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.
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_bl0likewise 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.
- 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.