[Bug 257796] backlight(8) gets confused on power change

From: <bugzilla-noreply_at_freebsd.org>
Date: Thu, 12 Aug 2021 21:21:46 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=257796

            Bug ID: 257796
           Summary: backlight(8) gets confused on power change
           Product: Base System
           Version: 13.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: mason@blisses.org

I've been using backlight(8) to set my backlight brightness, and it
generally works very well, but I just noticed that if I plug in or unplug
my laptop power adapter it will go to 100% brightness. The command output
will show 100% brightless, but it appears to remember the previous value,
as if I go back to, say, 50% with "backlight 50" it presumably already
thinks it's there and will do nothing. If I choose another value, whether
higher or lower, it will correctly set that new value, after which
backlight(8) will report that value.

So:

# backlight 50
-> unplug and see backlight brighten
# backlight
brightness: 100
# backlight 50
-> nothing happens
# backlight
brightness 100
# backlight 40
-> backlight goes to 40%
# backlight 
brightness: 40
# backlight 50
-> backlight goes to 50% this time
# backlight
brightness: 50

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