内核：Linux-3.4.2
驱动：drivers\media\video\uvc\uvc_driver.c
UVC 驱动整体调用流程：
/* 打开设备描述符 */
1. open:
uvc_v4l2_open
/* 查询设备属性 */
2. VIDIOC_QUERYCAP
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;
/* 枚举设备支持的格式 */
3. VIDIOC_ENUM_FMT
format = &video->streaming->format[fmt->index];
/* 得到设备当前所使用的 format 与 frame */
4. VIDIOC_G_FMT
uvc_v4l2_get_format
struct uvc_format *format = video->streaming->cur_format;
struct uvc_frame *frame = video->streaming->cur_frame;
/* Check if the hardware supports the requested format. */
5. VIDIOC_TRY_FMT
uvc_v4l2_try_format
/* 设置数据，此时并没有真正的设置，而是在启动视频流时将数据发送给设备 */
6. VIDIOC_S_FMT
uvc_v4l2_set_format
uvc_v4l2_try_format
video->streaming->cur_format = format;
video->streaming->cur_frame = frame;
/* 分配视频缓冲区 */
7. VIDIOC_REQBUFS
uvc_alloc_buffers
for (; nbuffers > 0; --nbuffers) {
mem = vmalloc_32(nbuffers * bufsize);
if (mem != NULL)
break;
}
/* 查询并获取到分配的缓冲区信息 */
8. VIDIOC_QUERYBUF
uvc_query_buffer
__uvc_query_buffer
/* 映射缓冲区地址到用户空间 */
9. mmap
uvc_v4l2_mmap
/* 将 V4L2 信息块放入队列 */
10. VIDIOC_QBUF
uvc_queue_buffer
list_add_tail(&buf->queue, &queue->irqqueue);
list_add_tail(&buf->stream, &queue->mainqueue);
/* 设置设备并启动视频流 */
11. VIDIOC_STREAMON
uvc_video_enable(video, 1)
/* Commit the streaming parameters. */
uvc_commit_video
/* 设置 format, frame */
uvc_set_video_ctrl
/* 启动：Initialize isochronous/bulk URBs and allocate transfer buffers. */
uvc_init_video(video, GFP_KERNEL);
uvc_init_video_isoc / uvc_init_video_bulk
usb_submit_urb
/* 休眠等待数据 */
12. poll
uvc_v4l2_poll
uvc_queue_poll
poll_wait(file, &buf->wait, wait);
/* 取出信息块 */
13. VIDIOC_DQBUF
uvc_dequeue_buffer
list_del(&buf->stream);
/* 关闭视频流 */
14. VIDIOC_STREAMOFF
uvc_video_enable(video, 0);
usb_kill_urb(urb);
usb_free_urb(urb);
驱动分析，首先找到 UVC 驱动的入口点：
struct uvc_driver uvc_driver = {
.driver = {
.name = "uvcvideo",
.probe = uvc_probe,
.disconnect = uvc_disconnect,
.suspend = uvc_suspend,
.resume = uvc_resume,
.reset_resume = uvc_reset_resume,
.id_table = uvc_ids,
.supports_autosuspend = 1,
},
};
usb_register(&uvc_driver.driver);
注册了 usb 驱动，如果系统中出现了与其 id_table 匹配的设备，则驱动会与它建立关系并调用 probe 函数：
我们选择比较重要的函数进行分析，在 probe 中的函数调用：
uvc_register_chains -> uvc_register_terms -> uvc_register_video(对类型为 UVC_TT_STREAMING 的video调用本函数)
uvc_register_video 函数如下：
static int uvc_register_video(struct uvc_device *dev,
struct uvc_streaming *stream)
{
struct video_device *vdev;
int ret;
/* 对 video 做一些初始化 */
ret = uvc_video_init(stream);
if (ret < 0) {
uvc_printk(KERN_ERR, "Failed to initialize the device "
"(%d).\n", ret);
return ret;
}
uvc_debugfs_init_stream(stream);
/* 分配一个 video device */
vdev = video_device_alloc();
if (vdev == NULL) {
uvc_printk(KERN_ERR, "Failed to allocate video device (%d).\n",
ret);
return -ENOMEM;
}
/* 配置 video device */
vdev->v4l2_dev = &dev->vdev;
vdev->fops = &uvc_fops;
vdev->release = uvc_release;
strlcpy(vdev->name, dev->name, sizeof vdev->name);
stream->vdev = vdev;
video_set_drvdata(vdev, stream);
/* 注册 video device */
ret = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
if (ret < 0) {
uvc_printk(KERN_ERR, "Failed to register video device (%d).\n",
ret);
stream->vdev = NULL;
video_device_release(vdev);
return ret;
}
atomic_inc(&dev->nstreams);
return 0;
}
在这里面有个重要的结构，即：
const struct v4l2_file_operations uvc_fops = {
.owner = THIS_MODULE,
.open = uvc_v4l2_open,
.release = uvc_v4l2_release,
.unlocked_ioctl = uvc_v4l2_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl32 = uvc_v4l2_compat_ioctl32,
#endif
.read = uvc_v4l2_read,
.mmap = uvc_v4l2_mmap,
.poll = uvc_v4l2_poll,
#ifndef CONFIG_MMU
.get_unmapped_area = uvc_v4l2_get_unmapped_area,
#endif
};
uvc_v4l2_ioctl 就是实现 V4L2 操作的函数集。
这个 unlocked_ioctl 属性的赋值需要注意一下，当它为 uvc_v4l2_ioctl（当前内核所使用）时，调用的是 uvc_v4l2.c 中现有的函数; 但如果将它赋值为 video_ioctl2 时，内核将调用我们在驱动中 vdev 的 ioctl_ops 属性所赋给的函数集进行操作。
__video_do_ioctl 中：
const struct v4l2_ioctl_ops *ops = vfd->ioctl_ops; 之后就调用 ops 中的一系列函数来操作 video
所以这一系列的函数可以由我们自己来编写。