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.
** Changed in: ubuntu
Status: Opinion => New
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https://bugs.launchpad.net/bugs/868208
Title:
Changing screen back-light freezes thinkpad t420
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