[alsa-devel] Asynchronous audio USB chips: choppy playback since 3.8-rc7

Daniel Mack zonque at gmail.com
Fri Apr 19 20:54:25 CEST 2013

On 03.04.2013 12:25, Takashi Iwai wrote:
> At Wed, 03 Apr 2013 12:15:25 +0200,
> David Henningsson wrote:
>> Hi ALSA developers,
>> Just to get your attention here on what seems to be an USB audio 
>> regression.
>> The bug is described in detail here:
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1136110?comments=all
>> Quoting the bug:
>> "
>> This bug seems to affect only a certain kind of hardware, which is 
>> called "Asynchronous USB Digital Audio Codec (DAC)". It's said that such 
>> a DAC hosts the clock itself (USB Device Host). An ordinary DAC, so 
>> called "Synchronous USB DAC", uses the clock hosted by the mother board, 
>> which is not affected by this bug.
>> When this bug affects an asynchronous USB DAC, the audio played by the 
>> DAC is constantly interrupted. The playback itself does not stop, but 
>> the output becomes discontinous, filling with constant crackling noises, 
>> destroying everything the DAC plays.
>> "
>> According to the bug reporter, which seems to have done quite a bit of 
>> research, this started between 3.8-rc6 and 3.8-rc7 as well as stable 
>> kernels and the bug also lists a few commits which could be the cause, 
>> none under sound/usb though.
> Yes, there is no commits regarding usb-audio itself between 3.8-rc6
> and rc7, so the likely culprit is in drivers/usb (usually either
> drivers/usb/host or drivers/usb/core).  There are a bunch of changes
> there, so further bisection would be appreciated.

Ok, Joseph Salisbury has build some bisection kernels, and Tyson Tan
relentlessly tested all of them, and it turns out that

3e619d0415 ("USB: EHCI: fix bug in scheduling periodic split transfers")

Is the first bad commit. Also, reverting this commit from the current
mainline head makes the problem disappear.

Alan, any idea?



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