[init] clear context 2 → Temp2      ✓
[init] set context 2  → vzwadmin    ✗  (modem returns ERROR)
  ↓ 5 retries exhausted
[jump] → index 41: AT+CFUN=1,1     → reboot
[init] restart from index 1 (isRebootCommandAlreadySent reset by outer loop)
  ↓ vzwadmin fails again
[jump] → index 41: CFUN, but isRebootCommandAlreadySent=true
  ↓ → MODEM_PROCESS_FAILURE
[outer] MODEM_CLEAN_DEVICE_FOR_FAILURE → MODEM_SERIAL_PORT_SCANNING → restart
  ↓ isRebootCommandAlreadySent reset again
  ↓ loop repeats forever

index 17 error → index 18  (next APN)
index 18 error → index 19
...
index 24 error → index 25  (AT+CTZU=1, first post-APN command)

// Accumulate approach:
modemResponseBuffer += ConvertedATResponseBuffer;
if (modemResponseBuffer.find(deviceAtCommand.atCmdExpectedResponse) != std::string::npos) {
    stateMachineStatus = AT_COMMAND_RESPONSE_OK;
} else if (modemResponseBuffer.find("ERROR") != std::string::npos) {
    stateMachineStatus = AT_COMMAND_RESPONSE_ERROR;
}
// else stay in RESPONSE_WAIT for next read

