I am new to KDE, recently having switched from Gnome. I am on a Thinkpad W530, which has an integrated Intel graphics controller, as well as a dedicated graphics card (NVIDIA Quadro K100M). They each have their own brightness settings, located in:

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/sys/class/backlight/intel_backlight/brightness
/sys/class/backlight/nv_backlight/brightness


If I have the backlight brightness set at 100%, I get:

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$ cat /sys/class/backlight/intel_backlight/brightness
4437
$ cat /sys/class/backlight/nv_backlight/brightness
100


Now if I use the function keys to reduce the brightness to 50%, I get:

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$ cat /sys/class/backlight/intel_backlight/brightness
4437
$ cat /sys/class/backlight/nv_backlight/brightness
50


However, my screen's brightness hasn't actually changed, presumably because the Intel card is the one in use. How do I point KDE to the correct graphics card?

Stumbled upon exactly same issue on Fedora 33 with KDE desktop on thinkpad with discrete nVidia graphics.
I don't think it's the KDE issue.

Anyway I managed to resolve it, adding acpi_backlight=video to kernel arguments.
It can be done via grubby utility:

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sudo grubby --update-kernel=ALL --args="acpi_backlight=video"