Introduction

I’ve got an unused Raspberry Pi Compute Module 4, that I’ve tried to flash with HomeAssistant OS. Since it’s a Pi with eMMC, I’ve had to some extra steps to get it writeable for flashing. Once I had it flashed with the OS, I’ve wanted to configure SSH using the suggested method: take a FAT32 formatted USB stick with your authorized_keys file on it and reboot. The OS should configure itself, but never saw my USB stick. A USB SSD showed a power light, but did also not work. So what’s the problem?

Waveshare CM4-IO-BASE-A and it’s USB ports

I’m using a Waveshare CM4-IO-BASE-A enclosure and board for the CM4. The wiki explains in a note:

9: USB2.0 is closed by default, if you need to open it, you need to add dtoverlay=dwc2,dr_mode=host.

Ok, the USB ports are disabled and doesn’t work without further configuration. This configuration needs to be done in the config.txt that every Raspberry Pi uses to control certain hardware features on startup. This file is normally placed in the boot partition of every installation. On a microSD Card I would have just pulled the card and placed it in a card reader, but I’ll have to go a different route.

Making the eMMC readable on macOS

My CM4 uses a soldered eMMC as memory. To be able to flash it, I’ll have to switch the BOOT selection to ON. On the Mac, I’ll followed Jeff Geerling’s instructions for flashing an eMMC.

Making changes to the config.txt

So how I can access the config.txt stored in the boot partition of the eMMC? I’ve started the pi from USB and made its drive readable for the Mac. I’ve tried diskutil list and found a DOS formatted partition on the Pi. This should be the boot partition. This blog explains it in more detail. The essential parts are:

mkdir /user/drive
# Create area to mount FAT partition 
mount -t msdos /dev/disk2s1 /user/drive

In /user/drive the complete content of the boot partition was readable. Open the config.txt and add these parameters in the all section, to enable the USB ports:

[all]
dtoverlay=dwc2,dr_mode=host

Save the file, unmount the partition and shut the Pi down. Switch the BOOT selection to OFF and reboot with the prepared USB stick.

Home Assistant imports the SSH key and a connection was finally possible.

Conclusion

Always read the documentation properly. I did not know, that I have to manually enable the USB ports, as a regular sized Pi doesn’t need this extra configuration. Now I’m able to use it like a regular one with all its provided ports.