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

[Alan Stern]

On Fri, 19 Apr 2013, Daniel Mack wrote:

> 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?
> 
>   https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1136110

No ideas as to the cause.

For debugging, it would help to see a usbmon trace from a kernel where 
the problem occurs, together with a trace from another kernel where the 
problem does not occur.

Alan Stern