On Tue, Jul 21, 2009 at 10:19 PM, Steve Castellotti<sc@xxxxxxxxxxxxx> wrote: > On 07/21/2009 06:42 PM, Devin Heitmueller wrote: >> >> On Tue, Jul 21, 2009 at 9:09 PM, Steve Castellotti<sc@xxxxxxxxxxxxx> >> wrote: >>> >>> We can confirm that a development system running Fedora 11 with the >>> latest stable kernel (2.6.29.5-191.fc11.i686.PAE), with identical em28xx >>> devices connected still exhibits the error message "v4l2: ioctl queue >>> buffer >>> failed: No space left on device" when attempting to display video input >>> on >>> two identical em28xx devices simultaneously. >>> >>> On the other hand, display is successful through either device when >>> trying to display individually (with both still connected). >>> >> >> Hello Steve, >> >> The issue occurs with various different drivers. Basically the issue >> is the device attempts to reserve a certain amount of bandwidth on the >> USB bus for the isoc stream, and in the case of analog video at >> 640x480 this adds up to about 200Mbps. As a result, connecting >> multiple devices can result in exceeding the available bandwidth on >> the USB bus. >> >> Depending on your how many devices you are trying to connect, what >> your target capture resolution is, and whether you can put each device >> on its own USB bus will dictate what solution you can go with. >> >> I've done a considerable amount of work with the mainline em28xx >> driver, so if you would like to discuss your desired configuration >> further and what we might be able to do to accommodate those >> requirements (including possibly optimizing the driver to better >> support more devices), feel free to email me off-list. >> >> Regards, >> >> Devin >> > > Devin- > > Thanks for the quick response. Happy to take the conversation off-list, > but first, to clarify what may be useful to future web searchers: > > So if I'm working with a USB 2.0 bus, which should have a theoretical > maximum of 480 Mbps, if the only two ports connected are both em28xx capture > devices running at (say) 640x480, shouldn't that be sufficient for > displaying both streams simultaneously? > > Talking in the general sense of course, perhaps some details vary from > system to system - any idea what sort of variables might affect that > however? > > I would assume most systems only have a single USB bus (regardless of > whether plugs are present on the front/back/side). If a given system has a > second USB bus or chipset, them perhaps plugging the second device into that > would solve the problem, but that surely that would be a rare situation? > > Most of the systems we use do not have expansion slots, so adding a PCI > USB board is not possible (in which case we would probably just add a PCI TV > Capture board anyway!). > > > That said, if you do have some thoughts or suggestions as to how we might > be able to investigate specific hardware, or there is some other way you > think you might be able to help address this particular problem (ideally in > a way that benefits the larger community too!) please let me know. > > > Thanks again > > Steve > > -- > > Steve Castellotti > sc@xxxxxxxxxxxxx > Technical Director > Eyemagnet Limited > http://www.eyemagnet.com I agree that *in theory* you should be able to do two devices. A back of the envelope calculation of 640x480 at 30fps in YUVY capture should be about 148Mbps. That said, I don't think the scenario you are describing has really been tested/debugged previously. If I had to guess, my suspicion would be a bug in the driver code that calculates which USB alternate mode to operate in, which results in the driver reserving more bandwidth than necessary. I would have dig into the code and do some testing in order to have a better idea where the problem is. Do you have a specific em28xx product in mind that you intend to use? Devin -- Devin J. Heitmueller - Kernel Labs http://www.kernellabs.com -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
