probe: stlink: lazily load and cache board ID to improve discovery an…

…d connect time (#1464)

pyocd/probe/stlink_probe.py

@@ -16,7 +16,7 @@
 # limitations under the License.
 
 from time import sleep
-from typing import (Any, Callable, List, Optional, Sequence, Union, TYPE_CHECKING)
+from typing import (Any, Callable, Dict, List, Optional, Sequence, Union, TYPE_CHECKING)
 
 from .debug_probe import DebugProbe
 from ..core.memory_interface import MemoryInterface
@@ -36,6 +36,12 @@
 class StlinkProbe(DebugProbe):
     """@brief Wraps an STLink as a DebugProbe."""
 
+    _board_id: Optional[str]
+
+    # Shared cache for the STLink Mbed board IDs read from MSD volumes.
+    # The dict maps the serial number to 4-character board ID, or None if no ID is available.
+    _mbed_board_id_cache: Dict[str, Optional[str]] = {}
+
     @classmethod
     def get_all_connected_probes(cls, unique_id: Optional[str] = None,
             is_explicit: bool = False) -> List["StlinkProbe"]:
@@ -55,29 +61,46 @@ def __init__(self, device: STLinkUSBInterface) -> None:
         self._is_connected = False
         self._nreset_state = False
         self._memory_interfaces = {}
-        self._mbed_info = None
-        self._board_id = self._get_board_id()
+        self._board_id = None
         self._caps = set()
 
+    @property
+    def board_id(self) -> str:
+        """@brief Lazily loaded 4-character board ID."""
+        if self._board_id is None:
+            self._board_id = self._get_board_id()
+        return self._board_id
+
     def _get_board_id(self) -> Optional[str]:
         # Try to get the board ID first by sending a command, since it is much faster. This requires
         # opening the USB device, however, and requires a recent STLink firmware version.
         board_id = self._link.get_board_id()
         if board_id is None:
-            # Try to detect associated board info via the STLinkV2-1 MSD volume.
-            detector = create_mbed_detector()
-            if detector is not None:
-                for info in detector.list_mbeds():
-                    if info['target_id_usb_id'] == self._link.serial_number:
-                        self._mbed_info = info
+            # Check the cache.
+            if self._link.serial_number in self._mbed_board_id_cache:
+                board_id = StlinkProbe._mbed_board_id_cache[self._link.serial_number]
+            else:
+                # Try to detect associated board info via the STLinkV2-1 MSD volume.
+                detector = create_mbed_detector()
+                if detector is not None:
+                    for info in detector.list_mbeds():
+                        usb_id = info['target_id_usb_id']
 
                         # Some STLink probes provide an MSD volume, but not the mbed.htm file.
                         # We can live without the board ID, so just ignore any error.
                         try:
-                            board_id = info['target_id_mbed_htm'][0:4]
+                            this_board_id = info['target_id_mbed_htm'][0:4]
                         except KeyError:
-                            pass
-                        break
+                            # No board ID is available for this board.
+                            StlinkProbe._mbed_board_id_cache[usb_id] = None
+                        else:
+                            # Populate the cache with the ID.
+                            StlinkProbe._mbed_board_id_cache[usb_id] = this_board_id
+
+                            # Use this ID if it's for our board.
+                            if usb_id == self._link.serial_number:
+                                board_id = this_board_id
+                                break
         return board_id
 
     @property
@@ -114,16 +137,16 @@ def capabilities(self):
 
     @property
     def associated_board_info(self) -> Optional["BoardInfo"]:
-        if (self._board_id is not None) and (self._board_id in BOARD_ID_TO_INFO):
-            return BOARD_ID_TO_INFO[self._board_id]
+        if (self.board_id is not None) and (self.board_id in BOARD_ID_TO_INFO):
+            return BOARD_ID_TO_INFO[self.board_id]
         else:
             return None
 
     def create_associated_board(self):
         assert self.session is not None
         board_info = self.associated_board_info
-        if board_info or self._board_id:
-            return MbedBoard(self.session, board_info=board_info, board_id=self._board_id)
+        if board_info or self.board_id:
+            return MbedBoard(self.session, board_info=board_info, board_id=self.board_id)
         else:
             return None