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

Question

This appears to be a variation on the venerable "why-don't-brightness-controls-work-after-resume?" issue, but with an interesting twist.

The behaviour described below occurs on a Macbook Air 6.1 with an absolutely stock install of 16.10 (literally the only changes are to reboot, run sudo apt-get update and to change the power settings so that closing the lid triggers suspend).

Description

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

To reproduce

1. Boot.
2. Increase & decrease brightness. Observe.
3. Suspend (either by closing the lid or via the UI).
4. Wake.
5. Increase & decrease brightness. Observe.

Results

Normal brightness control

At step 2, brightness controls work correctly in every way. I can dim the screen through twenty steps from sunburn-bright to off.

Broken brightness control

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

• brightness is reset to zero (or at least the screen is dark),
• repeatedly tapping brightness fn key returns brightness, but at a very high level.
• there are only four steps of brightness, roughly 'off', 'bright', 'very bright', and 'very, very bright'.

Other

Digging around, I noticed I have two different items in /sys/class/backlight (I have no idea if this is normal):

• acpi_video0 and
• intel_backlight

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

Answer 1

I started this as a question, but found the answer right where I should have been looking for it in the first place. On my hardware, the kernel module on that page solves every part of the problem I describe here.

May it save somebody else a few hours of futzing around (and also, may it teach me to rtfm!)