[Bug 98677] New: [NVAC] iMac9, 1 effective backlight brightness range changes after S3

[bugzilla-daemon-CC+yJ3UmIYqDUpFQwHEjaQ@public.gmane.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=98677

            Bug ID: 98677
           Summary: [NVAC] iMac9,1 effective backlight brightness range
                    changes after S3
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/nouveau
          Assignee: nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org
          Reporter: jefferym-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
        QA Contact: xorg-team-go0+a7rfsptAfugRpC6u6w@public.gmane.org

Created attachment 127900
  --> https://bugs.freedesktop.org/attachment.cgi?id=127900&action=edit
dmesg output after the resume with some nouveau trace options

I have an iMac 9,1 where the behaviour of the backlight of the built in screen
works differently between the first boot and after a suspend to ram.
It appears to be darker at lower values before a suspend than after.

kernel: 4.8.6-300.fc25.x86_64
lspci adapter: VGA adapter NVIDIA Corporation C79 [GeForce 9400] [10de:0867]
DMI: Apple Inc. iMac9,1/Mac-F2218EA9
I've been booting in EFI mode.
I do not know of a regression. I've only tried 4.4 and this 4.8 kernels.
I've observed this with X running but I've been testing this without starting a
graphical environment.

I've been testing by setting setting backlight values via the
/sys/backlight/nv_backlight/brightness interface.
Before a suspend I can set values such as 40, 70, 100 and the brightness seems
to be reasonable on the screen.

I then suspend to ram via the /sys/power/state interface.
Upon resuming the backlight appears to be at 100 percent. If I set the
brightness to values such as 30 and 60 the screen remains dark. It is barely
visible as a flicker at 70. At 100 it is visible.

I've observed that the value of NV50_PDISP_SOR_PWM_DIV (0x0061c080) is 0x1
after a boot but is set to 0x84 after a resume.
It appears there's an init script in the bios dump which sets it to the value
of 0x84. I have confirmed if I use the nouveau option
nouveau.config=NvForcePost=1 that the behavior after booting is the same as
after a resume. The value of 0x61c080 is 0x84.

If I `nvapoke 0x061c080 0x1` after a resume the backlight seems to work fairly
well. I created patch to test this. I expect it isn't the proper solution.

I'm attaching the dmesg which includes a resume with the nouveau driver.
I'll also attach the vbios dump, acpidump, dmidecode, lspci, 

I've gathered an mmiotrace of the binary driver as it sets a few brightness
values as well.

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