[Bug 85861] backlight control doesn't work on Lenovo t440p after upgrading to 3.17 [bisected]

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=85861

--- Comment #14 from Jani Nikula <jani.nikula at intel.com> ---
Thanks for testing. My only guess is that the userspace (could I be more
vague?) expects actual_brightness to always match brightness in
/sys/class/backlight/intel_backlight when its changed. Can you check whether
the backported commit makes a difference here?

acpi_backlight usually did not go all the way to black. intel_backlight used to
do that. However that broke some setups (by physically pulling other lanes down
on the eDP connector), so we had to start respecting the minimum. But we're
bringing 0 = black back in 3.18 with a different mechanism...

There's disagreement whether the non-linear scaling should be done in userspace
or kernel. Userspace could be more clever about it, but kernel does have some
extra information to use through the VBT for this purpose.

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