[Bug 108225] 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 #12 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to Basil Eric Rabi from comment #11)
> How can I do a modeset?

Try 'xrandr --output eDP-1 --off --output eDP-1 --auto'

If that helps when the bug is on, it would really be helpful if you could
provide the output of the following 1) after boot, 2) after running the above.

# intel_reg read 0xc8250 0xc2004 0xc8254 0x61250 0x61254

intel_reg is a tool in the intel-gpu-tools, or IGT.

I suspect the BIOS or GOP sets up different backlight settings from what i915
does, and after fastboot the backlight adjustment writes have no effect.

(Note to my future self and other developers: My hunch says GOP does not set
BLM_PCH_OVERRIDE_ENABLE and uses the CPU register to adjust backlight, while
i915 uses PCH override and uses the PCH register to adjust backlight. Without
the override, the PCH register does not have effect. PCH override is the
default after Lynxpoint, so we use that mode also for LPT.)

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