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[Marius Gedminas <marius@xxxxxxxxx>]

I did not have a DisplayPort cable before January 2014.

Plugging/unplugging VGA never caused any brightness issues.  And still
doesn't -- I just tested with current 14.04 kernel (had to go back to it
because I need VirtualBox for work).

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https://bugs.launchpad.net/bugs/1378863

Title:
  ThinkPad X220: unplugging DisplayPort breaks backlight (intel video)

Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  Steps to reproduce:

  1.  Plug in an external monitor using a DisplayPort -> HDMI cable
  2. Unplug the cable

  What happens about 30% of the time:

  - Brightness of laptop's internal LCD drops to lowest level
  - Brightness keys (Fn-Home/End on this Lenovo ThinkPad X220) produce the gnome-shell on-screen display of the changing brightness meter, but actual LCD brightness doesn't change.

  There are a couple of workarounds:

  1. Ctrl+Alt+F1 and use Fn-Home/End to control brightness, then
  Ctrl+Alt+F7 back to X (where brightness keys continue to not work)

  or

  2. Plug/unplug DisplayPort again, this usually fixes the brigthness
  (maybe 60% of the time)

  
  I've been having this problem on this laptop since January of 2014.  I was running Ubuntu 13.10 then, I'm running 14.04 now.

  Sometimes the brightness-drop-and-you-can't-fix-it thing happens on
  DisplayPort hotplug rather than on unplug.

  ProblemType: Bug
  DistroRelease: Ubuntu 14.04
  Package: linux-image-3.13.0-37-generic 3.13.0-37.64
  ProcVersionSignature: Ubuntu 3.13.0-37.64-generic 3.13.11.7
  Uname: Linux 3.13.0-37-generic x86_64
  ApportVersion: 2.14.1-0ubuntu3.4
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  mg         3318 F.... pulseaudio
  CurrentDesktop: GNOME
  Date: Wed Oct  8 17:35:50 2014
  EcryptfsInUse: Yes
  HibernationDevice: RESUME=UUID=bf592a79-5f4e-486d-b69d-7525d492dcc6
  InstallationDate: Installed on 2012-07-25 (805 days ago)
  InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
  Lsusb:
   Bus 002 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
   Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
   Bus 001 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
   Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
  MachineType: LENOVO 4291WJF
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-37-generic root=UUID=36079681-53fc-4ca2-80ac-98321c07e8d2 ro quiet splash vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-3.13.0-37-generic N/A
   linux-backports-modules-3.13.0-37-generic  N/A
   linux-firmware                             1.127.7
  RfKill:
   0: phy0: Wireless LAN
   	Soft blocked: no
   	Hard blocked: no
  SourcePackage: linux
  UpgradeStatus: Upgraded to trusty on 2014-04-18 (173 days ago)
  dmi.bios.date: 07/18/2013
  dmi.bios.vendor: LENOVO
  dmi.bios.version: 8DET69WW (1.39 )
  dmi.board.asset.tag: Not Available
  dmi.board.name: 4291WJF
  dmi.board.vendor: LENOVO
  dmi.board.version: Not Available
  dmi.chassis.asset.tag: No Asset Information
  dmi.chassis.type: 10
  dmi.chassis.vendor: LENOVO
  dmi.chassis.version: Not Available
  dmi.modalias: dmi:bvnLENOVO:bvr8DET69WW(1.39):bd07/18/2013:svnLENOVO:pn4291WJF:pvrThinkPadX220:rvnLENOVO:rn4291WJF:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
  dmi.product.name: 4291WJF
  dmi.product.version: ThinkPad X220
  dmi.sys.vendor: LENOVO

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