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[Bug 868208] [NEW] Changing screen back-light freezes thinkpad t420


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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

"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.

** Affects: linux (Ubuntu)
     Importance: Low
         Status: Incomplete

Changing screen back-light freezes thinkpad t420
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