Add CM4 supporting for kernel 4.19.y (back porting)

[micronrobot]

Hello,
Due to personal reason, I must use kernel 4.19.y for my system with CM4 module. I have tried to add CM4 supporting to kernel 4.19.y by creating the bcm2711-rpi-cm4.dts file from bcm2711-rpi-4-b.dts file. I've patched my bcm2711-rpi-cm4.dts file based on the differences between bcm2711-rpi-cm4.dts and bcm2711-rpi-4-b.dts in kernel 5.4.y. My CM4 system can boot with new bcm2711-rpi-cm4.dtb file but I pcie and usb doesn't work.
Does anyone have tip what I should do more to get CM4 fully work in kernel 4.19.y?
Thank you.

[wirelessOJ]

I am trying to do the same. I need to use kernel 4.19y, but need PCIe to work. Did you ever manage to get this to work?

[JamesH65]

It's likely this will never be possible without a lot of effort. You should find out why you need such an ancient kernel (2018 release, although EOL wasn't until Dec 2024) . Fixing that would be a better use of time.

[wirelessOJ]

I am trying to use a patch from the WiFi PTP project repo. It is an ath9k kernel patch designed specifically for kernel 4.19. It seemed like it might have been simpler at first to modify the kernel to enable PCIe rather than trying to modify the patch. But perhaps I will look into modifying the patch for a newer kernel. Thank you.

[popcornmix]

But perhaps I will look into modifying the patch for a newer kernel

That sounds like a much more useful solution.

I'll close this as we'll obviously not be backporting anything to 4.19 kernel.

[pelwell]

Although this forum isn't meant for general support, if you do get stuck with the port we may be able to help.

[wirelessOJ]

It seems that I might not be backporting as some people think it is not possible to enable PCIe this way. Do you have other thoughts on this?

[pelwell]

If you can do it on 6.12 then you can do it on 4.19, but you may have to back-port so many commits that you end up with a kernel looking suspiciously like 6.12.

[wirelessOJ]

hmmmm. I am no expert so I am not sure if it is worth to try that or to modify the kernel patch designed for 4.19 to be adapted for a later kernel

[pelwell]

You should definitely try the forward-port to 6.12.

[wirelessOJ]

what do you mean if you can do "it" on 6.12. you can do "it" on 4.19. what is "it"?

[6by9]

Post a link to this atk9k patch that you're trying to use.

If https://github.com/zlab-pub/wifi-ptp/blob/main/ath9k.diff, then it looks relatively simple, but will depend on how much churn there has been to the underlying driver.
Sticking on 4.19 forever isn't a viable solution.

[wirelessOJ]

yes it is that one. I agree, I would love to be able to update it for newer kernel.

[6by9]

Almost all of that forward ports with minimal intervention.

The only bit that doesn't is the adjfreq hook has been merged into a adjfine, and been removed with 75ab70e.
There are a number of other conversions from adjfreq to adjfine for various cards (d8fae25, 5a55423, ccd3bf9, 3626a69, etc), but I know nothing about that API or hardware to be immediately able to do the conversion.

I've thrown the basic forward port to 6.12 at #6882, but I am NOT proposing that this would be merged into our tree.
The function of adjfine has been disabled with #if 0, so needs fixing up in order to get PTP to work.

The first test would be to see if that works at all without PTP to confirm that no other parts are broken. If everything appears to be working, then someone with a little more knowledge or time could look into the implementation of that function.
You might ask via an issue on https://github.com/zlab-pub/wifi-ptp/issues in the first case, although the repo README also lists two email contacts for questions.