usbCamera相同规格摄像头不同时出预览图的一种解决思路

最近在做一项目，正常点亮一拖四usbCamera，开发版是用的是MTK8173，应用方面参考https://github.com/saki4510t/UVCCamera

单个摄像头点亮无问题，不同厂商，型号的四个摄像头同时点亮，预览，拍照，录像也是正常的。但当相同厂商，型号，也即读取的摄像头descriptor完全相同，问题就出现了，只有先挂载的可以正常预览，相同规格摄像头的第二个就无法出预览图。

出现问题的log中，明显的错误如下：

01-02 04:24:45.286 3519 3572 I libUVCCamera: [3572*UVCPreview.cpp:492:prepare_preview]:frameSize=(640,480)@MJPEG

01-02 04:24:45.294 3519 3572 E libuvc/stream: [3572*stream.c:1605:uvc_stream_start_bandwidth]:fail

01-02 04:24:45.294 3519 3572 W libUVCCamera: [3572*diag.c:88:uvc_perror]:failed start_streaming:Unknown error (-99)

提示uvc_stream_start_bandwidth出错。同样，在github上能找到的issue里，最接近的是

https://github.com/saki4510t/UVCCamera/issues/8

遗憾的是从头看到尾，目前作者没有给到很好的解决方案。

没办法，还得从代码去找问题，追logcat却也看不出来问题，就想到从kernel角度看下，其log如下：

<6>[ 235.960068] (1)[1394:CameraThread]usb 1-2.1.4: zzz,usbdev_do_ioctl: SETINTERFACE

<3>[ 235.961266] (1)[1394:CameraThread]usb 1-2.1.4: zzz111,proc_setintf come here

<3>[ 235.962165] (1)[1394:CameraThread]usb 1-2.1.4: zzz222,proc_setintf come here

<3>[ 235.963069] (1)[1394:CameraThread]usb 1-2.1.4: zzz333,proc_setintf come here

<3>[235.963956] (1)[1394:CameraThread]usb 1-2.1.4: zzz,proc_setintfproc_setintf here setintf.interface=1,setintf.altsetting=1

<6>[ 235.965366] (1)[1394:CameraThread]usb 1-2.1.4: zzz,usb_set_interface come,interface=1,alternate=1

<6>[ 235.966485] (1)[1394:CameraThread]usbfs 1-2.1.4:1.1: zzz,intf->altsetting[i].desc.bAlternateSetting=0, altnum=1

<6>[ 235.967797] (1)[1394:CameraThread]usbfs 1-2.1.4:1.1: zzz,intf->altsetting[i].desc.bAlternateSetting=1, altnum=1

<6>[ 235.969389] (1)[1394:CameraThread]usb 1-2.1.4: zzz,usb_set_interface product=USB 2.0 HD Camera ,manufacturer=(null),serial=(null),devnum=7,desc.bInterfaceNumber=1

<6>[ 235.971774] (5)[1394:CameraThread]usb 1-2.1.4: zzz,cur_alt && new_alt is true come here

<3>[ 235.972890] (5)[1394:CameraThread]xhci-hcd xhci-hcd.1.auto: zzz3,xhci_mtk_add_ep_quirk() type:1, speed:3, mpkt:1024, dir:1, ep:ffffffc076e62800

<3>[ 235.974508] (5)[1394:CameraThread]xhci-hcd xhci-hcd.1.auto: zzz,Not enough bandwidth!

<6>[ 235.975501] (5)[1394:CameraThread]usb 1-2.1.4: zzzzz,Not enough bandwidth for altsetting 1,ret=-28

<4>[ 236.049537] (0)[104:kworker/u12:1]mfg_2d: Power-off latency exceeded, new value 162616 ns

<4>[ 242.132057] (1)[140:kworker/u12:4]mfg_2d: Power-off latency exceeded, new value 174231 ns<222 -> 1338>

同样是提示带宽不足，错误序号-28（/* No space left on device */）：

<3>[ 235.974508] (5)[1394:CameraThread]xhci-hcd xhci-hcd.1.auto: zzz,Not enough bandwidth!

