fix(split): Use our managed queue for input report to avoid deadlock

- Use our service work queue to process input report to avoid deadlock on the system work queue
- Add config `CONFIG_ZMK_INPUT_SPLIT_MSG_QUEUE_SIZE`

app/src/split/bluetooth/Kconfig

@@ -102,6 +102,17 @@ config BT_GAP_AUTO_UPDATE_CONN_PARAMS
 
 endif # !ZMK_SPLIT_ROLE_CENTRAL
 
+if ZMK_INPUT_SPLIT
+
+config ZMK_INPUT_SPLIT_MSG_QUEUE_SIZE
+    int "Max number of input split messages to queue for sending over BLE"
+    default 32
+    help
+      Sets the maximum number of input split events that can be queued for BLE notifications.
+      Increasing this reduces drops during high-rate input bursts at the cost of additional RAM.
+
+endif # ZMK_INPUT_SPLIT
+
 endmenu
 
 endif # ZMK_SPLIT_BLE

app/src/split/bluetooth/service.c

@@ -312,6 +312,27 @@ static int zmk_split_bt_sensor_triggered(uint8_t sensor_index,
 
 #if IS_ENABLED(CONFIG_ZMK_INPUT_SPLIT)
 
+struct input_event_notify_item {
+    uint16_t attr_index;
+    struct zmk_split_input_event_payload payload;
+};
+
+K_MSGQ_DEFINE(input_event_msgq, sizeof(struct input_event_notify_item),
+              CONFIG_ZMK_INPUT_SPLIT_MSG_QUEUE_SIZE, 4);
+
+static void send_input_event_callback(struct k_work *work) {
+    struct input_event_notify_item item;
+    while (k_msgq_get(&input_event_msgq, &item, K_NO_WAIT) == 0) {
+        int err = bt_gatt_notify(NULL, &split_svc.attrs[item.attr_index], &item.payload,
+                                 sizeof(item.payload));
+        if (err) {
+            LOG_ERR("Error notifying input event %d", err);
+        }
+    }
+}
+
+static K_WORK_DEFINE(service_input_notify_work, send_input_event_callback);
+
 static int zmk_split_bt_report_input(uint8_t reg, uint8_t type, uint16_t code, int32_t value,
                                      bool sync) {
 
@@ -326,7 +347,24 @@ static int zmk_split_bt_report_input(uint8_t reg, uint8_t type, uint16_t code, i
                 .sync = sync ? 1 : 0,
             };
 
-            return bt_gatt_notify(NULL, &split_svc.attrs[i], &payload, sizeof(payload));
+            struct input_event_notify_item item = {.attr_index = (uint16_t)i, .payload = payload};
+            int err = k_msgq_put(&input_event_msgq, &item, K_NO_WAIT);
+
+            if (err == 0) {
+                k_work_submit_to_queue(&service_work_q, &service_input_notify_work);
+                return 0;
+            } else {
+                LOG_WRN("Input event queue full, dropping one and retry");
+
+                struct input_event_notify_item discarded;
+                k_msgq_get(&input_event_msgq, &discarded, K_NO_WAIT);
+                err = k_msgq_put(&input_event_msgq, &item, K_NO_WAIT);
+
+                if (err != 0) {
+                    LOG_WRN("Failed to queue input event (%d)", err);
+                    return err;
+                }
+            }
         }
     }
     return -ENODEV;