Backlight on Dell G15 with Linux Mint

Problem

Since I upgraded Linux Mint 21.1 Vera with kernel 6.8 on my Dell G15 5525, I encountered an issue: missing brightness control.

Searching the Internet didn’t help much in resolving the problem, but fortunately, a long session with ChatGPT (GPT-4o, August 2025 release) led to significant progress. I’m sharing here a brief set of instructions developed during that session. I still believe that having some technical knowledge was crucial — for properly describing the problem, understanding the hints, evaluating uncertainties, and reporting results back to ChatGPT.

Prompt

My current challenge I’m facing is: I’ve installed Linux Mint 21.1 Vera on Dell G15 5525 and upgraded kernel to 6.8.0-57. The problem is not working brightness adjustment for the display. The function keys present a reaction on the screen but brightness remains the same.
Answering use simple language for a DevOps, linux power-user. Provide simple solution, in points.

Below, find the summary from the whole session:

Collecting evidence

Check backlight interface

$ ls /sys/class/backlight/
acpi_video0

Check for correct driver (e.g., amdgpu)

$ lspci -nnk | grep -A3 VGA
pcilib: Error reading /sys/bus/pci/devices/0000:00:08.3/label: Operation not permitted
01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GA107BM [GeForce RTX 3050 Mobile] [10de:25e2] (rev a1)
	Subsystem: Dell GA107BM [GeForce RTX 3050 Mobile] [1028:0b5e]
	Kernel driver in use: nvidia
	Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia
--
05:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Rembrandt [1002:1681] (rev c9)
	Subsystem: Dell Device [1028:0b5e]
	Kernel driver in use: amdgpu
	Kernel modules: amdgpu

Diagnosis

If only acpi_video0 appears in /sys/class/backlight/, then it’s not activating the NVIDIA WMI EC backlight device, or the BIOS/firmware doesn’t expose it the way the kernel expects.

The goal

…is to see see either: amdgpu_bl0, nvidia_backlight, nvidia_wmi_ec_backlight
instead of just acpi_video0

Correction

Carefully, now you edit something sensitive:

$ sudo vim /etc/default/grub

Replace current line with (this is one line):

…
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash resume=UUID=… nvidia-drm.modeset=1 acpi_backlight=native"
…

save and apply:

$ sudo update-grub
$ sudo reboot

Verification after reboot

After reboot:

$ ls /sys/class/backlight/ -1
amdgpu_bl1
nvidia_0

Functional keys (F6, F7) present reactions by poping-up slider but brightness remains the same.

System likely defaults to controlling the wrong interface — probably nvidia_0, but the screen is driven by AMD iGPU (amdgpu_bl1).

NVIDIA is very often headless (not directly connected to display) in Optimus laptops. Brightness changes must target amdgpu_bl1.

Workaround – by commandline

For time being I am accepting to control brightness by issuing commands at terminal.
This is to find the devices for brightness controls:

$ brightnessctl -l -c backlight
Available devices:
Device 'nvidia_0' of class 'backlight':
        Current brightness: 100 (100%)
        Max brightness: 100

Device 'amdgpu_bl1' of class 'backlight':
        Current brightness: 78 (31%)
        Max brightness: 255

And setting brightness directly:

$ brightnessctl -d amdgpu_bl1 s 50%

…or by a step up/down:

$ brightnessctl -d amdgpu_bl1 s +10%
$ brightnessctl -d amdgpu_bl1 s 10%-

To do

Make sure function keys (and system UI sliders) control amdgpu_bl1, not the unused nvidia_0.