Linux摄像头驱动学习之：（四）UVC-摄像头驱动框架分析

UVC: USB Video Class
UVC驱动：drivers\media\video\uvc\

uvc_driver.c分析:
1. usb_register(&uvc_driver.driver);
2. uvc_probe
        uvc_register_video
            vdev = video_device_alloc();
            vdev->fops = &uvc_fops;
            video_register_device

在www.usb.org下载 uvc specification,
UVC 1.5 Class specification.pdf : 有详细描述
USB_Video_Example 1.5.pdf    : 有示例

通过VideoControl Interface来控制，
通过VideoStreaming Interface来读视频数据，
VC里含有多个Unit/Terminal等功能模块，可以通过访问这些模块进行控制，比如调亮度
           
分析UVC驱动调用过程：
const struct v4l2_file_operations uvc_fops = {
 .owner  = THIS_MODULE,
 .open  = uvc_v4l2_open,
 .release = uvc_v4l2_release,
 .ioctl  = uvc_v4l2_ioctl,
 .read  = uvc_v4l2_read,
 .mmap  = uvc_v4l2_mmap,
 .poll  = uvc_v4l2_poll,
};

1. open:
        uvc_v4l2_open
2. VIDIOC_QUERYCAP   // video->streaming->type 应该是在设备被枚举时分析描述符时设置的
  if (video->streaming->type == V4L2_BUF_TYPE_VIDEO_CAPTURE)
   cap->capabilities = V4L2_CAP_VIDEO_CAPTURE
       | V4L2_CAP_STREAMING;
  else
   cap->capabilities = V4L2_CAP_VIDEO_OUTPUT
       | V4L2_CAP_STREAMING;
3. VIDIOC_ENUM_FMT // format数组应是在设备被枚举时设置的
        format = &video->streaming->format[fmt->index];
4. VIDIOC_G_FMT
        uvc_v4l2_get_format  // USB摄像头支持多种格式fromat, 每种格式下有多种frame(比如分辨率)
             struct uvc_format *format = video->streaming->cur_format;
             struct uvc_frame *frame = video->streaming->cur_frame;
5. VIDIOC_TRY_FMT
        uvc_v4l2_try_format
            /* Check if the hardware supports the requested format. */

         /* Find the closest image size. The distance between image sizes is
          * the size in pixels of the non-overlapping regions between the
          * requested size and the frame-specified size.
          */
6. VIDIOC_S_FMT  // 只是把参数保存起来，还没有发给USB摄像头
        uvc_v4l2_set_format
            uvc_v4l2_try_format
         video->streaming->cur_format = format;
         video->streaming->cur_frame = frame;
7. VIDIOC_REQBUFS
        uvc_alloc_buffers
            for (; nbuffers > 0; --nbuffers) {
          mem = vmalloc_32(nbuffers * bufsize);
          if (mem != NULL)
           break;
         }
8. VIDIOC_QUERYBUF
        uvc_query_buffer
            __uvc_query_buffer
                memcpy(v4l2_buf, &buf->buf, sizeof *v4l2_buf);  // 复制参数
9. mmap
        uvc_v4l2_mmap
           
10. VIDIOC_QBUF
        uvc_queue_buffer
         list_add_tail(&buf->stream, &queue->mainqueue);
         list_add_tail(&buf->queue, &queue->irqqueue);

11. VIDIOC_STREAMON
        uvc_video_enable(video, 1)  // 把所设置的参数发给硬件,然后启动摄像头
            /* Commit the streaming parameters. */
            uvc_commit_video
                uvc_set_video_ctrl  /* 设置格式fromat, frame */
                     ret = __uvc_query_ctrl(video->dev /* 哪一个USB设备 */, SET_CUR, 0,
                      video->streaming->intfnum  /* 哪一个接口: VS */,
                      probe ? VS_PROBE_CONTROL : VS_COMMIT_CONTROL, data, size,
                      uvc_timeout_param);
                   
            /* 启动：Initialize isochronous/bulk URBs and allocate transfer buffers. */
            uvc_init_video(video, GFP_KERNEL);
                    uvc_init_video_isoc / uvc_init_video_bulk
                        urb->complete = uvc_video_complete; (收到数据后此函数被调用,它又调用video->decode(urb, video, buf); ==> uvc_video_decode_isoc/uvc_video_encode_bulk => uvc_queue_next_buffer => wake_up(&buf->wait);)
                       
                    usb_submit_urb                     
12. poll
        uvc_v4l2_poll           
            uvc_queue_poll
                poll_wait(file, &buf->wait, wait);  // 休眠等待有数据

13. VIDIOC_DQBUF
        uvc_dequeue_buffer
         list_del(&buf->stream);

14. VIDIOC_STREAMOFF           
        uvc_video_enable(video, 0);
      usb_kill_urb(urb);
      usb_free_urb(urb);
       
分析设置亮度过程：
ioctl: VIDIOC_S_CTRL
            uvc_ctrl_set
            uvc_ctrl_commit
                __uvc_ctrl_commit(video, 0);
                    uvc_ctrl_commit_entity(video->dev, entity, rollback);
                   ret = uvc_query_ctrl(dev  /* 哪一个USB设备 */, SET_CUR, ctrl->entity->id  /* 哪一个unit/terminal */,
                    dev->intfnum /* 哪一个接口: VC interface */, ctrl->info->selector,
                    uvc_ctrl_data(ctrl, UVC_CTRL_DATA_CURRENT),
                    ctrl->info->size);
                       
    
总结：
1. UVC设备有2个interface: VideoControl Interface, VideoStreaming Interface
2. VideoControl Interface用于控制，比如设置亮度。它内部有多个Unit/Terminal(在程序里Unit/Terminal都称为entity)
   可以通过类似的函数来访问：
                   ret = uvc_query_ctrl(dev  /* 哪一个USB设备 */, SET_CUR, ctrl->entity->id  /* 哪一个unit/terminal */,
                    dev->intfnum /* 哪一个接口: VC interface */, ctrl->info->selector,
                    uvc_ctrl_data(ctrl, UVC_CTRL_DATA_CURRENT),
                    ctrl->info->size);
3. VideoStreaming Interface用于获得视频数据，也可以用来选择fromat/frame(VS可能有多种format, 一个format支持多种frame， frame用来表示分辨率等信息)
   可以通过类似的函数来访问：
                     ret = __uvc_query_ctrl(video->dev /* 哪一个USB设备 */, SET_CUR, 0,
                      video->streaming->intfnum  /* 哪一个接口: VS */,
                      probe ? VS_PROBE_CONTROL : VS_COMMIT_CONTROL, data, size,
                      uvc_timeout_param);
4. 我们在设置FORMAT时只是简单的使用video->streaming->format[fmt->index]等数据，
   这些数据哪来的？
   应是设备被枚举时设置的，也就是分析它的描述符时设置的。

5. UVC驱动的重点在于：
   描述符的分析
   属性的控制: 通过VideoControl Interface来设置
   格式的选择：通过VideoStreaming Interface来设置
   数据的获得：通过VideoStreaming Interface的URB来获得

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