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基于V4L2的视频驱动开发(2)

2024/7/19 14:06:08发布19次查看
基于v4l2 的视频驱动开发(2) 作者:刘洪涛 , 华清远见嵌入式学院 讲师。 三、 v4l2 api 及数据结构 v4l2 是 v4l 的升级版本,为 linux 下视频设备程序提供了一套接口规范。包括一套数据结构和底层 v4l2 驱动接口。 1 、常用的结构体在内核目录 include/lin
基于v4l2的视频驱动开发(2)
作者:刘洪涛,华清远见嵌入式学院讲师。
三、 v4l2 api及数据结构
v4l2是v4l的升级版本,为linux下视频设备程序提供了一套接口规范。包括一套数据结构和底层v4l2驱动接口。
1、常用的结构体在内核目录include/linux/videodev2.h中定义
struct v4l2_requestbuffers        //申请帧缓冲,对应命令vidioc_reqbufs
        struct v4l2_capability        //视频设备的功能,对应命令vidioc_querycap
        struct v4l2_input        //视频输入信息,对应命令vidioc_enuminput
        struct v4l2_standard        //视频的制式,比如pal,ntsc,对应命令vidioc_enumstd
        struct v4l2_format        //帧的格式,对应命令vidioc_g_fmt、vidioc_s_fmt等
        struct v4l2_buffer        //驱动中的一帧图像缓存,对应命令vidioc_querybuf
        struct v4l2_crop        //视频信号矩形边框
        v4l2_std_id        //视频制式
2、常用的ioctl接口命令也在include/linux/videodev2.h中定义
vidioc_reqbufs //分配内存
        vidioc_querybuf         //把vidioc_reqbufs中分配的数据缓存转换成物理地址
        vidioc_querycap        //查询驱动功能
        vidioc_enum_fmt        //获取当前驱动支持的视频格式
        vidioc_s_fmt        //设置当前驱动的频捕获格式
        vidioc_g_fmt        //读取当前驱动的频捕获格式
        vidioc_try_fmt        //验证当前驱动的显示格式
        vidioc_cropcap        //查询驱动的修剪能力
        vidioc_s_crop        //设置视频信号的矩形边框
        vidioc_g_crop        //读取视频信号的矩形边框
        vidioc_qbuf        //把数据从缓存中读取出来
        vidioc_dqbuf        //把数据放回缓存队列
        vidioc_streamon        //开始视频显示函数
        vidioc_streamoff        //结束视频显示函数
        vidioc_querystd         //检查当前视频设备支持的标准,例如pal或ntsc。
3、操作流程
v4l2提供了很多访问接口,你可以根据具体需要选择操作方法。需要注意的是,很少有驱动完全实现了所有的接口功能。所以在使用时需要参考驱动源码,或仔细阅读驱动提供者的使用说明。
下面列举出一种操作的流程,供参考。
(1)打开设备文件
                  int fd = open(devicename,mode);
                  devicename:/dev/video0、/dev/video1 ……
                  mode:o_rdwr [| o_nonblock]
如果使用非阻塞模式调用视频设备,则当没有可用的视频数据时,不会阻塞,而立刻返回。
(2)取得设备的capability
          struct v4l2_capability capability;
                  int ret = ioctl(fd, vidioc_querycap, &capability);
看看设备具有什么功能,比如是否具有视频输入特性。
(3)选择视频输入
          struct v4l2_input input;
                  ……初始化input
                  int ret = ioctl(fd, vidioc_querycap, &input);
一个视频设备可以有多个视频输入。如果只有一路输入,这个功能可以没有。
(4)检测视频支持的制式
          v4l2_std_id std;
                  do {
                                ret = ioctl(fd, vidioc_querystd, &std);
                   } while (ret == -1 && errno == eagain);
                switch (std) {
                case v4l2_std_ntsc:
                                //……
                case v4l2_std_pal:
                                //……
                }
(5)设置视频捕获格式
          struct v4l2_format fmt;
                  fmt.type = v4l2_buf_type_video_output;
                  fmt.fmt.pix.pixelformat = v4l2_pix_fmt_uyvy;
                  fmt.fmt.pix.height = height;
                  fmt.fmt.pix.width = width;
                  fmt.fmt.pix.field = v4l2_field_interlaced;
                  ret = ioctl(fd, vidioc_s_fmt, &fmt);
                  if(ret) {
                          perror(vidioc_s_fmt\n);
                          close(fd);
                          return -1;
                  }
(6)向驱动申请帧缓存
          struct v4l2_requestbuffers req;
                   if (ioctl(fd, vidioc_reqbufs, &req) == -1) {
                          return -1;
                   }
v4l2_requestbuffers结构中定义了缓存的数量,驱动会据此申请对应数量的视频缓存。多个缓存可以用于建立fifo,来提高视频采集的效率。
(7)获取每个缓存的信息,并mmap到用户空间
          typedef struct videobuffer {
                          void *start;
                          size_t length;
                  } videobuffer;
          videobuffer* buffers = calloc( req.count, sizeof(*buffers) );
                  struct v4l2_buffer buf;
          for (numbufs = 0; numbufs 映射所有的缓存
                          memset( &buf, 0, sizeof(buf) );
                          buf.type = v4l2_buf_type_video_capture;
                          buf.memory = v4l2_memory_mmap;
                          buf.index = numbufs;
                          if (ioctl(fd, vidioc_querybuf, &buf) == -1) {//获取到对应index的缓存信息,此处主要利用length信息及offset信息来完成后面的mmap操作。
                                  return -1;
                          }
                  buffers[numbufs].length = buf.length;
                          // 转换成相对地址
                          buffers[numbufs].start = mmap(null, buf.length,
                                  prot_read | prot_write,
                                  map_shared,
