[pulseaudio-discuss] HDA HDMI audio PCM names

[Tanu Kaskinen]

On Tue, 2019-08-27 at 17:39 +0300, Kai Vehmanen wrote:
> 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?

I don't recall, in fact I don't even remember hearing about the "backup
PCM" concept before. As far as I can tell, PulseAudio imposes or has
imposed the following limitations:

 * PCMs can't be added and removed dynamically on a card. PulseAudio
doesn't have support for dynamic PCMs.

 * Previously HDMI jack detection required that specific PCM numbers
are used, but as you noted, we have autodetection for this now.

 * The "hdmi:x,y" PCM devices must exist in the alsa configuration,
bare "hw:x,z" won't be detected properly.

-- 
Tanu

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