Serial debugging/Serial Gadget
For devices where getting a physical UART cable is not possible, or not feasible, the kernels USB serial gadget can be a good option for debugging when you know that the kernel is booting.
Enabling serial gadget
Turn on the following kernel config options:
CONFIG_USB_G_SERIAL=y CONFIG_U_SERIAL_CONSOLE=y CONFIG_USB_U_SERIAL=y
Add the following to your kernel cmdline to enable logging to the gadget console:
console=ttyGS0,115200
Using the gadget
Now when booting your device, you should see a new serial device appear on your host, usually it's /dev/ttyACM0.
This one liner can make sure you don't miss anything (although usually the output is buffered until you open the device).
Sometimes the gadget buffer breaks down and it sends a lot of the same few characters over and over, when this includes newlines it can cause you to lose a lot of the logs, hence the grep below.
Leave this running in a console and the boot logs will be printed every boot.
while true; do test -e /dev/ttyACM0 && sleep 0.1 && cat /dev/ttyACM0 | grep -vE "^\s?\n$"; done
Initramfs debugging
You may also drop a shell in the initramfs by adding pmos.debug-shell to your kernel cmdline. A shell will automatically be spawned on the USB serial gadget device.