Hardware workflow

The repeatable loop for Raspberry Pi 4 testing: build the image, stage it safely, watch UART, keep recovery available, and know which medium will boot next.

Preflight

Before rebooting into bare metal, check the active boot source. If EEPROM BOOT_ORDER prefers USB before SD and a recovery USB device is attached, the board may boot Linux from USB instead of the SD image you just staged.

lsblk -o NAME,TRAN,RM,SIZE,MOUNTPOINTS
sudo rpi-eeprom-config | grep BOOT_ORDER

Build and stage

Examples produce boot/kernel8.img or install/boot/kernel8.img. Stage the image and matching config.txt from Linux, then install them onto the active FAT boot partition. Preserve known-good Linux copies so recovery remains deterministic.

UART first

UART is the acceptance channel. A hardware test should capture the boot banner, command prompt, and the command output relevant to the driver under test. For UART RX itself, test fast command bursts, because slow manual typing can hide FIFO overflow bugs.

Returning to Linux

The preferred path is a monitor command that rewrites the active boot config for Linux handoff and reboots. If the monitor is not alive, use recovery media or mount the SD card elsewhere and restore config.txt.linux.bak plus kernel8.img.linux.bak.