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.
brightness controls do change /sys/class/backlight/intel_backlight/brightness value, but it doesn't affect the actual brightness of the panel.
Plugging an external monitor immediately sets the panel brightness to the value of /sys/class/backlight/intel_backlight/brightness and further control becomes possible.
Is it the same with vanilla sources?
The culprit turned out to be the i915.fastboot=1 Please see https://bugs.freedesktop.org/show_bug.cgi?id=103781
