I am using a KDE Plasma Desktop on Excalibur (via Daedalus) on x86 and
have not found how to adjust screen brightness. What needs to be
changed from the defaults so that the display brightness can be adjusted?

Perhaps it is related, but the external display cannot be found either.

I've tried the Display Configuration widget but that only shows greyed
out options for the layout of the screens which it cannot find. xrandr
also cannot find the external display, just the built-in one.

I've tried ddcui but it does not find any of the displays, and even
gives this warning:

    "Display hotplug detection requires
     DRM enabled video drivers"

I presume that something is missing with the Direct Rendering Manager?

What are the solutions or work-arounds to be able to control the display
brightness in KDE Pklasma? Any clues are welcome.

/Lars

$ lsb_release -rd
Description:    Devuan GNU/Linux 6 (excalibur/ceres)
Release:    6

$ uname -sr
Linux 6.12.25-amd64

$ kscreen-doctor -o
Output: 568 default
    enabled
    connected
    priority 1
    Unknown
    Modes:  569:2560x1600@93*!
    Geometry: 0,0 2560x1600
    Scale: 1
    Rotation: 1
    Overscan: 0
    Vrr: incapable
    RgbRange: unknown
    HDR: incapable
    Wide Color Gamut: incapable
    ICC profile: incapable
    Color profile source: incapable

$ xrandr
Screen 0: minimum 2560 x 1600, current 2560 x 1600, maximum 2560 x 1600
default connected primary 2560x1600+0+0 0mm x 0mm
    2560x1600     93.00*

# lshw -c display -sanitize
   *-display
        description: VGA compatible controller
        product: Phoenix3
        vendor: Advanced Micro Devices, Inc. [AMD/ATI]
        physical id: 0
        bus info: pci@0000:66:00.0
        logical name: /dev/fb0
        version: c5
        width: 64 bits
        clock: 33MHz
        capabilities: pm pciexpress msi msix vga_controller cap_list fb
        configuration: depth=32 latency=0 resolution=2560,1600
        resources: iomemory:7c0-7bf memory:7c00000000-7c0fffffff 
memory:dc000000-dc1fffff ioport:e000(size=256) memory:dc500000-dc57ffff