After upgrading from xfce4-power-manager-1.6.0 to xfce4-power-manager-1.6.1 or later (I tested all versions up to xfce4-power-manager-1.6.5), I cannot change the backlight using xfpm-power-backlight-helper anymore. It seems xfpm-power-backlight-helper 1.6.5 is picking the wrong device to apply the backlight changes. At least, when I pass an invalid value as the brightness, I get the following message: $ pkexec /tmp/xfce4-power-manager-1.6.5/src/xfpm-power-backlight-helper --set-brightness 100000 Could not set the value of the backlight: writing '100000' to /sys/class/backlight/acpi_video0/brightness failed When performing the same test on v1.6.0, another device is listed: $ pkexec /tmp/xfce4-power-manager-1.6.0/src/xfpm-power-backlight-helper --set-brightness 100000 Could not set the value of the backlight: writing '100000' to /sys/class/backlight/intel_backlight/brightness failed Manually setting the backlight via /sys/class/backlight/intel_backlight/brightness works fine, while /sys/class/backlight/acpi_video0/brightness does not work.
This commit changed how the interface detection works: https://git.xfce.org/xfce/xfce4-power-manager/commit/?id=36b2c96e91feeb118a4e8170c5d608795f00d5f2 The following GNOME bug report may be relevant: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/issues/53
Thank you for the reference. The work around of adding the following to my X.org config, worked for me: Section "Device" Identifier "Intel" Driver "intel" Option "Backlight" "intel_backlight" EndSection
In case this is a linux kernel bug and you are still using an older version, can you check if 4.19+ resolves the problem?
The problem is fixed when using kernel version 5.3.7.
Thank you for confirming it. Closing this report.