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

[Hui Wang]

On 2/24/21 6:17 PM, Kai Vehmanen wrote:
> 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

Indeed, today I found a thunderbolt dock station which plays DP-MST hub 
and I connected 2 monitors on it, they are 2 sub-devices belong to a 
physical pin, and they are assigned different converters. And the procfs 
output is very clear to show it.

Thanks,

Hui.

>
> 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