I just tested it on my Thinkpad X1 (haswell) and the fix works.
However, there is a side effect. Connecting and Disconnecting the
Power resets the brightness to maximum.  Not sure how to go about
working around this.

anmar

On 26 February 2014 23:14, madbiologist <email address hidden> wrote:
> The upstream 3.14-rc4 kernel also contains this commit:
>
> author Rafael J. Wysocki
> committer Rafael J. Wysocki
> commit a6940190ac15c361862a1a8f50a2072db7184749
> tree c73be15646c6418f77a94fede3afc7eabd589dd6
> parent 0e9f81d3b7cd0649a3bc437391b6a0650f98f844
>
> Revert "ACPI: Blacklist Win8 OSI for some HP laptop 2013 models"
>
> This reverts commit 2d4054d84224 (ACPI: Blacklist Win8 OSI for some HP
> laptop 2013 models) that is not necessary any more after previous commit
> 1811fcb029fa (ACPI / video: Add systems that should favour native
> backlight interface).
>
> Requested-by: Takashi Iwai
> Tested-by: Mika Westerberg
> Signed-off-by: Rafael J. Wysocki
>
> --
> You received this bug notification because you are subscribed to a
> duplicate bug report (1121951).
> https://bugs.launchpad.net/bugs/1098216
>
> Title:
>   Regression in brightness control on Lenovo Thinkpad X230 (+tablet) and
>   X1 Carbon
>
> Status in The Linux Kernel:
>   In Progress
> Status in “linux” package in Ubuntu:
>   Triaged
> Status in “linux” package in Fedora:
>   Unknown
>
> Bug description:
>   == Raring SRU Justification ==
>
>   Impact: A number of ThinkPad models have a workaround in the ACPI
>   backlight implementation for Windows 8 that more or less completely
>   breaks backlight control.
>
>   Fix: Add an OSI quirk to no longer claim to be Windows 8 on these
>   machines, causing the firmware to revert to the older, working behavior.
>
>   Test Case: Verified on LP #1098216 and bugzill.kernel.org #51231.
>
>   ---
>
>   Something changed between the 3.5 and the 3.7 kernel, leading to
>   broken brightness control on my Thinkpad x230.
>
>   Here are some test results:
>   == 3.5.0-21-generic ==
>   === Post-boot values ===
>   /sys/class/backlight:
>     - intel-backlight
>       + actual_brightness => 4438
>       + bl_power => 0
>       + brightness => 4438
>       + max_brightness => 4438
>     - acpi_video0
>       + actual_brightness => 15
>       + bl_power => 0
>       + brightness => 15
>       + max_brightness => 15
>
>   === Tests ===
>   echo 0 > acpi_video0/brightness => works
>   echo 0 > intel_backlight/brightness => works
>
>   == 3.8.0-0-generic ==
>   === Post-boot values ===
>   /sys/class/backlight:
>     - intel-backlight
>       + actual_brightness => 4438
>       + bl_power => 0
>       + brightness => 4438
>       + max_brightness => 4438
>     - acpi_video0
>       + actual_brightness => 100
>       + bl_power => 0
>       + brightness => 100
>       + max_brightness => 100
>
>   === Tests ===
>   echo 0 > acpi_video0/brightness => doesn't do anything
>   echo 0 > intel_backlight/brightness => works
>   ---
>   ApportVersion: 2.8-0ubuntu1
>   Architecture: amd64
>   CheckboxSubmission: 12108b8b8b67d760cdd84f1b9574e7f3
>   CheckboxSystem: bb422ca46d02494cdbc459927a98bc2f
>   DistroRelease: Ubuntu 13.04
>   InstallationDate: Installed on 2012-09-02 (129 days ago)
>   InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Alpha amd64 (20120902)
>   MarkForUpload: True
>   Package: linux (not installed)
>   ProcEnviron:
>    LANGUAGE=en_US:en
>    TERM=xterm
>    PATH=(custom, no user)
>    LANG=en_US.UTF-8
>    SHELL=/bin/bash
>   ProcVersionSignature: Ubuntu 3.8.0-0.1-generic 3.8.0-rc3
>   Tags:  raring
>   Uname: Linux 3.8.0-0-generic x86_64
>   UnreportableReason: The running kernel is not an Ubuntu kernel
>   UpgradeStatus: No upgrade log present (probably fresh install)
>   UserGroups:
>
>   WORKAROUND:
>
>   Add to /etc/default/grub
>   in the variable "GRUB_CMDLINE_LINUX_DEFAULT":
>
>   acpi_osi=\"!Windows 2012\"
>
>   e.g:
>
>   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=\"!Windows 2012\""
>
>   Then run "sudo update-grub".
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/linux/+bug/1098216/+subscriptions