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

[Valerio Passini]

Hi Michael,

Il Ven 7 Feb 2020, 19:05 Michael Biebl <biebl at debian.org> ha scritto:

> Control: tags -1 = moreinfo
>
> What exactly was not working? What did you do, what did you expect, what
> did (not) happen.
>

Screen brightness controls (the two keyboard buttons designed to tune
brightness and the sliders in GNOME and KDE) are ineffective. If I press
the buttons or I move the slider, the OSD shows that brightness is changing
while actually it's not. I expect that brightness is being regulated by my
actions.

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
>

I know that it seems unrelated to systemd-backlight, that service should
store and restore brightness through reboots, but here:
https://devtalk.nvidia.com/default/topic/1061387/linux/screen-brightness-resets-to-max-after-reboot-linux-/
I have found that systemd-backlight is keeping brightness to maximum and so
I tried to disable it just to see if my problem would be fixed, and so it
is.
The real culprit might be something else as you said previously, but at the
moment it is a decent workaround for me and it might be worth investigating
it further.

Tell me if you need more info. Best regards

Valerio
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