[Bug 85861] New: backlight control doesn't work on Lenovo t440p after upgrading to 3.17 [bisected]

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=85861

            Bug ID: 85861
           Summary: backlight control doesn't work on Lenovo t440p after
                    upgrading to 3.17 [bisected]
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.17
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: dion at inhex.net
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

Created attachment 152911
  --> https://bugzilla.kernel.org/attachment.cgi?id=152911&action=edit
dmesg

After upgrading from 3.16 to 3.17 I can't adjust brightness in KDE using Fn-*
hotkeys. 

Only intel_backlight is present in /sys/class/backlight. And adjusting value
here works.

I've bisected it and got followed commit:

commit 6dda730e55f412a6dfb181cae6784822ba463847
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Tue Jun 24 18:27:40 2014 +0300

    drm/i915: respect the VBT minimum backlight brightness

    Historically we've exposed the full backlight PWM duty cycle range to
    the userspace, in the name of "mechanism, not policy". However, it turns
    out there are both panels and board designs where there is a minimum
    duty cycle that is required for proper operation. The minimum duty cycle
    is available in the VBT.

    The backlight class sysfs interface does not make any promises to the
    userspace about the physical meaning of the range
    0..max_brightness. Specifically there is no guarantee that 0 means off;
    indeed for acpi_backlight 0 usually is not off, but the minimum
    acceptable value.

    Respect the minimum backlight, and expose the range acceptable to the
    hardware as 0..max_brightness to the userspace via the backlight class
    device; 0 means the minimum acceptable enabled value. To switch off the
    backlight, the user must disable the encoder.

    As a side effect, make the backlight class device max brightness and
    physical PWM modulation frequency (i.e. max duty cycle)
    independent. This allows a follow-up patch to virtualize the max value
    exposed to the userspace.

    Signed-off-by: Jani Nikula <jani.nikula at intel.com>
    Reviewed-by: Jesse Barnes <jbarnes at virtuousgeek.org>
    [danvet: s/BUG_ON/WARN_ON/]
    Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Reverting it fixes issue for me.

00:02.0 VGA compatible controller: Intel Corporation 4th Gen Core Processor
Integrated Graphics Controller (rev 06)

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