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

[bugzilla-daemon at freedesktop.org]

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

--- Comment #9 from Jeffery Miller <jefferym at gmail.com> ---
I tried a few more brightness tests with the nvidia binary blob. There's a
similar mmiotrace already attached.

I've observed that the nvidia driver sets the values of 
I observed that the nvidia driver sets both 0x0061c080 and 0x0061c084 on each
brightness change.
It always sets 0x0061c080 to 0x17 and 61c084 with the following
percent values:

% setting    |  0x0061c084 poke
---
21              0x800000d8
22              0x800000e2
30              0x80000134
50              0x80000201
70              0x800002ce
78              0x80000320
100             0x80000401

I noticed that if I change value to 21% (d8) or below the screen is dark/black.
It then stays dark at levels such as 30%, 50%, and such that worked previously.
If I set it to 78% or 100% the screen will appear again. Once it reappears the
50%, 30% values work as they did previously.

I patched the nouveau backlight code to always set the PWM_DIV to 0x17. The
nouveau driver then behaved the same for the various percent values mentioned
above. It also had the off at low requiring a high % to have it show again.

I reverted to my original patch which would always set the DIV to 0x1 on each
change. This makes it consistent with the fresh boot value. This has the side
effect that after a post (and a resume from suspend) the value is always the
same.
The visible range is wider for the 0x1 case. The screen doesn't turn fully dark
until around 14%. Additionally it would return to the previous value without
having to jump all the way up to a high value. It could go from the 14% off
case back to 50% and the screen had the same brightness. I am observing the
brightness with my eye.
The percent to DUTY value being set to 0x61c084 is similar in each driver.
Nouveau sets 0x401 for 100%, 0x200 for 50%.

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