Bug#950609: systemd: backlight service messes with screen brightness

[Michael Biebl]

Control: tags -1 = moreinfo

On Tue, 04 Feb 2020 09:26:58 +0100 Valerio Passini
<valerio.passini at unicam.it> wrote:
> Package: systemd
> Version: 244.1-2
> Severity: normal
> Tags: patch
> 
> Dear Maintainer,
> 
> Brightness controls on keyboard (and any other mean except xrandr) have not 
> been working reliably since the day I installed Debian on my laptop: sometime 
> it was possible to tune brightness after a fresh boot, sometime only after a 
> S2R and resume.

What exactly was not working? What did you do, what did you expect, what
did (not) happen.

> Trying to find a solution, I have discovered that masking the backlight 
> service fixes the problem:
> 
> sudo systemctl mask systemd-backlight at backlight:nvidia_0.service
> 
> I have double checked that enabling the service again restores the issue.
> The relevant information on my system are:
> Laptop model: HP Omen 15-dh0002nl, GPU Nvidia
> Software: Debian/Unstable Xorg Nvidia proprietary drivers (don't mind about my
> custom kernel, the issue is the same with Debian stock kernel)
> 

systemd-backlight at .service simply stores the current backlight value on
shutdown and restores it on boot.
It reads from /sys/class/backlight/*/brightness and writes accordingly.
If that somehow confuses the hardware, this smells like a driver/kernel
issue.

Michael

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