[Bug 87286] [i915 intel_backlight] can't regulate brightness at GDM login prompt if HDMI monitor connected

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87286

--- Comment #20 from Aaron Lu <aaron.lu at intel.com> ---
(In reply to Luca Boccassi from comment #19)
> Hello Aaron,
> 
> That package ships a binary, xf86-video-intel-backlight-helper, which
> apparently is used to regulate the brightness. I'm not familiar with the
> code myself or the project itself, but with a quick Google search this Xorg
> mailer thread came up and it does shed some light on how this is used:
> http://lists.x.org/archives/xorg-devel/2014-February/040575.html

Thanks for the pointer. The helper is used to write a value to the
/sys/class/backlight/X/brightness and the fact that it now works means that the
native interface works and the original problem may be due to broken user space
tool. You can verify by doing this(when the native interface is still there):
# cd /sys/class/backlight/intel_backlight
# cat max_brightness
XXX
# echo some_value_less_than_max > brightness
See if backlight level changes. Of source you will also need to verify if the
interface still works after hotplug as your original scenario.

If the backlight indeed works, then we have mistakenly disabled the native
interface and I think we should revert that patch.

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