2020–09–11: Ways to help improve Pinephone kernel

There are a bunch of open questions about Pinephone's HW. When answered, these could help improve the kernel behavior. Here's one of them.

Minimal display brightness is too bright

This gets asked quite a bit, but just maybe 2 people helped do anything about it, so here's a more detailed guide on how to help.

Sadly fixing this issue requires gathering a lot more samples than 2, regardless of whether your Pinephone has the problem or doesn't. Trouble is that if only people who have this issue do the following test, the results will be useless. Even people who's backlight seems to work correctly need to take part. So it's a hard problem to solve. :)

If enough people report at what brightness level the backlight turns on on their Pinephone, we could improve the default minimum brighness level, so that most users can reduce their Pinephone screen brightness to lower levels.

There's an open question on how to set the backlight brightness values on post 1.0 revision phones, since lower PWM duty cycles lead to backlight being basically off. It would be nice if more people can test the various backlight levels on 1.1 and 1.2 revision with this change in dts in the backlight node:

/backlight {
        brightness-levels = <0 1000>;
        num-interpolated-steps = <1000>;
};

The above change can be made by fdtput, which is part of dtc package:

fdtput -t u your.dtb /backlight brightness-levels 0 1000
fdtput -t u your.dtb /backlight num-interpolated-steps 1000
fdtput -t u your.dtb /backlight default-brightness-level 500

You don't need to recompile the kernel or anything. Just identify what dtb file is your OS using (it will be in /boot directory) backup it up and apply the above commands to the original (just replace yout.dtb with the name of your dtb file).

This change will make it so that PWM backlight driver sets up 1000 linear backlight steps, so you can enter values 1–1000 to /sys/class/backlight/backlight/brightness and these are directly mapped to PWM duty cycle. So level 1 is 0.1% level 100 is 10%, and so on.

What's improtant is to report:

lowest level at which the backlight still works and the screen doesn't flicker too much (please test in dark environment)
what Pinephone varian this was tested on