4.5 regression: ACPI hangs when backlight hotkeys are pressed

Andy Lutomirski
2016-01-18T20:53:17Z

I'm running 4.4 with the pm+acpi-4.5-rc1-1 merged in. After pressing
the backlight hotkeys a couple times, things start to break. Poking
at hung thread stacks, it appears that calls to
acpi_video_handles_brightness_key_presses never return.

It appears that the culprit is:

commit aecbd9b1bff6afbe349921b406b99d285b412820
Author: Adrien Schildknecht <adrien+dev@schischi.me>
Date: Mon Jan 4 23:22:28 2016 +0100

ACPI / video: driver must be registered before checking for keypresses

Reverting that patch makes my laptop work again.

I don't know what that patch is trying to do, but it looks highly
questionable to me. Why should an accessor wait for a potential
different driver to register something? I see no guarantee that it
will eve happen.

This is a Dell XPS 13 9350.

--Andy