Created attachment 125102 [details]
capture 1

SUMMARY
Some screens dimmed after a set period to do not return to original brightness level.

I have a Dell Inc. Precision 7710 with four external monitors configured as follows: [Dell Laptop monitor][Dell P2412H DP-1][Dell P2417H DP-2][Dell P2417H DP-0][Dell P2412HVGA-0]

- The first thing to note is that the brightness levels of the monitors vary for the different part numbers so a universal Screen brightness level does not work well at all as sometime I want to dim some monitors individually.

- I'd suggest moving the screen brightness control to the Display and Monitor KCM where brightness levels can be associated per display with a disable option for broken controls.

STEPS TO REPRODUCE
1. Open System Settings and navigate to Power Management KCM
2. Set Screen brightness as desired
3. Enable Dim screen
4. Apply configuration changes.
5. Wait for screens to dim.
6. Move mouse or press keyboard key to wake.

OBSERVED RESULT
After some duration that I did not time, all screens dim. When move the mouse, or press a keyboard key, or switch to a virtual console and back, the brightness level of some combination of external screens is not restored. They remain at the dim levels.

- I get a few [260529.831841] i2c i2c-2: sendbytes: NAK bailout. kernel errors when the external displays and if I keep adjusting the screen brightness eventually I get all displays at the desired brightness level. Perhaps the restore code could check the results of the I2C request via appropriate API
and try a few times before giving up. On giving up, the control for individual display would be disabled for a failure. I have not noticed this before because the external monitors used not be dimmed.

EXPECTED RESULT
Brightness level of all screens is restored.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.17.80 master
KDE Frameworks Version: 5.66.0 master
Qt Version: 5.14.0

ADDITIONAL INFORMATION