MTU에 따른 BLE 패킷 분할 - Piezo echo ADC 응답 패킷

- 협상된 MTU 기준으로 reb: 응답 패킷 크기 판단
- 기존 reb:가 MTU에 들어가지 않으면 rec: chunk 분할 전송

src/ble/ble_service.c

@@ -37,6 +37,7 @@ static bool dfu_advertising_mode;
 static bool advertising_unlimited;
 static uint8_t conn_param_update_attempts;
 static int64_t last_rx_conn_param_update_ms;
+static uint16_t current_notify_mtu = 23U;
 
 #if IS_ENABLED(CONFIG_BT_SMP)
 #define BLE_REQUIRED_SECURITY_LEVEL    BT_SECURITY_L4
@@ -55,6 +56,33 @@ static struct k_work adv_restart_work;
 static struct k_work_delayable adv_timeout_work;
 static struct k_work_delayable conn_param_update_work;
 
+/*
+ * Store the UATT MTU used by normal GATT/NUS notifications.
+ * NUS notifications can carry application payload up to UATT MTU - 3 bytes.
+ */
+static void att_mtu_updated(struct bt_conn *conn, uint16_t tx, uint16_t rx)
+{
+    uint16_t uatt = bt_gatt_get_uatt_mtu(conn);
+
+    if (uatt != 0U)
+    {
+        current_notify_mtu = uatt;
+    }
+    else
+    {
+        current_notify_mtu = (tx < rx) ? tx : rx;
+    }
+
+    DBG_PRINTF("[BLE] ATT MTU updated tx=%u rx=%u uatt=%u used=%u notify_payload=%u\r\n",
+               tx, rx, uatt, current_notify_mtu,
+               (current_notify_mtu > 3U) ? (current_notify_mtu - 3U) : 0U);
+}
+
+static struct bt_gatt_cb gatt_callbacks =
+{
+    .att_mtu_updated = att_mtu_updated,
+};
+
 /* 로컬 BLE MAC 주소 로그 */
 static void ble_log_local_identity(void)
 {
@@ -424,6 +452,7 @@ static void disconnected(struct bt_conn *conn, uint8_t reason)
     }
 
     ble_connection_st = false;
+    current_notify_mtu = 23U;
     if (dfu_advertising_mode)
     {
         // DFU 광고는 미연결 10분 타임아웃 적용
@@ -578,6 +607,7 @@ int ble_service_init(ble_data_rx_cb_t rx_cb)
 
     ble_log_local_identity();
 
+    bt_gatt_cb_register(&gatt_callbacks);
 #if IS_ENABLED(CONFIG_BT_SMP)
     err = ble_security_init();
     if (err)
@@ -790,3 +820,9 @@ bool ble_is_connected(void)
 {
     return (current_conn != NULL);
 }
+
+/* ATT MTU used to calculate notify payload size */
+uint16_t ble_current_mtu(void)
+{
+    return current_notify_mtu;
+}