Set PWM mode and frequency for quieter fan

[agspoon]

Using an oscilloscope and my sensitive hearing ;), I achieved much lower noise and slower minimum speed, by setting the PWM mode and frequency to "Mark-Space" and 25KHz respectively. This is with 30mm & 40mm fans mounted on a RPi-4b Hat.

The wiringPi library defaults to "Balanced Mode", and a resulting high frequency signal at the PWM controller input. By changing the frequency close to the more standard 25KHz, and using a fixed square-wave, the resulting noise artifacts are considerably reduced. I am also able to achieve very low minimum fan rotation speeds under these conditions.

Note, this should be a backwards compatible change with any existing configuration in /etc/conf.d/kvmd-fan, as the default 1024 Range is not changed here. Any user that has specified a different value for --pwm-high, was not changing the actual signal.

I'm using this - https://www.tindie.com/products/jeremycook/ez-fan2-tiny-raspberry-pi-fan-controller/
as my controller interface between the RPi GPIO PWM signal, and the two-wire fan.

I'm running kvmd-fan on a PiKVM clone (Geekworm PiKVM-A3), but I believe this change would be applicable to any small fan PWM controller.

[mdevaev]

Hello. It's interesting. I'll check. But if it doesn't make improvements on official V3 and V4, I won't accept the changes.

[agspoon]

Certainly. I was sitting on a similar PR to #2, but was not happy that it required so much re-work and broke backwards compatibility. Then I realized it wasn't necessary to hit exactly 25KHz, it just needed to be in the ballpark. This allowed me to keep the 1024 step Range, and thus backwards compatibility with both the code base and people's existing configuration parameters.

Given it a listen in a quite space, and if it works better great.