LINUX&UVC处理单元描述符

不同的UVC版本，其UVC处理单元描述格式稍有不同。
关于UVC处理单元描述符各字段的详细说明可详见：https://www.usbzh.com/article/detail-84.html

对于UVC1.1/1.5版本，其内容如下：

  UINT8  bLength;
  UINT8  bDescriptorType;
  UINT8  bDescriptorSubtype;
  UINT8  bUnitID;
  UINT8  bSourceID;
  UINT16 wMaxMultiplier;
  UINT8 bControlSize;
  UINT8 bmControls[3];
  UINT8 iProcessing;
  UINT8 bmVideoStandards;

而对于UVC1.0则其内容如下：

  UINT8  bLength;
  UINT8  bDescriptorType;
  UINT8  bDescriptorSubtype;
  UINT8  bUnitID;
  UINT8  bSourceID;
  UINT16 wMaxMultiplier;
  UINT8 bControlSize;
  UINT8 bmControls[2];
  UINT8 iProcessing;

UVC1.0相对于UVC1.1/1.5,其bControlSize的值从2变成了3,故bmControls数组变从2变成了3字节。
另一个就是多了一个bmVideoStandards字节。
所以，UVC1.1/UVC1.5总的长度bLength为13字节，而UVC1.0为11字节。

case UVC_VC_PROCESSING_UNIT:
        n = buflen >= 8 ? buffer[7] : 0;
        p = dev->uvc_version >= 0x0110 ? 10 : 9;

        if (buflen < p + n) {
            uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
                "interface %d PROCESSING_UNIT error\n",
                udev->devnum, alts->desc.bInterfaceNumber);
            return -EINVAL;
        }

        unit = uvc_alloc_entity(buffer[2], buffer[3], 2, n);
        if (unit == NULL)
            return -ENOMEM;

        memcpy(unit->baSourceID, &buffer[4], 1);
        unit->processing.wMaxMultiplier =get_unaligned_le16(&buffer[5]);
        unit->processing.bControlSize = buffer[7];
        unit->processing.bmControls = (u8 *)unit + sizeof(*unit);
        memcpy(unit->processing.bmControls, &buffer[8], n);
        if (dev->uvc_version >= 0x0110)
            unit->processing.bmVideoStandards = buffer[9+n];

        if (buffer[8+n] != 0)
            usb_string(udev, buffer[8+n], unit->name, sizeof(unit->name));
        else
            sprintf(unit->name, "Processing %u", buffer[3]);

        list_add_tail(&unit->list, &dev->entities);
        break;