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Running 20.04 on an early-2008 iMac.

$ cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-5.4.0-31-generic root=UUID=3acf183b-0264-4c56-a18e-0fec33417d68 ro quiet splash acpi_osi=Linux acpi_backlight=vendor vt.handoff=7

I'm having a two-sided problem with monitor brightness.

  1. When the system unsuspends, the screen is always at about 1/3 brightness -- not very usable. None of the controls I know of can amend it: the GUI slider under the system menu, which usually functions (see 2), shows at 100% or fully to the right, and will only take away brightness. Running # echo 255 > /sys/class/backlight/radeon_bl0 likewise does nothing, but usually works away from this scenario. It shows full values:

    $ for i in /sys/class/backlight/*; do echo $i; cat $i/brightness; cat $i/actual_brightness; cat $i/max_brightness; done 
    /sys/class/backlight/radeon_bl0
    255
    255
    255
    

xbacklight has never worked on this system, and is no help in this situation.

Shutting down rather than suspending, side-steps the problem, and rebooting fixes it, but I'd rather not do that all the time.

  1. That slider under the system menu is, so to speak, too powerful: it makes the screen black, and I believe it's actually causing the lamp to go off. (The first time I used it, I accidentally let go the mouse button, and not being able to do anything about the black screen, had to reboot; unfortunately, the blacked-out/off state was preserved between boots, and so I had to re-install.)

Any help or insights out there? Many thanks.

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