New PWM cooling fan overlay for Pi 3/4

[herbingk]

I recently discovered that there is a new kernel based software PWM solution available since November 2024. A software-based PWM kernel module is available since then, back-ported from the Linux kernel 6.11 to the Raspberry Pi OS Bookworm kernel 6.6.62. This made me write a new pwm-gpio-fan overlay for my own use and after extensive testing on a Pi 3, I have published it for the community.

Moderator 6by9 on the Raspberry Pi forum suggested that I make a pull request, so that it gets merged for all Pi users.

Using the new kernel-based software PWM imposes several advantages over other implementations I have found on the internet and this forum:

• Compared with the existing gpio-fan overlay, the fan runs much quieter, often not even noticeable, due to the use of real PWM duty cycles.
• No hardware PWM (PWM0 or PWM1) of the Pi is occupied, so there is no conflict with the Pi's analogue audio output anymore.
• Reliable on even high CPU loads, as it's part of the kernel.
• Doesn't consume noticeable CPU power even on a Pi 3.