# AT+CGDCONT / vzwadmin APN: Modem Init Failure Investigation **Date:** 2026-03-13 **Branch:** VOD-HWCamera-Dash-MTK --- ## Symptom Modem initialization never completes when `vzwadmin` is present in the APN list (`/etc/mvcamera/config.json`). Renaming the APN to any other string causes initialization to succeed. The system enters an infinite reinit loop. --- ## Root Cause: `vzwadmin` Rejection → Infinite Init Loop The modem rejects `AT+CGDCONT=2,"IPV4V6","vzwadmin"\r` with `ERROR` (carrier-reserved APN name). This triggers a failure cascade: ``` [init] clear context 2 → Temp2 ✓ [init] set context 2 → vzwadmin ✗ (modem returns ERROR) ↓ 5 retries exhausted [jump] → index 41: AT+CFUN=1,1 → reboot [init] restart from index 1 (isRebootCommandAlreadySent reset by outer loop) ↓ vzwadmin fails again [jump] → index 41: CFUN, but isRebootCommandAlreadySent=true ↓ → MODEM_PROCESS_FAILURE [outer] MODEM_CLEAN_DEVICE_FOR_FAILURE → MODEM_SERIAL_PORT_SCANNING → restart ↓ isRebootCommandAlreadySent reset again ↓ loop repeats forever ``` The clearing phase succeeds (`Temp2` works), but after the `CFUN` reboot the modem has `Temp2` in context 2, not `vzwadmin`. Validation in `isQuectelModemInit` then sees the mismatch and triggers re-init, compounding the loop. --- ## Design Issues ### 1. CGDCONT errors are fatal — they should not be (primary issue) `ModemManager.cpp:72-79`: Every APN-set command has `nextCommandIndexIfError = MODEM_AT_COMM_INDEX_AFTER_CFUN`. One rejection by the modem for a single APN aborts the entire sequence and reboots. There is no way to skip a failed APN and continue setting the rest. ### 2. No response accumulation — single read decides success/failure `AtCommandInterface.cpp:93-103`: When `select()` signals data, there is one `read()` call. If the modem's `OK` arrives in a second serial fragment, that single read yields no `OK` → immediate `AT_COMMAND_RESPONSE_ERROR`, before the timeout even expires. This makes response detection brittle for larger payloads (e.g., `AT+CGDCONT?` query responses). ### 3. Clearing APNs before writing creates a destructive intermediate state `ModemManager.cpp:61-69`: All 8 contexts are cleared to `TempN` first. If a subsequent set command fails, the modem reboots with Temp names instead of the original APN configuration. The post-reboot validation then always fails because it compares against the JSON-expected names. ### 4. No "read before write" — blindly overwrites even correct APNs `AT+CGDCONT?` is only queried *after* reboot for validation. It is never queried at the *start* of initialization to check whether APNs are already correct. If they are (or pre-provisioned by carrier), the entire clear → set → reboot cycle is unnecessary and destructive. ### 5. Validation iterates `currentApnList` (modem) not `availableApnList` (config) `ModemManager.cpp:861-872`: The outer loop is over `currentApnList` (what the modem reported). If the modem reports fewer contexts than expected, or reports them in a different order, the index pairing between `currentApnList[n]` and `availableApnList[n]` breaks silently. --- ## Concrete Improvements ### Fix 1: Query `AT+CGDCONT?` at init start, skip APNs already correct Add `AT+CGDCONT?` as one of the first init commands with a callback that populates `currentApnList`. Then in `modemInterfaceInit`, before generating an APN-set command, check if `currentApnList[n]` already matches `availableApnList[n]`. If it does, skip both the clear and the set for that context and advance `initAPNlistIndex`. This avoids clearing pre-provisioned APNs (like `vzwadmin`) that the modem will reject being re-written. ### Fix 2: CGDCONT set failure → skip to next APN, not to CFUN Change the error routing for indices 17–24: ``` index 17 error → index 18 (next APN) index 18 error → index 19 ... index 24 error → index 25 (AT+CTZU=1, first post-APN command) ``` After all APNs have been attempted (skipping failures), add an explicit validation command (`AT+CGDCONT?`) to check that all critical APNs (especially IMS, required for VoLTE) are present. Only trigger CFUN if a critical APN is genuinely missing. ### Fix 3: Accumulate response bytes until terminal line In `AtCommandInterface.cpp:81-103`, stay in `AT_COMMAND_RESPONSE_WAIT` and append to `modemResponseBuffer` on each `read()` until either the expected response substring or `ERROR` is found — or the timeout fires. A single-read strategy works only if the modem always sends the full response atomically, which serial ports do not guarantee. ```cpp // Accumulate approach: modemResponseBuffer += ConvertedATResponseBuffer; if (modemResponseBuffer.find(deviceAtCommand.atCmdExpectedResponse) != std::string::npos) { stateMachineStatus = AT_COMMAND_RESPONSE_OK; } else if (modemResponseBuffer.find("ERROR") != std::string::npos) { stateMachineStatus = AT_COMMAND_RESPONSE_ERROR; } // else stay in RESPONSE_WAIT for next read ``` This also removes false-ERROR when the first fragment of a long response arrives without `OK` yet. ### Fix 4: Separate "clear" and "set" only for contexts that need changing Instead of blindly clearing all 8 contexts to `TempN`, only clear a context if its current value differs from expected AND the expected value can actually be set. If a clear succeeds but the set fails (as with `vzwadmin`), restore the original value (or leave it if it matches expected). This prevents the destructive Temp2-after-reboot problem. ### Fix 5: Validate by iterating `availableApnList` (config) not `currentApnList` (modem) In `isQuectelModemInit`, iterate the expected list and search the modem's reported list for each entry. This is resilient to the modem reporting APNs in a different order or reporting more/fewer contexts than expected. --- ## Recommended Immediate Fix (Minimal Change) The smallest change that breaks the infinite loop: change `nextCommandIndexIfError` for CGDCONT SET commands (indices 17–24) from `MODEM_AT_COMM_INDEX_AFTER_CFUN` to the next APN index. Add `AT+CGDCONT?` as a post-APN-set validation command with a callback that checks for critical APNs before deciding whether to proceed or reboot. This stops the loop while keeping the overall structure intact. --- ## Key Files | File | Relevance | |------|-----------| | `initmodem/src/ModemManager.cpp` | Init command array, APN generation, validation logic | | `initmodem/src/AtCommandInterface.cpp` | AT state machine, response parsing | | `initmodem/src/JsonFileParser.cpp` | APN config parsing from `/etc/mvcamera/config.json` | | `initmodem/src/ModemManager.h` | `ModemAPNList` struct, class interface | | `initmodem/include/user_config.h` | `MODEM_TOTAL_NUMBER_OF_APN_AVAILABLE`, timing constants |