Brightness controls act different after waking from suspend on dual-boot Macbook Air 6.1 - Ask Ubuntu

this appears variation on venerable "why-don't-brightness-controls-work-after-resume?" issue, interesting twist.

the behaviour described below occurs on macbook air 6.1 absolutely stock install of 16.10 (literally changes reboot, run sudo apt-get update , change power settings closing lid triggers suspend).

description

• after boot (good!):
  • pre-boot brightness level retained, and
  • brightness controls work expected.
• after wake (not good!):
  • pre-suspend brightness level forgotten, and
  • brightness controls work poorly (see below).

to reproduce

1. boot.
2. increase & decrease brightness. observe.
3. suspend (either closing lid or via ui).
4. wake.
5. increase & decrease brightness. observe.

results

normal brightness control

at step 2, brightness controls work correctly in every way. can dim screen through twenty steps sunburn-bright off.

broken brightness control

at step 5, brightness controls do still work, but:

• brightness reset 0 (or @ least screen dark),
• repeatedly tapping brightness fn key returns brightness, @ high level.
• there 4 steps of brightness, 'off', 'bright', 'very bright', , 'very, bright'.

other

digging around, noticed have two different items in /sys/class/backlight (i have no idea if normal):

• acpi_video0 and
• intel_backlight

the contents of /sys/class/acpi_video0/brightness , /sys/class/intel_backlight/brightness both change when adjust screen brightness, acpi_video0 on scale of 0-100, , intel_video on (non-linear?) scale 0-2777.

i started question, found answer right should have been looking in first place. on hardware, kernel module on page solves every part of problem describe here.

may save else few hours of futzing around (and also, may teach me rtfm!)