Changing hw.acpi.video.lcd0.brightness does nothing

[David Demelier]

Hello,

I have a thinkpad x1 carbon 4th gen. I wanted to give FreeBSD 12 a try 
and installed it in a USB hard drive.

The module acpi_video(4) is loaded and nodes under hw.acpi.video are shown.

However, changing values in hw.acpi.video.lcd0.brightness does nothing. 
It is correctly set but the screen brightness stays the same.

Even tried values like 2, 98, etc.

# sysctl hw.acpi.video.lcd0.brightness=2
# sysctl hw.acpi.video.lcd0.brightness=95

No change, it worked fine on my HP ProBook (which was more surprising 
since HP is not ACPI-compliant in many places).

What's strange is the value hw.acpi.video.lcd0.active, it is 0 and 
trying to change to 1 is impossible.

Any ideas?

-- 
David