dwt: mailbox_watch.py -- poll 0x27C190/0x16F0 across a boot; results are consumed before rdebug is reachable

dwt/mailbox_watch.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""Poll the VT1200 diagnostic handoff cells over rdebug during a boot.
+
+rdebug only exists once the OS/network stack is up, so this cannot see the
+level-1 tests themselves.  It *can* catch the window between the network stack
+starting and the Terminal Manager consuming the diagnostic result:
+
+  0x27C190  diagnostic mailbox pointer (Ghidra: FUN_rom_07f5a026 dereferences it)
+  0x16F0    level-1 dispatcher's transient status code slot
+
+If the mailbox pointer becomes non-zero, the nine fields it points at are
+dumped.  The log records every value change, so a value that exists only for a
+few polls is still captured.
+
+Usage:
+    python3 dwt/mailbox_watch.py --host 100.103.100.103 --seconds 180
+
+Start it, then power-cycle the terminal.
+"""
+from __future__ import annotations
+
+import argparse
+import struct
+import sys
+import time
+
+import exploit
+
+MAILBOX = 0x27C190
+SLOT = 0x16F0
+FIELDS = 9
+
+
+def u32(rd: exploit.RDebug, bit: int) -> int:
+    return struct.unpack("<I", rd.peek(bit, 4))[0]
+
+
+def main(argv):
+    ap = argparse.ArgumentParser(description=__doc__,
+                                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("--host", default="100.103.100.103")
+    ap.add_argument("--port", type=int, default=771)
+    ap.add_argument("--seconds", type=float, default=180.0)
+    ap.add_argument("--out", default=".vt1200-scratch/mailbox-watch.log")
+    a = ap.parse_args(argv[1:])
+
+    deadline = time.time() + a.seconds
+    log = open(a.out, "w")
+    rd = None
+    last_m = last_s = None
+
+    def emit(line: str) -> None:
+        log.write(line + "\n")
+        log.flush()
+
+    emit(f"watch {a.host}:{a.port} for {a.seconds:.0f}s -> {a.out}")
+    while time.time() < deadline:
+        if rd is None:
+            try:
+                rd = exploit.RDebug(a.host, a.port, 2.0, False)
+                rd.connect()
+                emit(f"{time.time():.3f} connected")
+            except Exception as exc:            # noqa: BLE001
+                time.sleep(0.4)
+                continue
+        try:
+            m = u32(rd, MAILBOX)
+            s = u32(rd, SLOT)
+            if m != last_m or s != last_s:
+                emit(f"{time.time():.3f} mailbox={m:#010x} slot={s:#010x}")
+                last_m, last_s = m, s
+            if m:
+                for i in range(FIELDS):
+                    try:
+                        v = u32(rd, m + 0x20 * i)
+                    except Exception:           # noqa: BLE001
+                        break
+                    emit(f"    +0x{0x20 * i:03x}: {v:#010x}")
+        except Exception as exc:                # noqa: BLE001
+            emit(f"{time.time():.3f} lost: {type(exc).__name__}: {exc}")
+            try:
+                rd.close()
+            except Exception:                   # noqa: BLE001
+                pass
+            rd = None
+            time.sleep(0.2)
+    if rd is not None:
+        rd.close()
+    log.close()
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))

dwt/notes/REAL_UNIT.md

@@ -214,3 +214,18 @@ real framebuffer is 1280x1024 with the 256-byte stride.
 The console refuses diagnostics while the DWT has active work — our injected
 xhello window is the blocker. The service guide requires no open windows before
 remote tests: power-cycle (or close all session windows) and retry.
+
+### Diagnostic results are not observable through rdebug
+
+`dwt/mailbox_watch.py` polled the mailbox pointer (`0x27C190`) and the level-1
+dispatcher slot (`0x16F0`) across a power cycle. The rdebug service was only
+connectable **after** the OS was up, at which point:
+
+```
+mailbox=0x00000000   slot=0x8ac00000   (OS data)
+```
+
+During the boot it was unusable (timeouts / AssertionError). So the level-1
+result is consumed before rdebug exists; the mailbox is a dialog/debug
+structure, not the boot handoff. The only durable record is the **screen**:
+the boot alert, and the Diagnostics dialog's on-screen results.