Fix came in with 5.8.6

https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.8.6

 Author: Alexander Monakov <amonakov@ispras.ru>
 Date: Tue Aug 4 23:13:13 2020 +0300
 
 drm/amd/display: use correct scale for actual_brightness
 
 commit 69d9f4278d0f9d24607645f10e5ac5c59c77a4ac upstream.
 
 Documentation for sysfs backlight level interface requires that
 values in both 'brightness' and 'actual_brightness' files are
 interpreted to be in range from 0 to the value given in the
 'max_brightness' file.
 
 With amdgpu, max_brightness gives 255, and values written by the user
 into 'brightness' are internally rescaled to a wider range. However,
 reading from 'actual_brightness' gives the raw register value without
 inverse rescaling. This causes issues for various userspace tools such
 as PowerTop and systemd that expect the value to be in the correct
 range.
 
 Introduce a helper to retrieve internal backlight range. Use it to
 reimplement 'convert_brightness' as 'convert_brightness_from_user' and
 introduce 'convert_brightness_to_user'.
 
 Bug: https://bugzilla.kernel.org/show_bug.cgi?id=203905
 Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1242
 Cc: Alex Deucher <alexander.deucher@amd.com>
 Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
 Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
 Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
 Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
 Cc: stable@vger.kernel.org
 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

I also tried it and for me it works just fine.

Fabis.cafe (talk) 12:19, 22 September 2020 (UTC)

Checked and working for me, tqvm! YNShung (talk) 12:45, 1 October 2020 (UTC)