hathach/cm4_netboot.md

We will need an LAN cable to CM4 and PC running tftpd-hpa for netbooting. Basically we will need to

Modify bootloader of CM4 to set TFTP_IP to our PC
Flash the new config
Install tftpd and change directory to our boot folder

1. Modify CM4 bootloader config

full doc can be found here https://www.raspberrypi.com/documentation/computers/compute-module.html#compute-module-4-bootloader . here is the summary

git clone https://github.com/raspberrypi/usbboot.git
then go into newly cloned usbboot folder and run make to compile.
then go to recovery folder to edit boot.conf to include TFTP_IP to your PC local IP, BOOT_UART=1 is nice to eanble as well. Here is mine

[all]
BOOT_UART=1
WAKE_ON_GPIO=1
POWER_OFF_ON_HALT=0

# Try  SD -> Network ->  Loop
BOOT_ORDER=0xf21

# Set to 0 to prevent bootloader updates from USB/Network boot
# For remote units EEPROM hardware write protection should be used.
ENABLE_SELF_UPDATE=1

TFTP_IP=192.168.0.117

run ./update-pieeprom.sh which is helper script to generate binary and signature for bootloader config

2. Flash the CM4 bootloader config

Insert Jumpber to one that says Fit Jumper to disable eMMC Boot as shown in the picture
plug usb micro usb cable from CM4IO to your pc and power on the cm4, it will boot as usb device with HID interface.
from usbboot folder run sudo ./rpiboot -d recover, it will then flash the new boot config to the board.
Then you are done with the CM4, remove the jumper, remove SD card, plug your LAN cable.

3. Install and Setup TFTP Server

Note this is for ubuntu 20.04, your PC can be different. Detailed install can be found here https://linuxhint.com/install_tftp_server_ubuntu/

sudo apt install tftpd-hpa
then edit its config sudo nano /etc/default/tftpd-hpa to change the TFTP_DIRECTORY to your preferred folder that hold all the boot files for CM4 (all the files in boot partition in SD card).
restart server with sudo systemctl restart tftpd-hpa. You could test the server using client from tft-hpa

You are all set, plug the LAN to CM4 without SD card, then it should try to boot with SD, then fall back to network. Just make sure kernel8.img is copied to your tft server directory when compiling.

TIPs: CM4 does not have power button, use an AC extension cord with on/off button to quickly cycle power on/off to test it out (not sure if this could cause any electrical issue with cpu).