[pulseaudio-discuss] HDA HDMI audio PCM names

[Kai Vehmanen]

Hi all,

we are considering removing the backup-PCM logic from snd_hda_intel 
HDMI/DP audio driver (note: this is used for wide variety of HDA 
controllers, not just Intel ones). See mail thread on alsa-devel:

https://mailman.alsa-project.org/pipermail/alsa-devel/2019-August/154561.html

In short, with DisplayPort-MST, additional monitors connected to a MST 
hub, and/or to monitor upstream DP ports, these would be routed to 
separate ALSA PCMs, e.g. PCM9 and PCM10, leaving the "standard" PCM 
numbers free for non-MST monitors.

This is getting highly problematic, especially when we support HDMI audio
with systems that have a DSP, and the PCM<->HDMI-audio-converter mapping
can be essentially arbitrary. The proposal is to just expose N pieces 
of PCMs (N number of converters in hw, e.g. 3 with i915), and any 
connected monitor (MST or not), is roured to one these PCMs.

Tanu added the ELD autodetect support in PA-12.x, and after that, HDA + PA 
works pretty nicely even without the backup PCMs, and a non-standard PCM 
device numbering for HDMI audio.

Now, I'm writing here as some kernel devs have referred to Pulseaudio 
as the source for the requirement to map MST monitors to the 
backup/virtual PCMs. Can anyone of you recall what was this about?