Dear Laurent, Laurent Pinchart wrote: > On Monday 09 November 2009 00:04:36 Paulo Assis wrote: > >> Hi, >> I tested this in my system and was able to grab two simultaneous YUYV >> 1600x1...@5fps video streams. >> The cameras are two different uvc logitech models connected to two >> separate usb controllers, my board has plenty of them :) >> Connecting both cameras to the same controler (e.g. the two front >> ports) will cause the second stream to fail. >> >> I don't know about windows since I don't use it, but are you sure you >> aren't getting a zoomed (1600x1200) image from a lower resolution? >> maybe the windows driver is playing a trick on you. >> > > That's one possibility. > > If I remember correctly Windows might be violating the USB spec by allowing > devices to use more than 80% of the available bandwidth for periodic > (isochronous and interrupt) transfers. Or it might calculate the bandwith > slightly differently than Linux. > > In any case, the problem under Linux comes from the fact that the USB core > rejects the uvcvideo driver's request to allocate bandwidth for the second > camera because not enough bandwidth is available. The uvcvideo driver can't > do > much about it. > > Now, the camera might lie about the bandwidth it really requires, and it > might > be possible to hack the driver to request less bandwidth and still get away > with it. I have some doubts that this would work though, but it might be > worth > a try. >
This sounds like very likely explanation of my different Linux/Windows experience. Thanks also for the hint how to hack the driver. Fortunately, I can avoid hacking the driver as I solved the problem by using two separate USB controllers. But I'll look to the code just for curiosity... Thanks anyway for your reply. Regards, Vojtech > As you've written your own userspace software I assume you're familiar with C > programming so I'll just give you a few pointers to hack on the driver :-) > > USB bandwidth allocation is done in the uvc_init_video() function > (uvc_video.c) by a call to usb_set_interface(). You can check which alternate > setting the driver requests by printing the value of variable 'i'. > > A look at the camera's USB descriptors (lsusb -v) will show you a list of > alternate settings for the video streaming interface. Each alternate setting > has different packet sizes for the isochronous endpoint. Locate the alternate > setting with the biggest packet size below the one requested by the driver, > and modify uvc_init_video() to hardcode that alternate setting. If that's > still too much bandwidth, repeat the procedure until you can start streaming > on both cameras. > > The driver will then probably get corrupted data (lost packets), but it could > be worth a try. > > >> BTW with some new camera models that support MJPG in high resolution >> you should able to squeeze two MJPG 1600x1...@10fps video streams from >> the same usb controller. >> > > -- -------------------------------------------------------------- Vojtech Franc, Ph.D. Research fellow Center for Machine Perception Department of Cybernetics Faculty of Electrical Engineering Czech Technical University in Prague email: xfra...@cmp.felk.cvut.cz www: http://cmp.felk.cvut.cz/~xfrancv phone: +420 22435 7665 -------------------------------------------------------------- _______________________________________________ Linux-uvc-devel mailing list Linux-uvc-devel@lists.berlios.de https://lists.berlios.de/mailman/listinfo/linux-uvc-devel
