[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 #26 from Tolga Cakir <cevelnet at gmail.com> ---
(In reply to Jani Nikula from comment #22)
> Do you have LynxPoint (LPT) PCH? Look for something like this in the dmesg:
> 
> [    5.224781] [drm:intel_pch_type [i915]] Found LynxPoint PCH
> 
> Does the trick from comment #12 help? What's the output of the intel_reg
> reads in the two scenarios? (See comment #13 for example.)

My platform is Lynx Point (Intel HM87 chipset, Intel Core i7-4702HQ),
eventhough I don't see that specific line. I've done the tests on an Arch Linux
system, kernel 4.19.2.

After running the mentioned xrandr commands, my screen turns off for a second,
comes back on and backlight control works again. My output intel_reg read
output:

Fresh boot w/ fastboot (non-working backlight control):
BLC_PWM_PCH_CTL1 (0x000c8250): 0x80000000 (enable 1, override 0, inverted
polarity 0)
            (0x000c2004): 0x00000004
BLC_PWM_PCH_CTL2 (0x000c8254): 0x131203b1 (freq 4882, cycle 945)
            (0x00061250): 0x00000000
            (0x00061254): 0x00000000

After running xrandr command (backlight control working):
BLC_PWM_PCH_CTL1 (0x000c8250): 0xc0000000 (enable 1, override 1, inverted
polarity 0)
            (0x000c2004): 0x00000004
BLC_PWM_PCH_CTL2 (0x000c8254): 0x131203b1 (freq 4882, cycle 945)
            (0x00061250): 0x00000000
            (0x00061254): 0x00000000

@Maarten Lankhorst: Thanks for the patches in comment #23 #24 #25! I'm setting
up a testing environment and will report back asap.

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