dim screen after kernel upgrade

[bsammon]

So it appears there's a problem when upgrading from older kernels to something around 5.5 or later.  The symptom is that when you boot the new kernel, the screen is very dim, possibly appearing completely black.
The problem seems to be that with the older kernels, screen brightness was on a 8-bit 0-to-255 scale, and the newer kernels use a 12-bit 0-to-4095 scale.  So the reasonably bright "224" (for example) brightness that you had with the old kernel becomes a rather dim "224" with the new kernel.

It seems that this issue occurs with a few different distributions' kernel packages.  I'm not sure if any distributions have any fixes (maybe a transition script) built into their kernel packages.

My current thinking is that the best approach (for individual users) is to verify that your Fn-F1 and Fn-F2 are working for adjusting your screen brightness before upgrading the kernel.  Then when you boot your new kernel and you can't read the screen you can use them to turn up the backlight.  I neglected to do this.

A hackish approach could be to poke at the files in /sys/class/backlight/edp-backlight/ (you could do this via SSH if you can't adjust brightness via the keyboard/trackpad/screen)

[bsammon]

I noticed while booting the new kernel that it starts out at a reasonable brightness, but at some point, around a second in, it switches to the lower brightness level.  My wild guess is that the kernel starts out with a compiled-in reasonable default, and then at some point in the bootup, it reads a config file from the filesystem and changes the brightness based on that.  (The downside to solid state storage is that this all scrolls by too fast for me to read what is printed when it changes the brightness).

I'm also wondering if this change affects intel-based laptops as well, or if it is limited to Arm64 laptops.  Or just the pinebook pro?

[xmixahlx]

i have replied a few times to this issue over the last few weeks. see danielt's thread. i solved this by:

login to a tty
brightnessctl s 100%
done

[bsammon]

"brightnessctl" is not installed on either of the installs on my PBP.  Guess it's not part of the default install.

However, it's easy enough to find as the package is called "brightnessctl" (at least in Debian).

[xmixahlx]

yes i use brightness-udev, brightnessctl and light on mainline kernel with good results.

[xyzzy]

With the default Manjaro install the PBP now comes with, "brightnessctl" isn't installed.

What was causing the dim screen for me was systemd-brightness.  It's a service that runs on startup and shutdown to restore and save the screen brightness.  In my case the save brightness was 0, so this happened:

[root@pine systemd]# systemctl status systemd-backlight@backlight:edp-backlight.service
● systemd-backlight@backlight:edp-backlight.service - Load/Save Screen Backlight Brightness of backlight:edp-backlight
    Loaded: loaded (/usr/lib/systemd/system/systemd-backlight@.service; static)
    Active: active (exited) since Tue 2020-11-17 15:16:57 PST; 3h 12min ago
      Docs: man:systemd-backlight@.service(8)
  Main PID: 614 (code=exited, status=0/SUCCESS)
     Tasks: 0 (limit: 4462)
    Memory: 0B
    CGroup: /system.slice/system-systemd\x2dbacklight.slice/systemd-backlight@backlight:edp-backlight.service

Nov 17 15:16:57 pine systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:edp-backlight...
Nov 17 15:16:57 pine systemd-backlight[614]: edp-backlight: Saved brightness 0 too low; increasing to 204.
Nov 17 15:16:57 pine systemd[1]: Finished Load/Save Screen Backlight Brightness of backlight:edp-backlight.

The save valued is 0, which is too low, so it raises it to 5%.  Which is also really low.  The saved value can be changed by putting a new value in "/var/lib/systemd/backlight/platform-edp-backlight:backlight:edp-backlight".

I'm not sure how the value of 0 got saved in the first place though.  I wonder if the process was something like: close lib, screen turns off, battery low, shutdown, save brightness since screen is off.