I made a couple more tests and here is what I've found out.
When the backlight is managed by radeon, writing to
/sys/class/backlight/radeon_bl0/brightness doesn't do anything. Although
changing brightness with Fn+Left/Fn+Right works but the value in
/sys/class/backlight/radeon_bl0/brightness doesn
I don't think this is radeon or drm bug: the problem appears with legacy
fglrx driver, in custom kernel built without radeon and drm support in
rescue mode, and in systems that don't use any of them.
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The problem still appears in the latest mainline kernel.
Linux unti1x-laptop 5.11.0-051100rc4-generic #202101180234 SMP Mon Jan
18 02:36:59 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
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https
ge: linux-image-5.4.0-59-generic 5.4.0-59.65
ProcVersionSignature: Ubuntu 5.4.0-59.65-generic 5.4.78
Uname: Linux 5.4.0-59-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.14
Architecture: amd64
AudioDevicesInUse:
USERPID ACCESS COMMAND
/dev/snd/controlC0: unti1x 112
