Fresh install, can't adjust display brightness

User Support
1

I just installed R4.2.4 on a P14s Gen 5 Thinkpad (Intel Meteor Lake CPU with integrated Intel ARC Graphics only).
I’m using kernel-latest. I can login to dom0 with XFCE. Touchpad, wifi, audio (and the keyboard volume controls) are all working as expected.

At first, nothing happened when I pressed the brightness control keys.
Due to a hint in dmesg * (see below), I then added this parameter to GRUB_CMDLINE_LINUX in the grub config: i915.enable_dpcd_backlight=3. After a reboot, the brightness control keys are now correctly registered, and the GUI popup on the top right shows that the brightness bar is changing as expected. The dmesg now also no longer shows the dmesg hint.

However, the actual display brightness remains unchanged.
Manually changing the value in /sys/class/backlight/intel_backlight/brightness also doesn’t change the actual display brightness.

Now I’m out of ideas. Is this a driver problem? Thank you for any hints in advance.

Note: A known workaround is to use xrandr to change the software screen brightness, as described here or at the end here.

*dmesg line: “Panel is missing HDR static metadata. Possible support for Intel HDR backlight is not used. If your backlight controls don’t work try booting with i915.enable_dpcd_backlight=3. needs this, please file a new bug report on drm/i915, see https://drm.pages.freedesktop.org/intel-docs/how-to-file-i915-bugs.html for details”

2

So you’re confirming that this works, correct?
If it does, that means it isn’t a firmware/driver issue…

3

Yes, using xrandr I can change the screen brightness. Correct me if I’m wrong, but I think xrandr only adjusts the brightness on the software level, which makes the screen appear dimmer? I think the actual physical display brightness of the laptop remains unchanged when using xrandr.

4

TLDR: adding i915.modeset=1 as kernel parameter fixes the problem.

I just booted up a CachyOS live iso and the brightness controls worked there. It used these two kernel parameters: i915.modeset=1 nvme_load=yes. I added them to my grub, and now the brightness controls work as expected in QubesOS.

Edit: After some more testing, it seems that i915.modeset=1 alone seems to do the trick. Using this option together with i915.enable_dpcd_backlight=3 prevents the brightness keys from working again. So the latter option needs to be removed. Now the dmesg warning/hint about the Intel HDR backlight appears again, but at least the brightness controls are working.

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