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Laurent Pinchart wrote:
> That's bad. The USB EHCI driver will reject 2x 3072 bytes per microframe as 
> exceeding the available bandwidth.

Yep, I suspected something like that ...

> I suppose you'll object that the cameras work on Windows, so I'll try to 
> address that :-)

Thank you :)

> 
> I see three possible reasons why Windows would stream video from both cameras 
> at the same time.
> 
> - The Windows UVC driver might query the cameras slightly differently and 
> receive a different bandwidth. A USB sniffer would help confirm or infirm 
> this explanation.

I can check this if I manage to get a sniffer to work. I will report
back once that far.

> 
> - The Windows UVC driver might ignore the requested bandwidth and compute a 
> value itself.
> 
> - Windows might accept 2x 3072 bytes per microframe. I seem to remember this 
> might be the case, and that that behaviour is buggy according to the USB 2.0 
> spec. You would have to contact the linux-usb mailing list for more 
> information on that.

Hard to say which one is correct. It could be even both - Windows
accepts the value (doesn't report error) and the driver does some kind
of black magic afterwards to get things actually working.

I am getting different resolutions offered on Windows than on Linux and
also the camera seems to run at a lower framerate if both cameras are
streaming. I didn't try to measure it, but the stereo output looks like
~10fps or so. It could be a software issue as well, though.

The documentation is also cautioning to plug the device ideally to a
separate host and not to use the USB connectors on the front panel -
they assume that those are not full-featured. So it could be indeed a
marginal design.

> It would be helpful if you could capture all USB control traffic from device 
> enumeration to video streaming using a USB analyser (a software one will do). 
> It would show what bandwidth the device requests, and what bandwidth the 
> Windows UVC driver selects.

As I said above, I will try to get a sniffer working and will report
back once that far.

Regards (and thanks),

Jan

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