Re: POE Hat and dtoverlays in Debian 12

[Sebastian Kuzminsky]

On 7/13/23 13:53, Markus Falb wrote:

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


Most of the Raspberry Pi overlays are not in the mainline Linux kernel, 
they're only in the Raspberry Pi fork of the Linux kernel which you 
pointed to above.


Furthermore, I believe that the overlays in the Raspberry Pi kernel 
apply cleanly to the base device tree that ships with the Raspberry Pi 
kernel, but not to the base device tree in the mainline Linux kernel (or 
to the one in the Debian kernel).


Ubuntu is shipping the Raspberry Pi fork of the kernel (or something 
very close to it), including all their overlays.  Debian on the other 
hand ships a closer-to-mainline kernel which does not include the 
Raspberry Pi overlays (or a bunch of their kernel patches that make it 
easier to work with overlays at runtime).


This really limits the functionality of the Raspberry Pi running Debian.

I ran in to this problem when trying to enable SPI on the Raspberry Pi, 
which (on Raspberry Pi OS and Ubuntu) is done with a device tree overlay.



--
Sebastian Kuzminsky

POE Hat and dtoverlays in Debian 12

[Markus Falb]

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