USB摄像头驱动框架

写一个USB摄像头驱动程序
1.构造一个usb_driver
2.设置
probe:
2.1. 分配video_device:video_device_alloc
2.2. 设置
.fops
.ioctl_ops (里面需要设置11项)
如果要用内核提供的缓冲区操作函数，还需要构造一个videobuf_queue_ops
2.3. 注册: video_register_device
id_table: 表示支持哪些USB设备
3.注册： usb_register

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

两个概念：单元units ,终端terminals

单元：里面的

terminal:用于内外链接。