This allows to control the backlight level on certain Intel(R) GPUs,
including i915.

This has been taken from intel-gpu-tools which can be found
here: http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/

Originally ported to FreeBSD by "emmex" of the FreeBSD forums.

FreeBSD Build:
  pkg install libpciaccess libdrm
  make
  make install

OpenBSD Build:
  make
  make install

(this installs intel_backlight setuid root, so any user can execute it).

Usage:
  $ intel_backlight
  current backlight value: 30% (281/937)
  $ intel_backlight 50
  current backlight value: 15% (141/937)
  set backlight to 50% (469/937)
  $ intel_backlight incr
  current backlight value: 50% (469/937)
  set backlight to 51% (478/937)
  $ intel_backlight incr
  current backlight value: 51% (478/937)
  set backlight to 60% (562/937)
  $ intel_backlight incr
  current backlight value: 60% (562/937)
  set backlight to 70% (656/937)
  $ intel_backlight decr
  current backlight value: 70% (656/937)
  set backlight to 60% (562/937)
  $ intel_backlight decr
  current backlight value: 60% (562/937)
  set backlight to 51% (478/937)

The included example allows adjusting brightness automatically based on the
input of the isl driver (currently not part of standard FreeBSD).

Usage:
  ./isl_backlight.sh &


OpenBSD Notes:
Requires a kernel capable of concurrent opening of /dev/xf86, via
machdep.allowaperture=3.