brightd appears to fail to start with the default configuration on
Pinebook (an arm64 laptop).

Trying to start it manually, brightd just displays the help text and
exits:

  $ brightd -v -w 5
  brightd 0.4.1
  A X11-daemon for iBook-like brightness management
  Copyright (c) 2006-2008, Phillip Berndt

  Options:
   -v        Be verbose
   -d        Daemonize
   -P <file> Create pid file
   -u n      Drop privileges to this user (Defaults to nobody)
   -w n      Wait n seconds before reducing brightness (Defaults to 3)
   -b n      The brightness setting for the dark screen (Defaults zu 0)
   -f        Reduce brightness even if on the highest brightness level
             Specify twice to also do so when on AC
   -e n      Filter event sources using regexp n (on /dev/input/by-path
   -c n      Set the backlight class to use (defaults to the first
             subnode of /sys/class/backlight)
   -x        Don't query X11 Xss extension
   -r n      Create a FIFO, into which acpid may write the new level when the user
             changed display brightness

The exit code returned to the shell is 0, so it claims to have worked
correctly.

I've also tried running it as root with the same results, as this said
to be required in /usr/share/doc/brightd/README, though if that's
true, /etc/X11/Xsession.d/90brightd wouldn't possibly work.

/sys/class/backlight/backlight/brightness is writeable by the "video"
group, and the user is present in the "video" group.

The /sys/class/power_supply/AC doesn't exist, but fromt the attached
strace log, it doesn't appear to try to access it. I also built a
locally recompiled version patched hardcoding a different path for the
AC online status, but it behaved the same.

My hunch is some sort of missing assumed dependencies; this device
doesn't have ACPI for example.

live well,
  vagrant