If I build powerdevil with -DHAVE_DDCUTIL=On I still don’t get a brightness slider for my external display. I have the udev rule and the i2c_dev module loaded, and ddcutil works from the terminal. Furthermore, running /usr/lib/org_kde_powerdevil doesn’t give any errors. Any help will be appreciated.
Not a solution but an alternative: try to install a ddcci-driver-linux-dkms from AUR and use it instead. ddcutil support in powerdevil is still somewhat broken as far as I remember.
https://wiki.archlinux.org/title/backlight#External_monitors
That’s odd, according to this KDE bug it seems to be fixed. Nevertheless, is there any way to debug powerdevil?