Bug 641520

Summary:	i915.fastboot=1 breaks brightness control on my Thinkpad T540p (haswell)
Product:	Gentoo Linux	Reporter:	Anton Gubarkov <anton.gubarkov>
Component:	Current packages	Assignee:	Gentoo Kernel Bug Wranglers and Kernel Maintainers <kernel>
Status:	RESOLVED INVALID
Severity:	normal	CC:	herrtimson, hydrapolic
Priority:	Normal
Version:	unspecified
Hardware:	All
OS:	Linux
See Also:	https://bugs.freedesktop.org/show_bug.cgi?id=103781
Whiteboard:
Package list:		Runtime testing required:	---

Description Anton Gubarkov 2017-12-18 07:23:00 UTC

Booting with 4.13.X allows me to control my brightness.

Usual Fn-F5/F6 move the brightness slider, but the brightness stays at 100%. moving brightness slider in Gnome panel doesn' change brightness either.
I run X server with intel DDX with Gnome. Kernel Intel driver is compiled as module. 

Connecting external mointor via VGA restores brightness control. It continues working even when the external monitor is disconnected. 

The next boot w/o  external monitor brings the problem back again.

Comment 1 Anton Gubarkov 2017-12-18 07:31:55 UTC

brightness controls do change /sys/class/backlight/intel_backlight/brightness value, but it doesn't affect the actual brightness of the panel.

Comment 2 Anton Gubarkov 2017-12-18 07:34:32 UTC

Plugging an external monitor immediately sets the panel brightness to the value of /sys/class/backlight/intel_backlight/brightness and further control becomes possible.

Comment 3 Tomáš Mózes 2017-12-18 08:45:11 UTC

Is it the same with vanilla sources?

Comment 4 Anton Gubarkov 2018-01-10 12:18:13 UTC

The culprit turned out to be the i915.fastboot=1

Please see https://bugs.freedesktop.org/show_bug.cgi?id=103781