                                  fd, buf.m.offset);
                  if (buffers[numbufs].start == map_failed) {
                                  return -1;
                          }
(8)开始采集视频
          int buf_type= v4l2_buf_type_video_capture;
                  int ret = ioctl(fd, vidioc_streamon, &buf_type);
(9)取出fifo缓存中已经采样的帧缓存
          struct v4l2_buffer buf;
                  memset(&buf,0,sizeof(buf));
                  buf.type=v4l2_buf_type_video_capture;
                  buf.memory=v4l2_memory_mmap;
                  buf.index=0;//此值由下面的ioctl返回
                  if (ioctl(fd, vidioc_dqbuf, &buf) == -1)
                  {
                          return -1;
                  }
根据返回的buf.index找到对应的mmap映射好的缓存,取出视频数据。
(10)将刚刚处理完的缓冲重新入队列尾,这样可以循环采集
          if (ioctl(fd, vidioc_qbuf, &buf) == -1) {
                          return -1;
                  }
(11)停止视频的采集
          int ret = ioctl(fd, vidioc_streamoff, &buf_type);
(12)关闭视频设备
          close(fd);
四、 v4l2驱动框架
上述流程的各个操作都需要有底层v4l2驱动的支持。内核中有一些非常完善的例子。
比如:linux-2.6.26内核目录/drivers/media/video//zc301/zc301_core.c中的zc301视频驱动代码。上面的v4l2操作流程涉及的功能在其中都有实现。
1、v4l2驱动注册、注销函数
video核心层(drivers/media/video/videodev.c)提供了注册函数
              int video_register_device(struct video_device *vfd, int type, int nr)
                    video_device: 要构建的核心数据结构
                    type: 表示设备类型,此设备号的基地址受此变量的影响
                    nr: 如果end-base>nr>0:次设备号=base(基准值,受type影响)+nr;
                    否则:系统自动分配合适的次设备号
具体驱动只需要构建video_device结构,然后调用注册函数既可。
如:zc301_core.c中的
                    err = video_register_device(cam->v4ldev, vfl_type_grabber,
                              video_nr[dev_nr]);
          video核心层(drivers/media/video/videodev.c)提供了注销函数
                    void video_unregister_device(struct video_device *vfd)
2、struct video_device的构建
      video_device结构包含了视频设备的属性和操作方法。参见zc301_core.c
      strcpy(cam->v4ldev->name, zc0301[p] pc camera);
              cam->v4ldev->owner = this_module;
              cam->v4ldev->type = vid_type_capture | vid_type_scales;
              cam->v4ldev->fops = &zc0301_fops;
              cam->v4ldev->minor = video_nr[dev_nr];
              cam->v4ldev->release = video_device_release;
              video_set_drvdata(cam->v4ldev, cam);
大家发现在这个zc301的驱动中并没有实现struct video_device中的很多操作函数,如:vidioc_querycap、vidioc_g_fmt_cap等。主要原因是struct file_operations zc0301_fops中的zc0301_ioctl实现了前面的所有ioctl操作。所以就不需要在struct video_device再实现struct video_device中的那些操作了。
另一种实现方法如下:
static struct video_device camif_dev =
        {
                .name = s3c2440 camif,
                .type = vid_type_capture|vid_type_scales|vid_type_subcapture,
                .fops = &camif_fops,
                .minor = -1,
                .release = camif_dev_release,
                .vidioc_querycap = vidioc_querycap,
                .vidioc_enum_fmt_cap = vidioc_enum_fmt_cap,
                .vidioc_g_fmt_cap = vidioc_g_fmt_cap,
                .vidioc_s_fmt_cap = vidioc_s_fmt_cap,
                .vidioc_queryctrl = vidioc_queryctrl,
                .vidioc_g_ctrl = vidioc_g_ctrl,
                .vidioc_s_ctrl = vidioc_s_ctrl,
        };
        static struct file_operations camif_fops =
        {
                .owner = this_module,
                .open = camif_open,
                .release = camif_release,
                .read = camif_read,
                .poll = camif_poll,
                .ioctl = video_ioctl2, /* v4l2 ioctl handler */
                .mmap = camif_mmap,
                .llseek = no_llseek,
        };
注意:video_ioctl2是videodev.c中是实现的。video_ioctl2中会根据ioctl不同的cmd来调用video_device中的操作方法。
3、video核心层的实现
参见内核/drivers/media/videodev.c
(1)注册256个视频设备
static int __init videodev_init(void)
        {
                int ret;
                if (register_chrdev(video_major, video_name, &video_fops)) {
                        return -eio;
                }
                ret = class_register(&video_class);
                ……
        }
上面的代码注册了256个视频设备,并注册了video_class类。video_fops为这256个设备共同的操作方法。
(2)v4l2驱动注册函数的实现
int video_register_device(struct video_device *vfd, int type, int nr)
        {
                int i=0;
                int base;
                int end;
                int ret;
                char *name_base;
        switch(type) //根据不同的type确定设备名称、次设备号
                {
                        case vfl_type_grabber:
                                base=minor_vfl_type_grabber_min;
                                end=minor_vfl_type_grabber_max+1;
