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[CONFIRMED] [UPSTREAM] [REGRESSION] Intel HDMI stopped delivering 1080p@120Hz that it did before. "linux-lts" still works.

Description:

The latest "linux" kernel is completely failing when trying to set a certain resolution@refresh rate. The same command worked before and still works on the "linux-lts" kernel. More details below.

Specifically, this is the command that I have been using in the past:

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

On linux-6.14.9.arch1-1, before the upgrade, my system would successfully deliver the 1080p at 120Hz to this monitor (it's a 4k monitor which I occasionally set to 1080p for a higher refresh rate).

On linux-6.15.2.arch1-1, after the upgrade, the command above exits with 0 but 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 linux-6.15.2.arch1-1, if I run the same script but with --rate 60, the command exits successfully and the monitor works as intended, actually showing my desktop (but at 60 Hz).

If I use the linux-lts package, the command succeeds with either refresh rate.

Additional info:

  • My current linux version: 6.15.2.arch1-1 (that is not working)
  • My past linux version: 6.14.9.arch1-1 (that used to work)
  • The linux-lts version: 6.12.33-1 (that is still working)
  • The GPU is an iGPU from intel, specifically Intel Core Ultra 7 155H.
  • According to lsmod | grep -E "(i915|Xe)", I'm using the i915 kernel driver for the GPU.
  • link to upstream bug report, if any: none yet, I was recommended on IRC to raise an issue here first, to see if it should go upstream or not.
  • The monitor and the notebook are connected via an HDMI cable, the monitor itself is a 4k@120Hz monitor
  • 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.

Steps to reproduce:

  1. Start the system with one of the Linux kernels above
  2. Execute xrandr --output HDMI-1 --auto --scale 1x1 --mode 1920x1080 --rate 120 --pos 0x0 --output eDP-1 --off

Expected at this point: the command succeeds and the monitor is operating.

Actually at this point: the command exits with a 0 status but the monitor is not working.

Edited by Vas
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