🛠 Console série sur USB-OTG : dépanner sans réseau ni câble UART

Objectives

When Wi-Fi is misconfigured or the network is broken, the BPI-M2 Zero has a trump card: its micro-USB OTG port (dr_mode = "otg" in the device tree) can masquerade as a serial device. The board's Armbian configuration preloads the g_serial module and declares ttyGS0 as a console (SERIALCON="ttyS0,ttyGS0"): when connected directly to a PC, the board enumerates a CDC-ACM port — no USB-serial cable or network required. This is the rescue method to know before breaking your configuration.

Hardware

• BPI-M2 Zero running Armbian (ttyGS0 already declared as a console by the board definition)
• Micro-USB data cable (not a charge-only cable without data conductors)
• A Linux PC (on Windows: native CDC-ACM driver, then a terminal on the COM port)
• Initial access to the board (SSH, CON3, or an open session) to arm the gadget

Complete Code

# Sur la carte (via SSH ou CON3) : charger le gadget et exposer un getty
sudo modprobe g_serial
sudo systemctl enable --now serial-getty@ttyGS0.service
# Rendre le module permanent pour les prochains sauvetages
echo g_serial | sudo tee /etc/modules-load.d/gserial.conf

# Sur le PC : un /dev/ttyACM0 apparaît dès que le câble OTG est branché
sudo apt install -y picocom
picocom -b 115200 /dev/ttyACM0      # invite de login du BPI-M2 Zero

# En cas de plantage réseau, vérifier côté carte quel rôle prend le port
dmesg | grep -i -e gadget -e ttyGS0

Expected Verification

As soon as the cable is inserted into the OTG port, the PC detects a USB serial device (lsusb lists it); picocom displays the login prompt; after logging in, the who command shows a session on ttyGS0. The board remains accessible even with a misconfigured wlan0.

Common Errors

• Plugging the cable into the "Power only" micro-USB port: no data lines, so no ttyACM0 — the OTG port is the other one.
• Cheap charge-only cable without data pins: the gadget exists but nothing appears on the PC.
• Powering the board from the PC while a charger is connected to the other port: the Vbus lines of the two micro-USB connectors are linked, resulting in two sources competing for the 5 V.
• Forgetting to arm g_serial BEFORE losing network access: if the system no longer boots, the CON3 console (115200 8N1) remains available.
• Looking for ttyGS0 on the PC side: ttyGS0 is the name on the board side; on the PC side, it is ttyACM0.