[alsa-devel] BUG: USB audio discontinuities with 'UHCI: implement new semantics for URB_ISO_ASAP'

Clemens Ladisch clemens at ladisch.de
Fri Apr 19 18:25:46 CEST 2013

Alan Stern wrote:
> ...
> This trace shows that the frame numbers do not increase sequentially:
> 1057125, 1057126, 1057128, 1057129, 1053131, ...  This is because the
> new driver is a little more conservative than the old driver,
> requiring latencies to be larger than 1 ms.  You can see this
> explicitly in one of the new lines added by the commit you found:
> +		next = uhci->frame_number + 2;
> That 2 is the minimum latency, in frames (one frame per ms).

One frame worked fine with the old driver.  What is the reason for
this regression?

> Anyway, the solution is simple: The audio driver needs either to submit
> buffers that are at least 2 ms long, or to use more than two buffers
> (or both).

The audio driver already has the infrastructure to cope for hardware
restrictions like this, it just needs to *know* about them.  How can
it detect that it runs on the OHCI/UHCI drivers?

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