1. Firmware Flashing

The board that I gave you comes with 32gb of onboard eMMC storage. This is both a good and bad thing. The bad part is that it is a pain in the ass to flash and you cannot boot from an SD card.

The good part is that OpenWRT is an extremely good steward of whatever medium it’s installed on - it operates almost entirely in memory, the filesystem is mounted read only by default, and it’s only a couple hundred megs all said and done - so the storage should last a looooooong time.

In the previous step you flipped a DIP switch to enable rpiboot. This mounts the eMMC as a block device and exposes it over USB. To switch the CM4 into this mode, plug a USB-C cable into the port on the carrier board labelled “USB” (not the power one) and your computer. After that, you’ll need to build and execute the rpiboot program.

The repository (with build instructions) is here: https://github.com/raspberrypi/usbboot - it’s a pretty standard “install dependency, run make, get executable” ordeal. You’ll need to run it with root privileges, but then the CM4 will show up in your block devices.

Download the latest stable version of the OpenWRT image for a CM4 from their website - https://firmware-selector.openwrt.org/?version=21.02.0&target=bcm27xx%2Fbcm2711&id=rpi-4 - making sure you get the “Factory (ext4)” variant.

Flash the image to it using dd or rpi-imager or however you prefer to do this sort of thing, then ensure the device is unmounted before removing the cable. Flip the DIP switch for rpiboot back to the “off” position and remove the cable from the carrier board.

Up next, you’ll need to enable and install the drivers for the secondary ethernet controller.