<6>[ 235.975501] (5)[1394:CameraThread]usb 1-2.1.4: zzzzz,Not enough bandwidth for altsetting 1,ret=-28

至此，对于usb带宽和协议不是很了解的情况下，依然不晓得，应该怎样修复此问题。

从https://github.com/saki4510t/UVCCamera/issues/8得知，即使相同规格，有部分摄像头是可以满足同时出图的。恰好手头有深蓝科技的一组摄像头，就可以正常使用，其kernel log如下：

<3>[ 9617.289236] (1)[1498:CameraThread]usb 1-2.1.2: zzz,proc_setintfproc_setintf here setintf.interface=1,setintf.altsetting=2

<6>[ 9617.290630] (1)[1498:CameraThread]usb 1-2.1.2: zzz,usb_set_interface come,interface=1,alternate=2

<6>[ 9617.291982] (1)[1498:CameraThread]usbfs 1-2.1.2:1.1: zzz,intf->altsetting[i].desc.bAlternateSetting=0, altnum=2

<6>[ 9617.293637] (1)[1498:CameraThread]usbfs 1-2.1.2:1.1: zzz,intf->altsetting[i].desc.bAlternateSetting=1, altnum=2

<6>[ 9617.295322] (1)[1498:CameraThread]usbfs 1-2.1.2:1.1: zzz,intf->altsetting[i].desc.bAlternateSetting=2, altnum=2

<6>[ 9617.296613] (1)[1498:CameraThread]usb 1-2.1.2: zzz,usb_set_interface product=USB2.0_CAM1,manufacturer=D-vitec,serial=0000,devnum=9,desc.bInterfaceNumber=1

<6>[ 9617.298352] (1)[1498:CameraThread]usb 1-2.1.2: cur_alt && new_alt is true come here

<3>[ 9617.299343] (1)[1498:CameraThread]xhci-hcd xhci-hcd.1.auto: zzz3,xhci_mtk_add_ep_quirk() type:1, speed:3, mpkt:512, dir:1, ep:ffffffc06f0dd800

通过上述log，发现GET_MAX_PACKET(usb_endpoint_maxp(&ep->desc))这个值不一样，手头出错的 mpkt:1024，深蓝科技的mpkt:512，通过代码追踪，在libusb的驱动文件中stream.c的uvc_stream_start_bandwidth方法

uvc_error_t uvc_stream_start_bandwidth(uvc_stream_handle_t *strmh, uvc_frame_callback_t *cb, void *user_ptr, float bandwidth_factor, uint8_t flags) {


......

/* Find the endpoint with the number specified in the VS header */ 

for (ep_idx = 0; ep_idx < altsetting->bNumEndpoints; ep_idx++) {

endpoint = altsetting->endpoint + ep_idx; 

if (endpoint->bEndpointAddress==format_desc->parent->bEndpointAddress){ 

endpoint_bytes_per_packet = endpoint->wMaxPacketSize; LOGE("zzz11,endpoint_bytes_per_packet=%d",endpoint_bytes_per_packet); // wMaxPacketSize: [unused:2 (multiplier-1):3 size:11] 

// bit10…0: maximum packet size 

// bit12…11: the number of additional transaction opportunities per microframe for high-speed 

// 00 = None (1 transaction per microframe) 

// 01 = 1 additional (2 per microframe) 

// 10 = 2 additional (3 per microframe) 

// 11 = Reserved 

//*/new add here

if ((endpoint_bytes_per_packet & 0x07ff) > 512) {

 endpoint_bytes_per_packet = endpoint_bytes_per_packet/2;

 }

//*/

endpoint_bytes_per_packet = (endpoint_bytes_per_packet & 0x07ff) * (((endpoint_bytes_per_packet >> 11) & 3) + 1); LOGE("zzz22,endpoint_bytes_per_packet=%d",endpoint_bytes_per_packet); break; 

} 

}

......

}

从endpoint->wMaxPacketSize; 得到的值的前11位的值即为我们log中的mptk，在这边当此值大于512时，强行赋一半的值给它，则问题解决，两种完全相同规格的摄像头可以同时出预览图。

由于对驱动模块不是很熟悉，对USB协议相关的知识也知之甚少，以上仅从当前代码的角度，提供一拖四或一拖二usbCamera相同规格摄像头不能同时出图的一种思路。但是应该有更好的，更彻底的解决方案。

usbCamera相同规格摄像头不同时出预览图的一种解决思路

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