[Bug 108225] [LPT SPT KBP] Acer V3-772G screen brightness cannot be adjusted with i915.fastboot enabled

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108225

--- Comment #28 from Tolga Cakir <cevelnet at gmail.com> ---
(In reply to Maarten Lankhorst from comment #25)
> Created attachment 142472 [details] [review]
> (hopefully) Flicker free version of switching backlight method.
> 
> Can you test if this works? If not the previous patch should work.

I have tested the patch in comment #25 on Arch Linux and patched against
drm-intel-next branch.

My observations: when I power on my laptop, the default brightness is set
around 50% brightness. When i915 gets loaded, the brightness level changes to
my user value (saved during shutdown, restored on next boot). The brightness
level changes without any form of smoothing, but there is no flickering. It
changes strict from e.g. 50% to 75%, without the backlight turning off. After
the boot process, backlight controls fully work!

However, the flickering is not 100% gone, eventhough it's much better than
without fastboot=1. There is a very short screen content blanking, but the
backlight stays on. The contents of the screen blank for around ~1/10th of a
second and resume where they stopped.

Eventhough I still notice a very brief screen flickering, the result is
superior to fastboot=0.

I'm testing comment #24 next and will report back.

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