[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 #21 from Luca Boccassi <luca.boccassi at gmail.com> ---
(In reply to Aaron Lu from comment #20)
> (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.

Hello Aaron,

I just tested and it works as expected, brightness changes. Tested on Jessie
with both 3.18 and 3.19, and the 2.99.917-1 xserver-xorg-video-intel.

But please note that, although I believe the problem is the same since the GPU
family and drivers are the same, I have a different laptop: Dell Latitude
E5540, running an Intel Haswell i7-4600U with GPU HD Graphics 4400.
So we should have someone with the Samsung laptop (which has an HD4000 I think)
to confirm as well, to be sure.

Kind regards,
Luca Boccassi

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