drv/bluetooth_stm32_bluenrg: fix lost data in send

When sending large amounts of data quickly, some data was being lost.
Although we had a retry when BLE_STATUS_INSUFFICIENT_RESOURCES was
received, it seems that repeated calls immediately after that lead to
lost data (and sometimes duplicated data).

It turns out, the BlueNRG chip has an event that says when it is ready
again after a BLE_STATUS_INSUFFICIENT_RESOURCES, so we can use that
event to wait instead of doing multiple failed retries.

There are still some issues with data corruption/loss but this is a
significant improvement nonetheless.

Issue: pybricks/support#324

CHANGELOG.md

@@ -14,6 +14,7 @@
 ### Fixed
 - Fixed allocator interfering with motor control when memory usage is high ([support#956]).
 - Fixed `Stop.NONE` not working properly for some drivebase geometries ([support#972]).
+- Reduced data loss when printing in tight loop on Move hub ([support#324]).
 
 ### Changed
 - Methods like `control.limits()` now check the user input and raise a
@@ -28,6 +29,7 @@
 ### Removed
 - Removed `DriveBase.left` and `DriveBase.right` properties ([support#910]).
 
+[support#324]: https://github.com/pybricks/support/issues/324
 [support#484]: https://github.com/pybricks/support/issues/484
 [support#830]: https://github.com/pybricks/support/issues/830
 [support#989]: https://github.com/pybricks/support/issues/881

lib/pbio/drv/bluetooth/bluetooth_stm32_bluenrg.c

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2018-2022 The Pybricks Authors
+// Copyright (c) 2018-2023 The Pybricks Authors
 
 // Bluetooth for STM32 MCU with STMicro BlueNRG-MS
 
@@ -94,6 +94,8 @@ static uint16_t conn_handle;
 static uint16_t remote_handle;
 // handle to LWP3 characteristic on remote
 static uint16_t remote_lwp3_char_handle;
+// used to wait for Evt_Blue_Gatt_Tx_Pool_Available
+static bool tx_pool_available;
 
 // Pybricks GATT service handles
 static uint16_t pybricks_service_handle;
@@ -364,6 +366,8 @@ static PT_THREAD(send_value_notification(struct pt *pt, pbio_task_t *task))
     if (ret == BLE_STATUS_INSUFFICIENT_RESOURCES) {
         // this will happen if notifications are enabled and the previous
         // changes haven't been sent over the air yet
+        tx_pool_available = false;
+        PT_WAIT_UNTIL(pt, tx_pool_available);
         goto retry;
     }
 
@@ -948,6 +952,14 @@ static void handle_event(hci_event_pckt *event) {
                     aci_gatt_write_response_begin(subevt->conn_handle, subevt->attr_handle, !!err, err, subevt->data_length, subevt->data);
                 }
                 break;
+
+                case EVT_BLUE_GATT_TX_POOL_AVAILABLE: {
+                    // REVISIT: We might need to look at the event args for
+                    // connection handle if we need to handle this in multiple
+                    // places, e.g. for notifications and write without response.
+                    tx_pool_available = true;
+                }
+                break;
             }
         }
         break;