[PATCH] ALSA: hda/hdmi: let new platforms assign the pcm slot dynamically

[Kai Vehmanen]

Hey,

On Wed, 24 Feb 2021, Hui Wang wrote:

> On 2/24/21 1:51 AM, Kai Vehmanen wrote:
> > interface all pins are exposed. Each pin does provide functionality to
> > query whether a display is connected to it, and whether the connected
> > display has audio capability.
> > 
> > The maximum number of concurrent displays is described as converters.
> > On TGL this is 4.
> If a physical port supports DP-MST, does the 3 connections on this physical
> port share a single converter? And each connection has an independent pcm, 
> maybe the driver should create pcm pool according to num_converter * 3.

DP-MST is is reported per-pin, so basicly the interface can report display 
connection status for "numpins*3" endpoints so that would be 9*3 on 
Intel TGL systems.

However, this doesn't affect the converters. There is still four 
converters, so 4 PCMs are enough to cover all possible combination of 
plain DP/HDMI and DP-MST. User-space can query the ELD information to 
learn the mapping from a PCM to a specific display (like e.g. Pulseaudio 
does).

I sent a patch "ALSA: hda/proc - print DP-MST connections" to visualize 
these a bit better in procfs output. I put example output in the commit:
https://lore.kernel.org/r/20201208185736.2877541-1-kai.vehmanen@linux.intel.com

The existing driver provides a PCM for each pin, plus reserve two extra 
PCMs for DP-MST. There's merit to this design as well, but arguably 
the SOF approach is easier to understand on systems like TGL and ICL.

Br, Kai