UVC摄像头的关闭流程及抓包分析

通过UVC协议规范可以知道，UVC的数据传输支持USB四种传输中的批量传输和同步传输，所以对于UVC摄像头，当我们在摄像头正在工作时，需要停止摄像头工作，执行的操作是不同的。

在Linux的源代码中，摄像头的流关闭是由函数uvc_video_stop_streaming完成的。代码比较简单，我们直接给出原代码

linux-5.6.11\linux-5.6.11\drivers\media\usb\uvc\uvc_video.c:2080

void uvc_video_stop_streaming(struct uvc_streaming *stream)
{
    uvc_video_stop_transfer(stream, 1);

    if (stream->intf->num_altsetting > 1) {
        usb_set_interface(stream->dev->udev, stream->intfnum, 0);
    } else {
        /* UVC doesn't specify how to inform a bulk-based device
         * when the video stream is stopped. Windows sends a
         * CLEAR_FEATURE(HALT) request to the video streaming
         * bulk endpoint, mimic the same behaviour.
         */
        unsigned int epnum = stream->header.bEndpointAddress
                   & USB_ENDPOINT_NUMBER_MASK;
        unsigned int dir = stream->header.bEndpointAddress
                 & USB_ENDPOINT_DIR_MASK;
        unsigned int pipe;

        pipe = usb_sndbulkpipe(stream->dev->udev, epnum) | dir;
        usb_clear_halt(stream->dev->udev, pipe);
    }

    uvc_video_clock_cleanup(stream);
}

UVC摄像头同步传输的关闭

在UVC规范中，如果采用同步数据传输，那么数据流接口的转换接口为0的接口中默认是没有端点描述符中，是用于摄像头关闭或者不工作时；而转换接口非0的接口中含有对应的同步传输端点，用于数据传输。
如本人手中的一个摄像头，是一个多转换接口的UVC摄像头，在摄像头关闭时，抓包内容如下:

CTL    01 0b 00 00  01 00 00 00                  SET INTERFACE

可以看到，发送选择接口命令，接口ID=1,转换接口ID=0

视频流转换接口ID=1:

        ---------------- Interface Descriptor -----------------
bLength                  : 0x09 (9 bytes)
bDescriptorType          : 0x04 (Interface Descriptor)
bInterfaceNumber         : 0x01
bAlternateSetting        : 0x01
bNumEndpoints            : 0x01 (1 Endpoint)
bInterfaceClass          : 0x0E (Video)
bInterfaceSubClass       : 0x02 (Video Streaming)
bInterfaceProtocol       : 0x00
iInterface               : 0x06 (String Descriptor 6)
 Language 0x0409         : "Video Streaming"
Data (HexDump)           : 09 04 01 01 01 0E 02 00 06                        .........

        ----------------- Endpoint Descriptor -----------------
bLength                  : 0x07 (7 bytes)
bDescriptorType          : 0x05 (Endpoint Descriptor)
bEndpointAddress         : 0x81 (Direction=IN EndpointID=1)
bmAttributes             : 0x05 (TransferType=Isochronous  SyncType=Asynchronous  EndpointType=Data)
wMaxPacketSize           : 0x03FF
 Bits 15..13             : 0x00 (reserved, must be zero)
 Bits 12..11             : 0x00 (0 additional transactions per microframe -> allows 1..1024 bytes per packet)
 Bits 10..0              : 0x3FF (1023 bytes per packet)
bInterval                : 0x01 (1 ms)
Data (HexDump)           : 07 05 81 05 FF 03 01                              .......

通过BUSHOUND显示的设备端点信息：

UVC摄像头批量传输的关闭

批量传输是需要是需要停止当前的数据传输，故需要Reset。在Linux源代码中是通过usb_clear_halt函数实现的。

int usb_clear_halt(struct usb_device *dev, int pipe)
{
    int result;
    int endp = usb_pipeendpoint(pipe);

    if (usb_pipein(pipe))
        endp |= USB_DIR_IN;

    /* we don't care if it wasn't halted first. in fact some devices
     * (like some ibmcam model 1 units) seem to expect hosts to make
     * this request for iso endpoints, which can't halt!
     */
    result = usb_control_msg_send(dev, 0,
                      USB_REQ_CLEAR_FEATURE, USB_RECIP_ENDPOINT,
                      USB_ENDPOINT_HALT, endp, NULL, 0,
                      USB_CTRL_SET_TIMEOUT, GFP_NOIO);

    /* don't un-halt or force to DATA0 except on success */
    if (result)
        return result;

    /* NOTE:  seems like Microsoft and Apple don't bother verifying
     * the clear "took", so some devices could lock up if you check...
     * such as the Hagiwara FlashGate DUAL.  So we won't bother.
     *
     * NOTE:  make sure the logic here doesn't diverge much from
     * the copy in usb-storage, for as long as we need two copies.
     */

    usb_reset_endpoint(dev, endp);

    return 0;
}

抓包如下：

  44.3                                 RESET

从抓包分析可知，对该端点进行复位。
通过BUSHOUND显示该设备的端点信息.