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

Clemens Ladisch clemens at ladisch.de
Tue Apr 23 10:47:44 CEST 2013

Alan Stern wrote:
> On Fri, 19 Apr 2013, Joe Rayhawk wrote:
>> On Fri, Apr 19, 2013 at 02:18:24PM -0400, Alan Stern wrote:
>>> On Fri, 19 Apr 2013, Clemens Ladisch wrote:
>>>> Alan Stern wrote:
>>>>> +		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?
>>> Perhaps that was a mistake.  Joe, you can try changing the 2 above to a
>>> 1 to see if it fixes the problem.
>> Hey, that worked great! Audio's coming through continuously, now.
> This change could be added to the driver, but I would prefer not to.

Why do you think it is necessary to have a minimum latency of 2 ms?

Again, the old algorithm worked fine.  While such short queues are not
used by default, they are necessary to get low latencies for real-time
audio applications.  Keeping this change would keep this regression for
quite a few people.

> In any case, it would be best

What criteria are you using to evaluate the benefit of this?  Do you
want to reduce the chance of queue underruns?  Interrupts?  Power usage?

> if the usb-audio driver were changed to keep the pipeline length at
> least 2 ms at all times.

Why is having a queue of two URB with one packet each suddenly not

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