Use subsystem recovery for Sargo modem faults

Select RELATED before HAL startup and create oFono’s private state directory. A controlled modem restart recovered network registration without rebooting the phone; two persistent boots and native UI checks passed. The IMS firmware watchdog root cause remains unresolved.

tundra/runtime/modem-recovery.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MPL-2.0
+"""Keep a Sargo modem fault from immediately restarting the whole phone."""
+from pathlib import Path
+import time
+
+
+def configure(devices=Path('/sys/bus/msm_subsys/devices')):
+    matches = [p.parent for p in devices.glob('*/name')
+               if p.read_text().strip() == 'modem']
+    if len(matches) != 1:
+        raise RuntimeError('Expected one Sargo modem subsystem')
+    level = matches[0]/'restart_level'
+    previous = level.read_text().strip()
+    if previous not in ('SYSTEM', 'RELATED'):
+        raise RuntimeError('Unsupported modem restart policy: '+previous)
+    # This kernel supports coupled subsystem recovery. Firmware failures and
+    # unsuccessful recovery remain visible in the kernel log; do not suppress
+    # its watchdog or crash accounting.
+    if previous != 'RELATED':
+        level.write_text('RELATED\n')
+    if level.read_text().strip() != 'RELATED':
+        raise RuntimeError('Modem restart policy was not applied')
+    print('Sargo modem recovery: '+previous+' -> RELATED', flush=True)
+
+
+if __name__ == '__main__':
+    for attempt in range(30):
+        try:
+            configure()
+            break
+        except (FileNotFoundError, RuntimeError):
+            if attempt == 29:
+                raise
+            time.sleep(1)