what works:
network booting lk.elf on an rpi4 (just rename to start4.elf and host over tftp)
the full console with line editing and history
serial over pl011 (but no serial init, relies on previous bootloader stage)
reboot command
irq handling for timer compare and uart
one-shot timer
reporting cpu exceptions such as divide by zero
some parts are copy/pasted from https://github.com/cleverca22/rpi-open-firmware which itself is a fork of https://github.com/christinaa/rpi-open-firmware
what works: network booting lk.elf on an rpi4 (just rename to start4.elf and host over tftp) the full console with line editing and history serial over pl011 (but no serial init, relies on previous bootloader stage) reboot command irq handling for timer compare and uart one-shot timer reporting cpu exceptions such as divide by zero