POE Hat and dtoverlays in Debian 12
Hi, it’s Markus.
Some time ago there was a thread on this list about POE Hat in bullseye.
The outcome of this was the assumption that CONFIG_PWM_RASPBERRYPI_POE kernel module is necessary to have the fan working but in the end it was never discussed what is really necessary to have the fan of the POE Hat working.
I stumbled upon this as I have raspberry’s with POE Hats and I had the idea of putting debian on one of them. I soon realized that the fan was not working, I did some research on the topic, tried some things and I still have the fan not working.
What I tried is basically getting a
...
dtoverlay=rpi-poe
...
into /boot/firmware/config.txt
This led to the question where the overlays are located. I had a look at a working Ubuntu 20 and there is ls /boot/firmware/overlays/rpi-poe.dtbo. Where to find the overlays in Debian, if at all?
Because I could not find the overlays (maybe they are not shipped?) I compiled it myself
https://github.com/raspberrypi/linux/blob/1.20230405/arch/arm/boot/dts/overlays/rpi-poe-overlay.dts
There are warnings which I ignore for now. I put the overlay at some of the possible locations
/boot/firmware/rpi-poe.dtbo
/boot/firmware/overlays/rpi-poe.dtbo
/boot/overlays/rpi-poe.dtbo
but again I do not really know where to put it. I do not yet have a way to debug that properly. I tried
...
dtdebug=1
...
in config.txt but I don’t notice effect. Ultimately, I would love to see log entries that shows where the firmware looks for overlays and where it find files and which files it loads and so on.
I also put
pwm-raspberrypi-poe
pwm-fan
into the initramfs. The modules are loaded into the kernel but I do not see the Hat in the loaded devicetree.
In the end I try to load an overlay but I does not seem to have effect.
I think it boils down to the following questions
How do I load a dtoverlay into the kernel
How do I debug loading of dtoverlays
And last but not least, of course there could be fundamental flaws in the approach, I am by no means a kernel or boot sequence expert. General hints and help will be very welcomed and appreciated.
—
Best Regards, Markus
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