb_vidqueue->queue重新
放回到本地队列myvivi_vb_local_queue，等待被下一次使用。这样不断循环就实现了数据的读取、显示和处理完数据之后的buffer从新放回队列。

stop_capturing

static void stop_capturing (void)                                                                                                    {
enum v4l2_buf_type type;

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
errno_exit ("VIDIOC_STREAMOFF");
}

　　对应到也就是驱动中:myvivi_ioctl_ops-->myvivi_vidioc_streamoff,表示停止使用。

uninit_device/close_device

　　这两个函数对应的操作：接触所有的buffer映射和释放到buffer，然后调用myvivi_fops-->myvivi_close,来删除驱动中的定时器以及一些对应的清除工作。
这样一个v4l2的应用测试程序，以及它如何和驱动交互的分析就结束了。