On a Panasonic CF-AX2, installed with the latest (at this time of publishing)
version of Tumbleweed, the brightness keys appear to fail to work on KDE Plasma
despite showing the brightness indicator on-screen.

Probing with

    ls /sys/class/backlight/

showed two options: 

    intel_backlight panasonic

Using 

    echo 800 | sudo tee /sys/class/backlight/intel_backlight/brightness 

I was able to change the brightness on my screen, but not when the options were
passed to "panasonic" as the brightness keys were doing.

Further investigation using the `brightnessctl -l` command to see and set
brightness confirmed brightness values were being pushed to one device only
(panasonic), and not the one that actually affected brightness
("intel_backlight).

As an interesting aside, I installed the Enlightenment desktop environment
which as part of its boot-up process, popped up a message about acpi I couldn't
catch fast enough and automagically just worked for brightness control.

While trying to fix this bug, I encountered this entry for Powerdevil in KDE
which seems to indicate that this kind of behaviour should be fixed when
encountering multiple backlight devices.

https://invent.kde.org/plasma/powerdevil/-/commit/761fc8a4bf4bd70bcd9aca63fc67382c94ecf884

Fix:

Adding the following to the kernel boot option resolves the issue:

    acpi_osi=linux acpi_backlight=video

The second entry is probably the most important of the two options, but I
figured I might as well play it safe with both acpi options.

After confirming brightness toggle now works, 

    ls /sys/class/backlight/

Now shows:

    acpi_video0  intel_backlight  panasonic