[Bug 77831] New: i915: no backlight control any more (MacBook4,1)

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=77831

            Bug ID: 77831
           Summary: i915: no backlight control any more (MacBook4,1)
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.15
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: mcy at lm7.fr
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

Created attachment 139641
  --> https://bugzilla.kernel.org/attachment.cgi?id=139641&action=edit
kern.log drm.debug=0xe with v3.14-890-gc675949

Hi there,

Sorry Jani, I'm afraid this one is for you too. This is the bug I experienced
while testing the kernel from the drm-intel repo for bug 75791, and it is
confirmed in v3.15 on the same machine.

Symptoms:
- backlight level stays at maximum
- empty /sys/class/backlight
- this might be unrelated, but X (KDM) almost always initially starts to a
black screen, and I have to switch to a VT and back to the X VT to restore the
display; more rarely it's the other way round: KDM starts normally but when I
switch to a VT it is a black screen

Bisect result:

commit c675949ec58ca50d5a3ae3c757892f1560f6e896
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Wed Apr 9 11:31:37 2014 +0300

    drm/i915: do not setup backlight if not available according to VBT

The kern.log attached was obtained with this commit and drm.debug=0xe; you can
see the message "native backlight control not available per VBT" introduced by
this commit appear.

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