[Bug 111475] Brightness control inversion from VBT table is not taken into account in Ubuntu 18.10

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111475

--- Comment #10 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to Szymon from comment #5)
> 2. Once BIOS writes 0xC8250 MMIO register with bit 29, backlight logic is
> inverted correctly. However 10% minimum brightness level defined in VBT
> table is NOT taken into account in this case, i.e. Linux brightness slider
> set at maximum produces complete darkness instead of 10% brightness (as set
> as minimum).
> 
> 3. In comparison to point 2: Once backlight logic is inverted by kernel boot
> parameter acpi_backlight=vendor i915.invert_brightness=1, slider at max
> still respects minimum brightness level 10% defined in VBT.

Err, what? Slider set at max should produce max brightness instead of darkness,
right? Slider set at min should produce min brightness.

I'm at a loss with what you're trying to accomplish.

Perhaps we should take the GUI sliders out of the equation, and use
/sys/class/backlight/intel_backlight/ directly.

This should set the backlight to minimum brightness:

# echo 0 > brightness

This should set the backlight to maximum brightness:

# echo $(cat max_brightness) > brightness

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