I recently bought a Lenovo Legion Y540, and most options work except for the brightness control keys, the computer starts with 100% brightness and the keys (F5 and F6) seemingly do nothing, the brightness pop-up also does not show up. I have found that the following command seems to work
xrandr --output DP-4 --brightness 0.6
to lower the brightness.
I tried setting GRUB_CMDLINE_LINUX="acpi_backlight=vendor"
in the GRUB settings but that did not help things either. I did the installation with nouveau.modeset=0
and I do remember the keys to be working. Its after installing nvidia drivers that this problem seems to have risen (not absolutely sure). Any help would be appreciated.
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Grim Reaper
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I have the same problem (Y540 with nvidia drivers on Ubuntu 19.04), but for me the brightness pop-ups does show up, and the slider changes the value but the actual brightness does not change. – GuiGS Sep 02 '19 at 08:48
2 Answers
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I have This Issue with Lenovo Y540 that comes with NVIDIA GeForce GTX 1660Ti Graphics Card
Ubuntu version: 19.10
Also, I have upgraded to version 20.04 and working fine
you need to edit or create this file
sudo nano /usr/share/X11/xorg.conf.d/10-nvidia.conf
and add this line
Option "RegistryDwords" "EnableBrightnessControl=1;"
after this line Option "AllowEmptyInitialConfiguration"
so the file 10-nvidia.conf will look like this:
Section "OutputClass"
Identifier "nvidia"
MatchDriver "nvidia-drm"
Driver "nvidia"
Option "AllowEmptyInitialConfiguration"
Option "RegistryDwords" "EnableBrightnessControl=1;" # Line need to be added
ModulePath "/usr/lib/x86_64-linux-gnu/nvidia/xorg"
EndSection
and save the restart your laptop
and the brightness control keys will work

Ahmed Bermawy
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Setting the bios to switchable graphics works for me (y540 with gtx1650). Also setting "acpi_backlight=intel_backlight" as kernel option works but also seems to mess with resume from suspend.

cccplex
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