[alsa-devel] dmix produces garbled sound on ARM

[Takashi Iwai]

On Thu, 09 Jun 2016 13:38:19 +0200,
Ihar Filipau wrote:
> 
> Hi All!
> 
> When using the dmix on ARM, the sound comes out garbled, with as if
> overlapping, increasing-in-number echoes. That happens even with a
> single aplay instance.
> 
> In a forum discussion, it was suggested that ARM lacks optimized dmix
> implementation. But that implies that the generic dmix code is broken,
> since it is what is used on ARM/etc, but not on i386/amd64. (The forum
> discussion about my particular issue:
> http://unix.stackexchange.com/questions/288451/sound-corruption-echo-when-trying-to-use-alsa-dmix
> )
> 
> Does anybody has any suggestions how to fix/avoid the dmix issue on ARM?
> 
> Thanks.
> 
> P.S. Versions: Linux v3.18.24, alsa-lib/-utils v1.0.29. The hardware:
> FreeScale i.MX35 (ARM1136) with SGTL5000 sound chip (used in 48KHz
> mode).
> 
> P.P.S. The generic code has outdated functions for the "concurrent"
> implementation with cmpxchg. The code probably could be updated, and
> revised to use the C11's stdatomic.h when available.

This is likely the cache coherency issue.  For example, the generic
dmix code works fine on x86-64 if I modify the ifdef condition (I
tested now).

I suppose the mmap support is broken with your device / driver.
Does "aplay -Dhw -M --period-size=1024 somefile.wav" work?  This
should show a similar problem.


Takashi