                                name_base = video;
                                break;
                        case vfl_type_vtx:
                                base=minor_vfl_type_vtx_min;
                                end=minor_vfl_type_vtx_max+1;
                                name_base = vtx;
                                break;
                        case vfl_type_vbi:
                                base=minor_vfl_type_vbi_min;
                                end=minor_vfl_type_vbi_max+1;
                                name_base = vbi;
                                break;
                        case vfl_type_radio:
                                base=minor_vfl_type_radio_min;
                                end=minor_vfl_type_radio_max+1;
                                name_base = radio;
                                break;
                        default:
                                printk(kern_err %s called with unknown type: %d\n,
                                        __func__, type);
                                return -1;
                }
        /* 计算出次设备号 */
                mutex_lock(&videodev_lock);
                if (nr >= 0 && nr                         /* use the one the driver asked for */
                        i = base+nr;
                        if (null != video_device[i]) {
                                mutex_unlock(&videodev_lock);
                                return -enfile;
                        }
                } else {
                        /* use first free */
                        for(i=base;i                                if (null == video_device[i])
                                        break;
                        if (i == end) {
                                mutex_unlock(&videodev_lock);
                                return -enfile;
                        }
                }
                video_device[i]=vfd; //保存video_device结构指针到系统的结构数组中,最终的次设备号和i相关。
                vfd->minor=i;
                mutex_unlock(&videodev_lock);
                mutex_init(&vfd->lock);
        /* sysfs class */
                memset(&vfd->class_dev, 0x00, sizeof(vfd->class_dev));
                if (vfd->dev)
                        vfd->class_dev.parent = vfd->dev;
                vfd->class_dev.class = &video_class;
                vfd->class_dev.devt = mkdev(video_major, vfd->minor);
                sprintf(vfd->class_dev.bus_id, %s%d, name_base, i - base);//最后在/dev目录下的名称
                ret = device_register(&vfd->class_dev);//结合udev或mdev可以实现自动在/dev下创建设备节点
                ……
        }
从上面的注册函数中可以看出v4l2驱动的注册事实上只是完成了设备节点的创建,如:/dev/video0。和video_device结构指针的保存。
(3)视频驱动的打开过程
当用户空间调用open打开对应的视频文件时,如:
int fd = open(/dev/video0, o_rdwr);
对应/dev/video0的文件操作结构是/drivers/media/videodev.c中定义的video_fops。
static const struct file_operations video_fops=
        {
                .owner = this_module,
                .llseek = no_llseek,
                .open = video_open,
        };
奇怪吧,这里只实现了open操作。那么后面的其它操作呢?还是先看看video_open吧。
static int video_open(struct inode *inode, struct file *file)
        {
                unsigned int minor = iminor(inode);
                int err = 0;
                struct video_device *vfl;
                const struct file_operations *old_fops;
        if(minor>=video_num_devices)
                        return -enodev;
                mutex_lock(&videodev_lock);
                vfl=video_device[minor];
                if(vfl==null) {
                        mutex_unlock(&videodev_lock);
                        request_module(char-major-%d-%d, video_major, minor);
                        mutex_lock(&videodev_lock);
                        vfl=video_device[minor]; //根据次设备号取出video_device结构
                        if (vfl==null) {
                                mutex_unlock(&videodev_lock);
                                return -enodev;
                        }
                }
                old_fops = file->f_op;
                file->f_op = fops_get(vfl->fops);//替换此打开文件的file_operation结构。后面的其它针对此文件的操作都由新的结构来负责了。也就是由每个具体的video_device的fops负责。
                if(file->f_op->open)
                        err = file->f_op->open(inode,file);
                if (err) {
                        fops_put(file->f_op);
                        file->f_op = fops_get(old_fops);
                }
                ……
        }
以上是我对v4l2的一些理解,希望能对大家了解v4l2有一些帮助!
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