[alsa-devel] PulseAudio and softvol

Jaroslav Kysela perex at perex.cz
Wed May 15 12:53:30 CEST 2013

Date 15.5.2013 12:48, Takashi Iwai wrote:
> At Wed, 15 May 2013 12:26:51 +0200,
> Jaroslav Kysela wrote:
>> Date 15.5.2013 11:55, Arun Raghavan wrote:
>>> Hello,
>>> A number of users have intermittently(?) been hitting a crash in
>>> alsa-lib 1.0.27 [1, 2] related to the softvol plugin. I'm not able to
>>> reproduce this reliably, so can't find an easy way to debug/fix.
>> The problem is that the offsets are not in sync in this case [1]:
>> src_offset = 38560
>> dst_offset = 38568
>> frames = 16374
>> Could you reproduce this bug in any way? At least snd_pcm_dump() before
>> the failing snd_pcm_mmap_commit() call might help to determine what was
>> the status before the assert() was entered.
> Yep.  And this path is actually with volume 0dB, that is, a simply
> passthrough in softvol.  Thus the bug may hit essentially any
> plugins, not specifically softvol.
>>> However, this raises a tangential question - why do we need softvol to
>>> be plugged for 'front' at all? David explained to me that this is to
>>> guarantee the existence of a PCM control. Perhaps I don't fully
>>> understand this, because I'm unconvinced by the reason. Could someone
>>> explain/refute?
>>> This is especially bad for us, from PulseAudio's perspective, because we
>>> aren't getting a zero-copy path.
>> If the softvol is set to 0dB (no attenuation or gain), then the ring
>> buffer pointers are moved without any sample processing, so the
>> zero-copy functionality is kept.
> Yeah, a sort of.  The mmap is cleared in the slave PCM, so actually
> there will be copy operations in underlying layers even though softvol
> itself does zero copy.
> Actually it makes no sense to keep softvol for PA, but the problem is
> always the regression.  There are certainly users without PA, which
> might still rely on the softvol for such hardware without the amp
> control.
> Maybe We can add some flag to indicate whether to handle softvol or
> not, e.g. defaults.pcm.skip_softvol, and let PA set this in its config
> space.  Setting a config item itself would break anything, so it'll
> still work with old alsa-lib (but with softvol).

We have already SND_PCM_NO_SOFTVOL open mode for this purpose, so I
wonder, why PA does not use it..


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.

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