Comment # 17 on bug 1180591 from LAURENT LE POITTEVIN

Same problem for me.
I'm on a livecd tumbelweed on a 27" late2013 Imac.
The problem is the same, it is impossible for me to adjust the screen
brightness using the slider in the kde menu, nor in the energy settings. Screen
brightness is always at the absolute maximum.
Setting the automatic brightness to 1 doesn't change anything.

After adjust the brightness directly on /sys/class/backlight/acpi_video0 with
the command :
echo 7 > /sys/class/backlight/acpi_video0/brightness
Nothing changes, even with other values.

In the /sys/class/backlight folder, I only have acpi_video0.

However, I have tried to change the settings at Grub startup :
acpi_backlight=vendor, acpi_backlight=video,acpi_backlight=none, and
acpi_backlight=native, but without success, the slider in the gnome menu
disappears for (vendor, native and none).

The problem is the same with nvidia or nouveau.

If I go into sleep mode and then reactivate my machine, I can adjust the
brightness again, but I can't do this from boot-up without going into sleep
mode.

The problem does not exist with opensuse leap 15.2 all works perfectly with a
5.3 kernel.

To have a disk with Ubuntu 20.04, the problem is the same. With the 5.4 kernel,
I can adjust the brightness, but with the HWE kernel update to version 5.8, I
can't anymore.
I think that the problem comes indeed from the kernel.