內核：Linux-3.4.2
驅動：drivers\media\video\uvc\uvc_driver.c
UVC 驅動整體調用流程：
/* 打開設備描述符 */
1. open:
uvc_v4l2_open
/* 查詢設備屬性 */
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;
/* 枚舉設備支持的格式 */
3. VIDIOC_ENUM_FMT
format = &video->streaming->format[fmt->index];
/* 得到設備當前所使用的 format 與 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
/* 設置數據，此時並沒有真正的設置，而是在啟動視頻流時將數據發送給設備 */
6. VIDIOC_S_FMT
uvc_v4l2_set_format
uvc_v4l2_try_format
video->streaming->cur_format = format;
video->streaming->cur_frame = frame;
/* 分配視頻緩沖區 */
7. VIDIOC_REQBUFS
uvc_alloc_buffers
for (; nbuffers > 0; --nbuffers) {
mem = vmalloc_32(nbuffers * bufsize);
if (mem != NULL)
break;
}
/* 查詢並獲取到分配的緩沖區信息 */
8. VIDIOC_QUERYBUF
uvc_query_buffer
__uvc_query_buffer
/* 映射緩沖區地址到用戶空間 */
9. mmap
uvc_v4l2_mmap
/* 將 V4L2 信息塊放入隊列 */
10. VIDIOC_QBUF
uvc_queue_buffer
list_add_tail(&buf->queue, &queue->irqqueue);
list_add_tail(&buf->stream, &queue->mainqueue);
/* 設置設備並啟動視頻流 */
11. VIDIOC_STREAMON
uvc_video_enable(video, 1)
/* Commit the streaming parameters. */
uvc_commit_video
/* 設置 format, frame */
uvc_set_video_ctrl
/* 啟動：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
/* 休眠等待數據 */
12. poll
uvc_v4l2_poll
uvc_queue_poll
poll_wait(file, &buf->wait, wait);
/* 取出信息塊 */
13. VIDIOC_DQBUF
uvc_dequeue_buffer
list_del(&buf->stream);
/* 關閉視頻流 */
14. VIDIOC_STREAMOFF
uvc_video_enable(video, 0);
usb_kill_urb(urb);
usb_free_urb(urb);
驅動分析，首先找到 UVC 驅動的入口點：
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);
注冊了 usb 驅動，如果系統中出現了與其 id_table 匹配的設備，則驅動會與它建立關系並調用 probe 函數：
我們選擇比較重要的函數進行分析，在 probe 中的函數調用：
uvc_register_chains -> uvc_register_terms -> uvc_register_video(對類型為 UVC_TT_STREAMING 的video調用本函數)
uvc_register_video 函數如下：
static int uvc_register_video(struct uvc_device *dev,
struct uvc_streaming *stream)
{
struct video_device *vdev;
int ret;
/* 對 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);
/* 分配一個 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);
/* 注冊 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;
}
在這里面有個重要的結構，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 就是實現 V4L2 操作的函數集。
這個 unlocked_ioctl 屬性的賦值需要注意一下，當它為 uvc_v4l2_ioctl（當前內核所使用）時，調用的是 uvc_v4l2.c 中現有的函數; 但如果將它賦值為 video_ioctl2 時，內核將調用我們在驅動中 vdev 的 ioctl_ops 屬性所賦給的函數集進行操作。
__video_do_ioctl 中：
const struct v4l2_ioctl_ops *ops = vfd->ioctl_ops; 之后就調用 ops 中的一系列函數來操作 video
所以這一系列的函數可以由我們自己來編寫。