[REGRESSION][BISECTED] intel iGPU with HDMI PLL stopped working at 1080p at 120Hz 1efd5384

[Vas Novikov]

Hi dear LKML and community, this is my first post here, so I'd 
appreciate any guidance or redirection if it's due.

Starting from commit 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1efd5384277eb71fce20922579061cd3acdb07cf,
HDMI handling for certain refresh rates on my intel iGPU is broken.

The error is still present in 6.16rc1.

Specifically, this is the command that disambiguates the newer broken 
kernels:

         xrandr --output HDMI-1 --auto --scale 1x1 --mode 1920x1080 
--rate 120 --pos 0x0 --output eDP-1 --off

The important parts are 1920x1080 and 120Hz. When run on commits prior 
to the bisected above, it behaves as expected, delivering 1920x1080 @ 
120Hz. When run on kernel builds with the above commit included (that 
commit or later), the monitor goes completely blank. After about 30 
seconds, it shuts down entirely (which I assume means that from the 
monitor's perspective, HDMI got "disconnected").

On this link you can see my original report in the ArchLinux community, 
where Christian Heusel (@gromit) kindly guided me through the bisection 
process and built the bisection images for me to try: 
https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/145#note_277482 
  This link also contains the bisection history.

Additional info:

* The monitor and the notebook are connected via an HDMI cable, the 
monitor itself is a 4k at 120Hz monitor.

* According to `lsmod | grep -E "(i915|Xe)"`, I'm using the i915 kernel 
driver for the GPU.

* The GPU is an iGPU from intel, specifically `Intel Core Ultra 7 155H`.

* One symptom that disambiguates the working and non-working kernels, 
besides whether they actually have the bug, is that the broken kernels 
cause xrandr to additionally report the 144.05 refresh rate as possible 
for the monitor, whereas the non-broken kernels consistently cause 
xrandr to only list refresh rate 120 and below as possible. I'm only 
ever testing the refresh rate of 120, but the presence of the 144.05 
rate is correlated.

If any other information or anything is needed, please write.


Thank you,
Vas

----

#regzbot introduced: 1efd5384277eb71fce20922579061cd3acdb07cf
#regzbot title: intel iGPU with HDMI PLL stopped working at 1080p at 120Hz 
1efd5384
#regzbot link: 
https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/145

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