Re: [alsa-devel] PulseAudio and softvol

[Arun Raghavan <arun.raghavan@collabora.co.uk>]

On Thu, 2013-05-16 at 08:31 +0200, David Henningsson wrote:
> On 05/15/2013 06:28 PM, Takashi Iwai wrote:
[...]
> > Yes, but the invocation of PCM softvol isn't guaranteed to be first
> > before the reference to the already existing user ctl element.
> > snd_mixer_open() can be called before that.
> 
> So this is a transient problem, right? As soon as the first PCM is 
> opened, the TLV would be corrected, and then stay corrected for all 
> times to come.
> 
> And looking at the current PulseAudio code, it does open the pcm device 
> before it opens the mixer/ctl device.
> So, if this isn't possible to solve in a better way, maybe we need to be 
> pragmatic about it - PulseAudio is the only application we know that 
> would care, and it opens the pcm device first. So in practice, it looks 
> like the TLV approach would work.
> 
[...]
> > The very reason we'd like to filter out the mixer control created by
> > softvol is that this mixer element confuses PA as if it actually
> > changes the volume (e.g. "PCM") although PA ignores the softvol.  If

Actually, we don't currently ignore softvol. I guess we could add the
no-softvol flag once we're able to make sure we don't have any softvol
controls.

> > user creates PCM volume in alsaloop in a different fashion as PA
> > expected, the similar problem may happen.  How can we detect this
> > logically...?  In other words, how can PA adjust the mixer elements
> > for alsaloop properly?
> 
> So if alsaloop is run, only once, that could cause a control to be added 
> for all future, due to alsactl saving and restoring it?
> 
> If so, that looks like a problem with alsaloop. If it adds controls, it 
> should also remove them.
> 
> If no, I don't think we need to worry. Alsaloop is probably mostly used 
> on non-PA systems (as PA has module-loopback which does the same thing).

Seems this thread died out. I didn't quite understand the
alsaloop/alsactl-specific concerns, tbh. What do we need to do to take
this forwards?

Cheers,
Arun