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["Christopher M. Penalver" <christopher.m.penalver@xxxxxxxxx>]

Amir Mlk, thank you for reporting this bug to Ubuntu. Natty reached EOL on October 28, 2012.
See this document for currently supported Ubuntu releases: https://wiki.ubuntu.com/Releases

Is this an issue in a supported release? If so, could you please execute the following command, as it will automatically gather debugging information, in a terminal:
apport-collect 868208

** Package changed: ubuntu => linux (Ubuntu)

** Changed in: linux (Ubuntu)
   Importance: Undecided => Low

** Changed in: linux (Ubuntu)
       Status: Confirmed => Incomplete

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https://bugs.launchpad.net/bugs/868208

Title:
  Changing screen back-light freezes thinkpad t420

Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  I have installed Ubuntu 11.04 on a new Lenovo T420 laptop (Core i7,
  NVIDIA NVS 4200M Graphics with Optimus) and I'm observing strange
  behaviors most notably, changing backlight causes the laptop to
  freeze. Also, running on battery is not possible since if I unplug the
  power, Ubuntu freezes. I'm guessing this is an ACPI issue but haven't
  still figured out what is wrong.  thinkpad_acpi is loaded
  automatically. Boot-time crashes are also very commonplace.

  Update1: I noticed the following thinkpad_acpi output upon loading the
  module:

  "thinkpad_acpi: detected a 8-level brightness capable ThinkPad"
  Which is wrong. Maybe thinkpad_acpi is not properly detecting hardware's version and falling back to the legacy mode.

  Update2: I disabled UEFI in firmware control panel and set the boot method to "legacy mode only" (BIOS) and the problem went away. I believe this is due to buggy implementation of ACPI by UEFI in those machines. Occasional freezes also went away. The only thing that is not working now is back-light keys.
  At this point it seems this is not an Ubuntu related bug rather a bug in the firmware.

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