[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 #11 from Szymon <szymon.balcerak at ts.fujitsu.com> ---
(In reply to Jani Nikula from comment #10)
> 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

Sorry if I confused you by my description. Let me explain once again:
1. "Slider set at max should produce max brightness instead of darkness. Slider
set at min should produce min brightness." - this general rule is of course
true and expected.
2. However our board has a possibility to attach different LVDS panels. By
different I mean especially different brightness polarity. I.e. 100% PWM might
mean max brightness for LVDS with "normal" polarity and 100%PWM might mean min
brightness for LVDS with 'inverted' polarity.
3. To still satisfy general rule from point 1, we introduced BIOS SETUP setting
which allows user to change LVDS PWM logic (normal/inverted).
4. So now user having LVDS with "normal" polarity should use "normal" setting
and user with LVDS with "inverted" polarity should use "normal" setting.

To perform my testes with Linux I have LVDS with normal polarity, but set
"inverted" setting in SETUP (BIOS wrote 0xC8250 MMIO register with bit 29),
that is why I received less brightness with higher slider position and more
brightness with lower slider position. I hope now it's clearer to you.

Having this setup I received problems as in comment 5.

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