Creating UCM2 profiles for devices to split multichannel streams

[GitHub issues - opened]

alsa-project/alsa-ucm-conf issue #122 was opened from brndd:

Hey. Apologies for this question-issue, but I wasn't sure where to ask and figured it was better to do it here in public (so that others may benefit from the discussion) than ask by email or something.

I randomly stumbled upon #121 which is about creating a UCM2 profile for the TC-Helicon GoXLR device to split some multichannel streams to better match the physical layout of the device.

I had no idea this was possible with UCM2 (in fact I looked at UCM2 for this a few months ago and got the impression that it was *not* possible), and would be interested in creating a similar profile for my device, a Behringer UMC204HD. This device has a similar problem of reporting a stereo input stream when the physical layout is two mono inputs, and a 4-channel output stream when the physical layout is two stereo outputs. Thus far I've been using PulseAudio/PipeWire remappings to get around the issue, but a UCM2 profile would be a far superior solution, if indeed it is possible.

So, my questions are:
- Is being able to split streams like this a new feature of ALSA/UCM2? How do I know if my system (Fedora 34) supports this feature?
- Is there documentation somewhere to get me started with writing a config for my device? I have no idea what half the stuff in https://github.com/alsa-project/alsa-ucm-conf/commit/97beee6ded9e3dddfb56580eee360088fbfa8595 means so I'm expecting to fall into some pits during the process ;)

Issue URL     : https://github.com/alsa-project/alsa-ucm-conf/issues/122
Repository URL: https://github.com/alsa-project/alsa-ucm-conf