SDDM - maximum level brightness issue

Hello everyone, first of all, I apologize for my bad english. I have a small issue with sddm on the first login. I tried Fedora 40 with both Gnome and KDE Plasma, and with the latter, on the first login on sddm, it forgets the previous brightness level and sets it to the maximum. This doesn’t happen with gdm3 and Gnome, so it’s not a hardware issue. How can I solve this? It’s quite annoying on a laptop, and I would really like to use KDE instead of Gnome. Thank you very much for your help.

SDDM has many features that need to be tied to Plasma, like theming and brightness.

You can avoid hurting your eyes from that maximum brightness by forcing SDDM to show a dark color.

Simply create the following file /usr/share/sddm/themes/breeze/theme.conf.user, and add to it the following lines:


You can achieve the same by searching for a darker image and set it as SDDM wallpaper via Plasma Settings application.

Thanks for your reaply, it doesn’t work. Still with max level of brightness only with a black screen in background.

I had a look in the SDDM codebase. Using the GitHub search function, there appears to be no mention of brightness in the code itself. There is an open issue about adding brightness controls to SDDM, which so far has not been resolved. Given this, I highly doubt that it’s SDDM itself that changes your backlight brightness.

What I know is that systemd restores the backlight brightness from the previous boot. But if you’ve already adjusted that downward, I don’t see why it would come back up to full brightness. I wonder if there is some distribution-specific (systemd) service that runs at the same time as SDDM and sets the brightness to maximum.

One thing you could do, as a workaround if the source of the brightness change can’t be found, is to add your own systemd service to lower it back down. Changing brightness via command line is trivial, e.g. for my system I can do

cat /sys/class/backlight/intel_backlight/max_brightness

which reads 24242. Then take half of that number for 50% brightness and run (as root)

echo 12121 > /sys/class/backlight/intel_backlight/brightness

which adjusts the backlight immediately. My system’s intel_backlight and the maximum brightness could be different on your system, the rest is the same. If you add a systemd unit that responds to SDDM being started, and the unit executes this command as a one-shot service, that would get brightness down.

But obviously that’s a bad hack, and it would be much more interesting to know which program is maximizing the brightness to begin with on Fedora 40.

I’ll go to use this trick for my brightness, thanks.
Like you said also I really wanna know why this issue happens…
I’ve also tried with other distro debian based, arch based, SUSE… happens in everyone. The only one that work is Fedora 40 workstation with gnome but on the login screen (not the gdm3) this issue happens again and only if I lock my pc manually!
If I suspend that work properly, really funny (no it’s not).
Thanks for your help once again.