https://bugzilla.kernel.org/show_bug.cgi?id=81691

            Bug ID: 81691
           Summary: Regression: Thinkpad T420 brightness keys broken by
                    commit 751109aad, video.use_native_backlight=1
           Product: ACPI
           Version: 2.5
    Kernel Version: 3.16 (since commit 751109aad)
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Power-Video
          Assignee: acpi_power-vi...@kernel-bugs.osdl.org
          Reporter: andre.mul...@web.de
        Regression: No

Created attachment 145151
  --> https://bugzilla.kernel.org/attachment.cgi?id=145151&action=edit
output of dmidecode, xz compressed

In linux-3.16, this commit

> commit 751109aad5834375ca9ed0dcfcd85a00cbf872b5
> Author: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Date:   Thu Jun 5 22:47:35 2014 +0200
>
>    ACPI / video: Change the default for video.use_native_backlight to 1

breaks backlight control on the Lenovo Thinkpad T420.
That is, I can't change brightness via the Fn-Keys anymore.

Writing to
/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness
works, but the kernel should just handle it by itself.

Providing video.use_native_backlight=0 on the kernel command line
fixes the controls.

So this would need to get a quirk or a fix for native backlight.

I do attach the dmidecode output.
I'll happily provide more information, or test things.

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