[Bug 87286] New: [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

            Bug ID: 87286
           Summary: [i915 intel_backlight] can't regulate brightness at
                    GDM login prompt if HDMI monitor connected
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: jre.winesim at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 110819
  --> https://bugs.freedesktop.org/attachment.cgi?id=110819&action=edit
/var/log/dmesg with kernel option drm.debug=0x06

Bug description:
================

Brightness can only be regulated up to 5% while being at the GDM login
prompt if an external HDMI monitor is connected.

It works again after logging in to Gnome.
If the HDMI monitor is not connected it also works.

With kernel 3.14 (or later versions if I set the kernel option
video.use_native_backlight=0) this works just fine. (There I have
/sys/class/backlight/acpi_video0 next to intel_backlight.)


System environment:
===================

-- chipset: i915 (Intel HD Graphics 4000)
-- system architecture: x86_64
-- xf86-video-intel: xserver-xorg-video-intel 2:2.21.15-2+b2
-- xserver: xserver-xorg-core 2:1.16.1.901-1
-- mesa: 10.3.2-1
-- libdrm: libdrm-intel1:amd64 2.4.58-2
-- kernel: 3.18-1~exp1
-- Linux distribution: Debian Jessie (testing)
-- Machine or mobo model: Samsung NP900X4C-A07DE Notebook
-- Display connector: Internal + Micro HDMI

I tested it with the following official Debian packages:
linux-image-3.14-2-amd64       3.14.15-2
linux-image-3.16-2-amd64       3.16.3-2
linux-image-3.16-3-amd64       3.16.5-1
linux-image-3.16.0-4-amd64     3.16.7-2
linux-image-3.16.0-4-amd64     3.16.7-ckt2-1
linux-image-3.17.0-trunk-amd64 3.17.4-1~exp1
linux-image-3.18.0-trunk-amd64 3.18-1~exp1

... and with a self-compiled (3.18), based on
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git, head at
92a578b064d0227a3a7fbbdb9e29dbab7f8d400e (Linus Torvalds, 2014-12-11
06:17:00), using the Debian 3.16.0-4 kernel config, compiled with "make
deb-pkg").

$ ls /sys/class/backlight/
intel_backlight


Reproducing steps:
==================

The problem exists in most cases (perhaps 80 to 90%).


Additional info:
================

With kernel 3.16 the internal display was also completely dimmed (=black), but
since kernel 3.17 the initial brightness is ok again.

Due to bug #84372 I've enabled sna. Disabling it again makes no difference.

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