[Alsa-user] ALSA UCM: Per-channel volume mixer controls?

[Joshua L. <jo...@dr...>]

Hello all,

I am attempting to write some ALSA use case configuration for a device
using a TAS6424 audio amplifier.
The Linux driver for this amplifier exposes a dual-channel PCM sink.
There are four physical output channels - two for each input channel.

Each output channel's volume can be set with a dedicated ALSA mixer control:
- TAS6424 Speaker Driver CH1 Playback Volume
- TAS6424 Speaker Driver CH2 Playback Volume
- TAS6424 Speaker Driver CH3 Playback Volume
- TAS6424 Speaker Driver CH4 Playback Volume

How can I tell ALSA UCM to set all of these channels when changing the
volume? As far as I can tell, I can only set a single PlaybackVolume
channel.

At a minimum, I need to set all the channel volumes together.
If I can also set each individually by describing which logical stereo
channels map to which output channels, that would be a nice bonus.

This is what I have so far, acting on CH2 alone:

SectionDevice."Speaker" {
  Comment "Speakers"

  Value {
    PlaybackPCM "hw:''${CardId},0"
    PlaybackRate 48000
    PlaybackChannels 2
    PlaybackVolume "TAS6424 Speaker Driver CH2 Playback Volume"
  }
}

Thanks,
Josh