[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 #5 from Szymon <szymon.balcerak at ts.fujitsu.com> ---
Today I performed another tests a here are the results:

1. Confirmed that Linux does not take VBT backlight active low bit into
consideration. If this is the bug that is to be decided by Linux graphic driver
developers. NOTE: Windows graphic driver takes this setting into consideration.

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.

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