Changing screen back-light freezes thinkpad t420

[Amir Mlk]

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.

[Robert Huitl (freedesktop-huitl)]

I had the same problem on my T420s with graphics set to discrete in the BIOS. Comment 40 in bug 776999 solved it for me: disable the VT-d extensions in the BIOS.

[Rainer (rainerf)]

Disabling VT-d fixed the freezes for my T420s (with NVS 4200M set to discrete graphics), and with Option "RegistryDwords" "EnableBrightnessControl=1" as described in bug 776999 changing the brightness even works again.

I also get
[ 4.682864] thinkpad_acpi: detected a 8-level brightness capable ThinkPad
during bootup. Interestingly, this seems correct for increasing the brightness: there seem to be 8 levels. Decreasing, however, seems to be more fine-grained since there are 16 levels before nothing changes anymore. Don't know if this is important somehow, however ;).

BTW: For Kubuntu, the system hanging on changing the brightness means that it is completely unusable since KDE seems to try to lower the brightness right during the startup.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[penalvch (penalvch)]

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

[Launchpad Janitor (janitor)]

[Expired for linux (Ubuntu) because there has been no activity for 60 days.]