linux-kernel - figuring out which monitors and which audio devices correspond

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Message-ID: <6a40fcbf-7e73-4557-a79c-fea764d1ff3e@deltaq.org>
Date: Mon, 19 May 2025 18:00:05 -0500
From: Jonathan Gilbert <logic@...taq.org>
To: linux-kernel@...r.kernel.org
Subject: figuring out which monitors and which audio devices correspond

Hello :-)

When an HDMI cable is connected from my computer to a device that 
handles both video and audio, the system automatically sets up multiple 
logical devices for it. One of them represents the monitor for the 
purposes of display. I can list monitors with a tool like xrandr. The 
other one is an audio sink (correct term?) for delivering sound down 
that HDMI cable. These I can list with e.g. "aplay -l". Is there any way 
to figure out which ones correspond?

* Is there a way to enumerate these and, from the enumeration, procure 
pairs of monitor identifiers and audio device identifiers?

* Is there a way to find a corresponding audio device identifier 
starting with a monitor identifier? Vice versa?

* What is the authoritative way to refer to these devices? "xrandr 
--listactivemonitors" describes monitors with strings like "eDP" and 
"DisplayPort-8" on my system, and assigns them numbers. Meanwhile, 
"aplay -l" seems to identify sound sinks with the combination of a card 
number and a device number. From what I've read, the most direct way to 
refer to a device is a string like "hw:0,3". These don't seem to have 
anything in common whatsoever. Is there some centralized namespace where 
both of these things coexist? /dev? /proc? If so, is there a way to go 
between the authoritative name and device names/numbers that are used at 
the top end? I ask in case this would provide some way to figure out 
that the two logical devices of different types are actually part of the 
same physical device.

* Is the Device Tree relevant here?

Thanks,

Jonathan Gilbert