HDMI hotplug does not work - how to troubleshoot?

[Jimini]

Hello everyone,

I have a bunch of headless systems running Gentoo. They all use newer kernels (5.15.16, 5.15.23) and come without X or any other kind of graphical output - only CLI.
While connecting them at runtime to an external HDMI screen, I noticed, that
- one immediately shows some output
- two other systems need to have the HDMI screen connected at boot time, otherwise the screen does not detect any HDMI connection

I did some research, but most of the threads I found led to some xrandr stuff, which seems to be senseless without X.
Some other threads had the solution, to use a "video="-parameter in the grub.cfg, but I was unable to figure out the name of my display (/sys/class/drm/ only contains a file named "version").

All three systems do not use kernel modules - all drivers are compiled into the kernel.
In principle HDMI works, so I would exclude a missing driver or something similar as a possible root cause. HDMI support is enabled (CONFIG_HDMI=y).

What steps can I take to localize the problem?

Kind regards and thanks in advance,
Jimini
_________________
"The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents." (H.P. Lovecraft: The Call of Cthulhu)