[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 #22 from jre.winesim at gmail.com ---
1.)
I can confirm that indeed the xserver-xorg-video-intel backlight helper fixes
the issue (and not an unrelated change).

If I build my own packages based on the preceding commit [1] the problem still
exists. After applying the next 2 commits [2] everything works (there are some
more related commits in the repo, but this is the minimal changeset).

[1]:
git://anonscm.debian.org/pkg-xorg/driver/xserver-xorg-video-intel
commit a01548ccf192a5b1fa1f4a3e31e1634db39f6b39
    intel: export fd_set_cloexec / fd_set_nonblock

[2]:
commit b71f3d8bd4d6773899c1bdc903911cf240e68ead
    Backlight helper build fixes
commit 3d629c91cfa98b75c6685c2a2003e64fd1b612c4
    intel: Add a helper for setting backlight without root rights


2.)
I can also confirm that while the problem exists (GDM login screen black and
brightness not adjustable) I can adjust the brightness with e.g.:
# echo 4647 > /sys/class/backlight/intel_backlight/brightness

(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.

If I understood it correctly, the helper is used to write a value to the
/sys/class/backlight/X/brightness *with root rights*, because xf86-video-intel
requires them (contrary to all other drivers). Still I don't understand why
this is a problem here before login to Gnome, but not after. (I'm running a
"normal" Debian Jessie here with Xorg/gdm3 running as root.) So probably yes,
broken user space tool.


(In reply to Aaron Lu from comment #20)
> If the backlight indeed works, then we have mistakenly disabled the native
> interface and I think we should revert that patch.

Based on Linus "We do not break userspace" I disagree. Everything works fine
here with an older kernel <3.16. And it works again for every newer kernel I
tested if I disable intel_backlight. So there was a regression in the kernel
which had to be fixed.

Having said that the above mentioned thread (and it links) show that the
complete backlight implementation would gain from a redesign coordinated
between X/kernel/... . Until then IMO the ugly blacklist to disable native
blacklight has to remain.

Greets
jre

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