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
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