[alsa-devel] Question about the various mixer options in UCM

[Tanu Kaskinen]

+ pulseaudio-discuss at lists.freedesktop.org

On Sat, 2020-02-15 at 17:25 +0100, Jaroslav Kysela wrote:
> Dne 15. 02. 20 v 7:29 Tanu Kaskinen napsal(a):
> > What's the difference between PlaybackVolume, PlaybackMixerElem and
> > PlaybackMasterElem? Other than the obvious difference that
> > PlaybackVolume is used only to configure the volume control, whereas
> > PlaybackMixerElem and PlaybackMasterElem are used also to configure the
> > mute control.
> 
> At first, I don't really know if someone uses PlaybackVolume/PlaybackSwitch. 
> It was defined for the direct control interface (not the mixer interface). I 
> do not think that we should support this.

PlaybackVolume/Switch is currently defined for PandaBoard,
PandaBoardES, SDP4430 and sof-hda-dsp. Do you mean those definitions
should be removed and replaced with PlaybackMixerElem? It seems that
PlaybackVolume and PlaybackSwitch don't always match the same simple
mixer element (e.g. PandaBoard), so I'm not sure if it's possible to
use PlaybackMixerElem with those platforms.

As you know, PulseAudio added support for PlaybackVolume recently.
Should we remove the support? I'm not against that, if
PlaybackMixerElem is the canonical way to control volume.

AFAIK CRAS is a major user of UCM, are there others? I think the CRAS
developers' opinion would be very useful here.

> I defined new PlaybackMixerElem to select the simple mixer element which 
> controls both volume and switch (mute) in the ALSA API. The master volume 
> might be also in the chain (thus PlaybackMasterElem) was introduced.
> 
> It seems that it might be not enough and I play with an idea to build custom 
> mixer description to handle the special cases (like several speakers with the 
> different volume controls connected to the single stereo stream etc.).
> 
> To keep things simple, I would probably hide all functionality to 
> PlaybackMixer/PlaybackMixerElem and CaptureMixer/CaptureMixerElem . The 
> special mixer name will create the abstract mixer for the applications and 
> only one simple mixer element control will set the appropriate volume for
> the stream (like pulseaudio actually does for the legacy ALSA support - volume 
> synthetizer). So UCM will describe the mixer for alsa-lib and application will 
> use only abstract interface to set / get the volume and mute state.

Hiding everything behind an abstract mixer element sounds VERY good,
but how to handle a situation where the application uses two devices at
the same time and the devices share a volume control? For example, a
phone playing a ringtone to both headphones and speakers. If the
headphone output has its own abstract mixer element and the speaker
output has its own abstract mixer element, the application may imagine
that it can change the volumes independently, but if the outputs share
a master volume control that is used by both abstract volume elements,
the volumes aren't really independent, and the resulting volumes may
not be what the application intended.

> Actually, I am also trying to resolve the description of the speaker 
> configuration. It may not be only enough to give the PCM device, because we 
> don't know, if user connected the stereo or surround speakers to the sound 
> card output for example. I play with an idea to add device variants to UCM, 
> but the question is, how we can map this to pulseaudio profile/port schematics.
> 
> My quick idea is to export those variants via the verbs, so the exported verb 
> names might look like:
> 
> HiFi:Speaker-Stereo
> HiFi:Speaker-5.1
> 
> Where 'HiFi' is the verb name, 'Speaker' is the device name and 'Stereo' is 
> the variant name.
> 
> If we need to define multiple variants, all may be exported like:
> 
> HiFi:Speaker-5.1,Mic-4.0
> 
> Also, we can enhance this and store the configuration to a file, thus 'HiFi' 
> can refer to 'HiFi at Speaker-5.1,Mic-4.0' by default.

Verb + list-of-device-variants sounds like a good way to map UCM
devices to pulseaudio profiles (and if there's just one verb, which I
expect to be the common case, don't show it in the profile name). I
don't know how the variants should be configured in UCM, but I know
that device variants should be able to declare conflicts with other
devices (or device variants). For example, 5.1 speaker output may make
recording impossible, while stereo speaker output can be used toghether
with a mic. If this information is not provided by UCM, pulseaudio will
have to probe all variant combinations (like it currently does with the
legacy mixer system).

-- 
Tanu

https://www.patreon.com/tanuk
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