為了圖省事使用UVC攝像頭，但是板子是USB1.1接口的，故挑選兼容USB1.1的UVC驅動的攝像頭，最終選定兩個301V芯片的攝像頭，先使用一個攝像頭，因頻率不高，將采集頻率設到了最低5幀每秒，打開視頻流，正常！視頻緩沖出列，入列，正常！保存圖像，正常！再添加一個攝像頭，先打開一個視頻流（使用VIDIOC_STREAMON），在打開另一個，報錯了！
出錯代碼：
uvcvideo: Failed to submit URB 0 (-28).
經查疑似帶寬不足，
The uvcvideo driver requests bandwidth based on values reported by the camera.
There are two settings involved there. The first one is the alternate settings
for the video streaming interface. The video streaming interface has an
isochronous endpoint for video streaming, and each alternate setting has a
different maximum packet size for the endpoint, resulting in different
bandwidth requirements.
The second one is the dwMaxPayloadTransferSize value reported by the camera
when querying its video streaming control. The driver selects the alternate
setting with the lowest bandwidth that fulfills the dwMaxPayloadTransferSize
requirements.
If your camera has a single alternate setting (this can be checked using
lsusb) you're probably screwed, as the bandwidth is fixed. If it has multiple
alternate settings, it might be requesting a bandwidth higher than what it
really needs. In that case you could try to hardcode a lower bandwidth (see
the uvc_init_video function in uvc_video.c), or let the driver compute a
bandwidth estimation on its own by setting the UVC_QUIRK_FIX_BANDWIDTH quirk.
It's also possible to play with URB submission order to use higher bandwidths
and alternate capture between the different webcams. Dennis Muhlestein
investigated that and got interesting results. Search the list archives for a
thread called "Multiple camera framerate".
郁悶數日，度日如年啊！一頭霧水，未解，唉，菜鳥嗎！難道吃菜的鳥真的飛不高嗎？或許天生就不是搞代碼的料。
後來的後來，試了試另外的兩個攝像頭，301PL的，USB1.1接口，Linux2.6.28中包含了該類型攝像頭的驅動，同時打開兩個攝像頭，意想不到的是，報了3次錯後，打開了
zc3xx: probe 2wr ov vga 0x0000
zc3xx: probe 2wr ov vga 0x0000
gspca: usb_submit_urb [0] err -28
s3c2410-ohci s3c2410-ohci: leak ed ff1d2140 (#81) state 2
zc3xx: probe 2wr ov vga 0x0000
gspca: usb_submit_urb [0] err -28
s3c2410-ohci s3c2410-ohci: leak ed ff1d2180 (#81) state 2
zc3xx: probe 2wr ov vga 0x0000
gspca: usb_submit_urb [0] err -28
s3c2410-ohci s3c2410-ohci: leak ed ff1d21c0 (#81) state 2
zc3xx: probe 2wr ov vga 0x0000
this is vidCapture_thread of Left
the 1 of Left
state at:978442537 719669
this is vidCapture_thread of right
the 1 of right
state at:978442537 823039
不知為什麼，但是看驅動的源代碼，發現對於usb_submit_urb () 函數的返回值的處理是不同的，gspca中加了對錯誤代碼的判斷，將28號錯誤(ENOSPC)同其他的區別對待了
/* submit the URBs */
614 for (n = 0; n < gspca_dev->nurbs; n++) {
615 ret = usb_submit_urb(gspca_dev->urb[n], GFP_KERNEL);
616 if (ret < 0) {
617 PDEBUG(D_ERR|D_STREAM,
618 "usb_submit_urb [%d] err %d", n, ret);
619 gspca_dev->streaming = 0;
620 destroy_urbs(gspca_dev);
621 if (ret == -ENOSPC)
622 break; /* try the previous alt */
623 goto out;
624 }
625 }
而UVC中只要是提交沒成功就退出。
