New PWM cooling fan overlay for Pi 3/4

[herbingk]

This is my 3rd attempt, to make a pull request on the topic. I got stuck with your workflow scripts, that always checked older commits and failed with errors, so I started over again. Please bear with me, as I am entirely new to github and still on a learning curve. I you have an advice how to fix workflow errors with newer commits, without the need to start all over again, I greatly appreciate to learn.

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.

[peterharperuk]

I would suggest waiting for feedback rather than trying with new merge requests. The RPI engineers will advise if changes are required

[pelwell]

I've not tested this, but it looks reasonable and the auto-build checks are happy.

When you are making changes to a PR, I suggest the following:

1. Add new commits that fix the problems.
2. Verify that the problem is solved, ignoring checkpatch warnings about the original commits.
3. Use git rebase -i HEAD~n to squash the fixups into the original commits.
4. Use git push -f ... to force push the new commits to your branch, which will update the PR.