Document ADB in the car: TCP over the NCM link, one SSH hop via the Orin

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

README.md

@@ -24,5 +24,6 @@ flutter test
 flutter run --dart-define=YUKART_GW=ws://<host>:7700/      # default: the car Orin, ws://192.168.55.1:7700/
 ```
 
+- For debugging the app in the car (ADB over the Orin link), refer to [ADB in the car](docs/ADB-IN-CAR.md);  
 - For implementation practice guidelines, refer to the [guidelines](docs/GUIDELINES.md);  
 - For implementation plans and history, refer to [them](docs/impl/);

docs/ADB-IN-CAR.md

@@ -0,0 +1,98 @@
+---
+type: reference
+---
+# ADB in the car
+
+**In one line: ADB over TCP on the tablet's USB-Ethernet link to the Orin, reached
+from halogen through one SSH port-forward via the Orin.** TCP mode is armed once
+over a USB cable to halogen, and again after every tablet reboot.
+
+## Why USB debugging is unavailable in the car
+
+A USB link has exactly one host and one device. The tablet's single USB-C port can
+be either, decided by the USB-C CC resistors at plug-in:
+
+| Plugged into | Tablet role | USB debugging |
+|---|---|---|
+| halogen | device | yes: ADB is a USB function the tablet offers as a device |
+| the Orin (via the PD hub) | **host** | no: a host offers no USB functions |
+
+A data hub does not help: it adds more devices under the same host. The PD hub in
+the car only passes power through, so the tablet charges while it is the host.
+
+## What the link is
+
+The Orin runs Jetson's USB device mode (`l4t-usb-device-mode`) and presents itself
+as a USB network adapter, with a DHCP server on it. The tablet's kernel binds it as
+**NCM** and Android brings it up as an ordinary **Ethernet** network:
+
+```
+tablet 192.168.55.100  ⇄  USB NCM  ⇄  Orin 192.168.55.1
+```
+
+The app reaches `svc-gw` over this network. It requests the network explicitly and
+binds to it, because a network without internet never becomes Android's default
+while WiFi or LTE is up. `adbd` is a system daemon and needs none of that: once
+it listens on TCP, it listens on every interface, this one included.
+
+## The path from halogen
+
+halogen is not on `192.168.55.0/24`; the Orin is. halogen reaches the Orin over
+the LAN or the tailnet (the Orin has an LTE uplink), so one SSH local forward
+bridges the gap:
+
+```
+adb (halogen) → localhost:15555 ═SSH═▶ Orin → 192.168.55.100:5555 → adbd (tablet)
+```
+
+Authorisation is unchanged: the tablet still checks ADB's RSA key, and halogen's
+key was accepted at the USB pairing. The tablet's port is reachable only from the
+NCM link (and from WiFi, while TCP mode is on and WiFi is up).
+
+## Steps
+
+**1. Arm, at the desk** (USB cable to halogen; once per tablet boot):
+
+```bash
+adb -s 1f1530c87d84 tcpip 5555
+```
+
+**2. In the car** (tablet on the Orin, app link up):
+
+```bash
+ssh -N -o ExitOnForwardFailure=yes -o ServerAliveInterval=10 \
+    -L 15555:192.168.55.100:5555 mikkeli-orin-nano &
+adb connect localhost:15555
+adb devices                               # → localhost:15555  device
+flutter run -d localhost:15555 --dart-define=YUKART_GW=ws://192.168.55.1:7700/
+```
+
+The default gateway address already is `ws://192.168.55.1:7700/`; the define only
+matters if a desk build was installed last.
+
+**3. Disarm** (optional): `adb usb` returns ADB to USB only. A reboot does the same.
+
+Local port 15555, not 5555: halogen's adb server scans localhost 5555–5585 for
+emulators, and a tunnel there shows up a second time as `emulator-5554`.
+
+## When it does not work
+
+| Symptom | Check |
+|---|---|
+| `connect` refused or times out | From the Orin: `nc -zv 192.168.55.100 5555`. Refused means TCP mode is off: the tablet rebooted; re-arm over USB |
+| No route to `192.168.55.100` from the Orin | The USB link is down: cable, PD hub, or the tablet asleep (see the initial plan, "The USB link, as measured") |
+| `unauthorized` | Accept the prompt on the tablet; the halogen key was revoked or never accepted |
+| `bind: Address already in use` on 15555 | An old tunnel is still running: `pkill -f 15555:192.168.55.100` |
+
+## Alternatives
+
+- **adb on the Orin itself.** `adb connect 192.168.55.100:5555` there, no tunnel.
+  Needs `adb` installed on the Orin, and `flutter run` stays on halogen anyway.
+- **Wireless debugging** (Android 11+, pairing codes) survives better, but it only
+  runs over WiFi, so it does not use this link.
+
+## Status
+
+The NCM link and the app over it are verified (2026-10-09). The ADB path above is
+**untested in the car**; HyperOS may need its USB-debugging security options for
+installs over TCP as it did over USB.

docs/impl/plans/INITIAL.md

@@ -109,23 +109,12 @@ The app binds the process to the USB Ethernet network only when the gateway
 address is on `192.168.55.0/24`, so a desk build pointed at halogen over WiFi is
 not routed into a dead USB link.
 
-### ADB in the car, without a hub
+### ADB in the car
 
-The tablet's only USB port is the Orin link, and in it the tablet is the host —
-USB debugging is unavailable there. ADB over TCP is not, and it travels over the
-same NCM link:
-
-```bash
-adb tcpip 5555                                   # at the desk, over USB, once per tablet boot
-# in the car, from halogen, through the Orin over the tailnet:
-ssh -N -L 5555:192.168.55.100:5555 mikkeli-orin-nano &
-adb connect localhost:5555
-flutter run -d localhost:5555
-```
-
-The tablet keeps the RSA authorisation from the USB pairing. `adb usb` turns TCP
-mode off again; a reboot does too. Android's *Wireless debugging* (pairing) is
-WiFi-only and does not help here. Untested in the car so far.
+USB debugging is unavailable there: on the Orin link the tablet is the USB host.
+ADB runs over TCP on the NCM link instead, reached from halogen through one SSH
+forward via the Orin. Mechanism, steps and troubleshooting:
+[ADB in the car](../../ADB-IN-CAR.md).
 
 ### CI