[alsa-devel] Issues and/or possible bugs in alsa

[Alexander E. Patrakov]

25.02.2015 03:46, Yomi Ogunwumi wrote:
> Third. This is just a question. Is this :
> https://bugs.freedesktop.org/show_bug.cgi?id=86676 actually a pulseaudio
> bug, or is it an issue with alsa?
> I'm only asking since Raymond linked something that seemed to belong to the
> alsa project.
>

This is a bug both in ALSA and in PulseAudio.

The ALSA part (from the user viewpoint) is that the softvol plugin does 
not reprocess the already-submitted but buffered samples when the volume 
changes. But it can't, because that would require an additional thread 
for monitoring the software volume changes, and such thread does not exist.

The PulseAudio part of the bug is that it does not deactivate softvols, 
even though it can apply volume in software itself. In October 2014, in 
Dusseldorf, a general agreement has been reached on the following arguments:

  * ALSA has no API to definitely distinguish softvols from other controls.
  * ALSA has the snd_ctl_elem_info_is_user() API function that tells 
whether this is a userspace control.
  * All softvols are userspace controls.
  * There are other kinds of userspace controls, but they are rare.
  * If a control is named PCM Playback Volume and is a userspace 
control, then it's likely a softvol. Not bulletproof, but a good-enough 
heuristic.
  * On finding a softvol, PulseAudio should set it to 100% (so that it 
doesn't eat CPU) and don't touch from that point on.

But nobody has implemented this so far.

The relevant code can be placed in the src/modules/alsa/alsa-mixer.c 
file in PulseAudio source tree. I guess that element_probe() is a good 
place. If you know C programming, a contribution would be welcome.||

-- 
Alexander E. Patrakov