On my BIGTREETECH E3EZ i switched from the CB1 to the Raspberry PI CM4. Unfortunately on every boot the following message appeared:

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brcm-pcie fd500000.pcie: link down

The issue is the missing PCIe Port, hence you have to disable the feature to get rid of this message. Create the file disable-pcie-overlay.dts with the following command:

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wget https://gist.githubusercontent.com/SirUli/de94ddb751b91adfa6809f286a18a382/raw/af010907360153f74edc8f65d415d717378bf037/gistfile1.txt -O /tmp/disable-pcie-overlay.dts

You must then convert the file using the following command:

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sudo dtc -@ -I dts -O dtb -o /boot/overlays/disable-pcie-overlay.dtbo /tmp/disable-pcie-overlay.dts

Then delete the file with the following command:

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rm /tmp/disable-pcie-overlay.dts

Finally open the boot config file using the following command:

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sudo nano /boot/config.txt

And add the following line at the top:

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dtoverlay=disable-pcie-overlay

Then save using CTRL+X which will ask you to save. Enter Y and ENTER to save. On the next reboot that message from above will be gone.