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). ", 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). ", 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). ", 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。