Re: offering bounty for GPL'd dual em28xx support

[Steve Castellotti <sc@eyemagnet.com>]

On 07/21/2009 06:42 PM, Devin Heitmueller wrote:
> On Tue, Jul 21, 2009 at 9:09 PM, Steve Castellotti<sc@eyemagnet.com>  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@eyemagnet.com
Technical Director
Eyemagnet Limited
http://www.eyemagnet.com