Panasonic FZ-40 Mk2 backlight issues

Ask Fedora
1

So Fedora 44 KDE Plasma Edition on a Panasonic FZ-40 Mk2. I figured, it’s RHEL certified, so let’s give Fedora a go. I mean after all, it’s more usable than Windows 11 and offers a lot more customization and functional options. Naturally not all features seem to work (eg the EM7690 cellular modem and cannot change the color on the RGB keyboard), but the backlight is a concern.

The LCD backlight doesn’t change brightness until I sleep the laptop and then resume it. Then it works OK, except 0% brightness is not as dim as it gets (it resembles 5%). It also feels like full brightness isn’t as bright as Windows set to 100% brightness.

Following kernel command lines have the following results:

  • no “acpi_backlight” option → works as per above.
  • acpi_backlight=none → cannot change brightness.
  • acpi_backlight=native → works like acpi_backlight=none.
  • acpi_backlight=vendor → cannot change brightness.
  • acpi_backlight=video → can only change brightness after logging in, without sleep/wake up, however cannot attain more than about 10% of the brightness (adjusting beyond 35% reveals no changes - although 35% is slightly dimmer than 30%). It uses the actual full lower range 0%-10% though.

I am not sure where I should start diagnosing this, although Fedora and RHEL 9/10 are not exactly the same, surely there is some way to get stuff like this working on a unit which claims to be RHEL certified.

2

Is it the function buttons that don’t work?

You can also install a software such as light that can change the brightness via command line. Worth a try, it often works.

light can also interact with the keyboard backlight in some cases.

3

Yes, Fn+F1 (decrease) and Fn+F2 (increase). Those keys do work and the OSD displays. Running light as suggested the brightness reflects what the OSD does.

However the screen’s actual brightness doesn’t change (or behaves abnormally as per OP).

4

An update on this one: it seems the issue behind the backlight appears to have sort of resolved itself, although it seems there is some PWM driver needed to get to the very low portion of the range. In particular, the issue reoccurs if the backlight is set very low during system startup.
I switched to the new Xe driver (actually more to see if I can improve Fedora’s power usage, the battery life is abysmal on Fedora KDE stock install), the UI feels a bit smoother but it could be psychological.

Where should I start if I were to look for what PWM device is controlling the backlight at the lower end of the range, so I could work out a patch or driver?