Fluctuating brightness on kernel 6.4.9

After rebooting after installing some updates, I noticed that the display seemed to be gently fluctuating in brightness. I’m using a full-tower desktop computer connected to an LG TV; most results I found when searching for this issue online are related to laptops and being unable to adjust heir backlights, that’s not the issue going on here.

It seems like it’s not the backlights of the physical hardware that’s fluctuating (I can see the light bleed increase when the backlight warms up, it is not doing that here), but the brightness of the signal itself. Seems to be random, but consistently constantly random. It flickers by a slight amount, only noticeable when viewing bright, still images on the screen.

I rebooted again, this time holding Ctrl to access the GRUB menu to load an older kernel, and the constant brightness fluctuating no longer occurred, however there seems to still be some flickering only when objects on the screen are moving. So far I know this:

  • Kernel-6.4.9: Flickering
  • Kernel-6.4.7: Some flickering, mainly when moving objects are on the screen

By using the command sudo dnf versionlock add kernel-6.4.7-200.fc38 I’m keeping the older kernel installed and preventing auto-updates from knocking it off the list. I’ll keep this post updated if I find out anything more, and may make a bug report once I’m confident this is reproducible.

I’ve found the cause of the issue: adaptive sync. The newer kernel had only increased the frequency of the symptom, but it wasn’t the direct cause.

I was initially confused as disabling adaptive sync from Fedora’s system settings was one of the first things I tried, and that didn’t fix the issue. Only once I tried the TV’s settings (where I disabled freesync globally) it had stopped.

In summary, try troubleshooting all devices in the chain.