ThinkNode M1: USB CDC serial drops (`serial_disconnect`) when forwarding received packets

[tgaguy]

Describe the bug

The ThinkNode M1's USB CDC serial connection drops repeatedly while connected to a host, a few seconds after the node receives any mesh packet (advert, REQ, GRP_TXT, PATH, etc.) and forwards it to the host over USB serial. Drops occur at irregular intervals (3–30 s), sometimes immediately after a successful reconnect.

The device itself does not reboot — its clock stays monotonic through the drops and it answers device/battery queries as soon as the link is re-established. Only the USB CDC session drops.

To Reproduce

1. Flash a ThinkNode M1 with v1.17.0 (build 09-Aug-2026).
2. Connect via USB CDC-ACM (/dev/ttyACM0, VID 239a:4405) to a Linux host running the meshcore Python library (or meshcore-packet-capture).
3. Generate any mesh traffic from a second node (advert, message, request).
4. Observe serial_disconnect / "Serial connection lost" followed by host reconnect cycles.

Expected behavior

The serial connection remains stable while the node receives and forwards packets.

Environment

• Hardware: Elecrow ThinkNode M1 (nRF52840, native USB CDC-ACM)
• Firmware: v1.17.0-727fc05 (build 09-Aug-2026)
• Host: Linux, kernel 6.14.0-37-generic, cdc_acm driver
• Client: agessaman/meshcore-packet-capture, meshcore Python library 2.3.8

Diagnostics already ruled out

• Two different ThinkNode M1 units — both drop identically.
• Different USB cable and different USB port — still drops.
• Host reboot (fresh USB stack) — still drops.
• USB autosuspend disabled (power/control = on) — not power management.
• ModemManager never probes the device; only the analyser process opens the port.
• Device stays alive (no reboot); battery healthy (4.16 V).

Possible relation

May be related to #2819 ("kiss modem fixes for stalls under USB backpressure") — the drop persists on v1.17.0 despite that fix.

Example log excerpt

INFO  Captured packet #7: D type 4, 123 bytes, SNR: 11.75, RSSI: -44 (MQTT: 3/3)
INFO  Captured packet #8: F type 7, 57 bytes,  SNR: 12.5,  RSSI: -63 (MQTT: 3/3)
INFO  Captured packet #9: F type 0, 29 bytes,  SNR: 11.75, RSSI: -63 (MQTT: 3/3)
INFO  Captured packet #10: D type 4, 123 bytes, SNR: 12.0, RSSI: -48 (MQTT: 3/3)
WARNING  Disconnect event received: serial_disconnect
ERROR    Serial connection lost - cable may be disconnected

thinknode-packet-capture-log.txt