[root@brain-ThinkPad-X230 acpi_video0]# cat max_brightness 
100
[root@brain-ThinkPad-X230 acpi_video0]# cat brightness 
100
[root@brain-ThinkPad-X230 acpi_video0]# xbacklight 
100.000000
[root@brain-ThinkPad-X230 acpi_video0]# /usr/libexec/gsd-backlight-helper --get-brightness
100
[root@brain-ThinkPad-X230 acpi_video0]# /usr/libexec/gsd-backlight-helper --set-brightness 90
[root@brain-ThinkPad-X230 acpi_video0]# /usr/libexec/gsd-backlight-helper --get-brightness
90
[root@brain-ThinkPad-X230 acpi_video0]# xbacklight 
90.000000
[root@brain-ThinkPad-X230 acpi_video0]# cat brightness 
90
[brain@brain-ThinkPad-X230 acpi_video0]$ gdbus call  --session --dest org.gnome.SettingsDaemon --object-path /org/gnome/SettingsDaemon/Power --method org.gnome.SettingsDaemon.Power.Screen.StepDown
(uint32 85,)
[root@brain-ThinkPad-X230 acpi_video0]# xbacklight 
90.000000
[root@brain-ThinkPad-X230 acpi_video0]# cat brightness 
90

