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[Rüdiger Kupper <ruediger.kupper+launchpad@xxxxxxxxx>]

On my Thinkpad X230, the problem was present in May 2013 but now is gone
(3.8.0-25-generic). I cannot tell which kernel update brought the fix.

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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".

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