Add RTT console/capture tooling (tools/rtt.py), rtt skill, and HIL harness support (#3853)

Promote SEGGER RTT from an inline debugging technique to a standalone skill
backed by one stdlib-only implementation in tools/rtt.py: a CLI and importable
module for console/capture over J-Link (RTTTelnetPort) and OpenOCD (rtt server)
probes, with probe selection by serial or VID:PID, control-block address via
--elf or --addr, bidirectional console, post-mortem ring dump, and
--reset-before-attach for boot-time capture. The HIL harness reads a board's
console over RTT when its probe has no VCOM ("logger": "rtt" plus a LOGGER=rtt
variant define), covering device_info, pool-check aliveness, and CI wiring.
Validated on 22 boards across both backends; 26 unit tests run in pre-commit.

2 files changed, 66 insertions, 2 deletions

diff --git a/test/hil/helper/hil_pool_check.py b/test/hil/helper/hil_pool_check.py
index b92f0aee0..4623ce45f 100644
--- a/test/hil/helper/hil_pool_check.py
+++ b/test/hil/helper/hil_pool_check.py
@@ -360,7 +360,47 @@ def check_host_serial(board: dict, do_reset: bool = True, want_hello: bool = Fal
 
     do_reset=False listens to the firmware as-is: used right after a flash whose
     own reset already started it — a second openocd/JLink session back-to-back on
-    the same probe can fail transiently and leave the target halted."""
+    the same probe can fail transiently and leave the target halted.
+
+    "logger": "rtt" boards have no VCOM: the same check runs over the probe's RTT
+    console instead. The reset happens BEFORE the console opens (it owns the probe),
+    which also zeroes the .bss ring — so pre-reset backlog cannot count as life, and
+    without a reset Commander delivers the boot burst the preceding flash left."""
+    if board.get('logger') == 'rtt':
+        if do_reset:
+            # a failed reset leaves the previous run's ring intact: attaching anyway would
+            # score stale output as life, so bail to host_alive's board_test reflash ladder
+            rc, err = call_flasher(getattr(hil_flash, f'reset_{board["flasher"]["name"].lower()}'), board)
+            if rc:
+                say(f'{board["name"]:26} reset failed: {err}')
+                return None
+        try:
+            ser = hil_util.JlinkRtt(board, timeout=0.3)
+        except hil_util.RttError as e:
+            say(f'{board["name"]:26} no RTT console: {e}')
+            return None
+        try:
+            data = b''
+            deadline = time.monotonic() + SERIAL_WAIT
+            while time.monotonic() < deadline:
+                ser.write(b'U')
+                data += ser.read(256)
+                # JLinkExe's banner arrives whether or not the target is alive --
+                # judged unfiltered it scores a dead board 'alive'. Same shared filter
+                # as test_host_device_info; complete_only drops a trailing partial
+                # line, so a banner FRAGMENT split by this read boundary cannot count
+                # as target output either.
+                td = hil_util.strip_banner(data, complete_only=True)
+                if want_hello:
+                    if b'Hello from TinyUSB' in td:
+                        return td
+                elif td and not boardtest_output(td):
+                    return td
+            return hil_util.strip_banner(data)
+        except hil_util.RttError:
+            return None  # console died mid-poll (server exited, probe dropped)
+        finally:
+            ser.close()
     import serial
     try:
         port = hil_util.get_serial_dev(board['flasher']['uid'], None, None, 0)
diff --git a/test/hil/helper/hil_util.py b/test/hil/helper/hil_util.py
index 03d01270f..f279cfa77 100644
--- a/test/hil/helper/hil_util.py
+++ b/test/hil/helper/hil_util.py
@@ -1,7 +1,8 @@
 #!/usr/bin/env python3
 # SPDX-License-Identifier: MIT
 # Bottom layer of the HIL harness: the bounded command runner plus the shared helpers and
-# data every other module needs. Stays stdlib-only and imports nothing local -- everything
+# data every other module needs. Stays stdlib-only; its one local dependency is
+# tools/rtt.py (the RTT console, loaded by path below) -- everything
 # else imports this, including the unit tests on GitHub's bare runner; never import them
 # from here. Callers set the module global `verbose`.
 
@@ -499,6 +500,29 @@ def run_alongside(argv: list, work, timeout: int) -> subprocess.CompletedProcess
     return _reap()
 
 
+# The RTT console implementation lives in tools/rtt.py (importable classes + CLI,
+# stdlib-only, harness-critical — see its module docstring). Loaded by file path so
+# no sys.path entry for tools/ can shadow other imports; re-exported here so the
+# harness keeps addressing hil_util.JlinkRtt.
+import importlib.util as _ilu
+
+_rtt_path = TINYUSB_ROOT / 'tools' / 'rtt.py'
+if not _rtt_path.exists():
+    # name the real cause: a bare FileNotFoundError out of an exec_module here reads
+    # as a harness bug, when the actual problem is an incompletely staged tree
+    raise ImportError(f'{_rtt_path} is missing — the RTT console lives there and the '
+                      f'harness depends on it; stage it alongside test/hil (hil_ci.sh does)')
+_rtt_spec = _ilu.spec_from_file_location('tinyusb_tools_rtt', _rtt_path)
+_rtt = _ilu.module_from_spec(_rtt_spec)
+sys.modules[_rtt_spec.name] = _rtt   # registered: RttError must be picklable across the fork Pool
+_rtt_spec.loader.exec_module(_rtt)
+JlinkRtt = _rtt.JlinkRtt
+OpenocdRtt = _rtt.OpenocdRtt
+RttError = _rtt.RttError
+RTT_BANNER_RE = _rtt.RTT_BANNER_RE
+strip_banner = _rtt.strip_banner
+
+
 def _cmd_label(cmd) -> str:
     """A one-line name for a banner. An argv whose payload is a `python3 -c` program would
     otherwise dump the whole body into the CI log, where run_cmd's banners are already the