I got 2.4.109-1 a short while ago, and had old symptoms again --
screen flickering and kernel messages like

    i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
    i915 0000:00:02.0: [drm] *ERROR* CPU pipe A FIFO underrun

Switching monitor, cables, reboot, poweroff didn't help.

Downgrading libdrm-intel1 to 2.4.108-2 made the problems go away -
at least I could work now for 23 minutes, previously I had
flickering about every minute or so.

See also https://gitlab.freedesktop.org/drm/intel/-/issues/1512

Control: tag -1 moreinfo

It's great that you participated in that upstream issue, but my guess is that
it's not getting attention because the issue is closed.
If you can still reproduce this with the latest kernel and libdrm packages,
can you open a new issue upstream? And mention that in this bug?

ion because the issue is closed.

Well, right now with 6.5.0-1-amd64 I can't reproduce.
(Yeah, that's not the latest, I know)

Control: tag -1 -moreinfo

That sounds like you expect the issue to return 'any moment', so I'll refrain
from closing this bug. From https://bugs.debian.org/796087 it appears that
you're not the only one who sees issues come and go periodically.

The goal was to test it with a newer kernel/libdrm versions, so testing with
6.5.0-1 was good enough AFAIC.
If -4 would reintroduce the problem, that would be an interesting data point.

Looking at libdrm's commit log, I see periodically commits where they sync up
with the upstream linux kernel. Maybe these issues come and go when there is/
was a mismatch between the kernel and libdrm (which the sync up then fixes)?

At https://snapshot.debian.org/binary/libdrm-intel1/ you can find (most)
previous versions and if you still have the 5.15.0-3 kernel installed, could
you try if version 2.4.110-1 with it would also solve the problem?
And try newer libdrm-intel1 versions if not till you find one where it is fixed.