feat: ESP MTU 512 + dynamic Android chunk size based on negotiated MTU

android/app/src/main/java/nl/eddydevink/sylda/BleRelayManager.kt

@@ -59,6 +59,7 @@ class BleRelayManager(
     private var otaCharacteristic: BluetoothGattCharacteristic? = null
     private var espVersion: String? = null
     private var appVersion: String = ""
+    private var negotiatedMtu: Int = 23
 
     interface OtaNotifyCallback {
         fun onOtaNotify(status: Int, seq: Int)
@@ -229,6 +230,8 @@ class BleRelayManager(
 
     fun getEspVersion(): String? = espVersion
 
+    fun getNegotiatedMtu(): Int = negotiatedMtu
+
     @SuppressLint("MissingPermission")
     private fun connectToDevice(device: android.bluetooth.BluetoothDevice) {
         handler.post { callbacks.onStatusChanged(BleStatus.Connecting) }
@@ -274,6 +277,7 @@ class BleRelayManager(
         @SuppressLint("MissingPermission")
         override fun onMtuChanged(gatt: android.bluetooth.BluetoothGatt, mtu: Int, status: Int) {
             Log.d(TAG, "MTU changed to $mtu")
+            negotiatedMtu = mtu
             val service = gatt.getService(SERVICE_UUID)
             if (service != null) {
                 relayCharacteristic = service.getCharacteristic(CHARACTERISTIC_UUID)

android/app/src/main/java/nl/eddydevink/sylda/OtaManager.kt

@@ -18,15 +18,12 @@ class OtaManager(
 ) {
     companion object {
         private const val TAG = "OtaManager"
-        private const val OTA_CHUNK_DATA_SIZE = 248
         private const val OTA_CMD_START: Byte = 0x10
-        private const val OTA_STATUS_ACK: Byte = 0x00
-        private const val OTA_STATUS_COMPLETE: Byte = 0x10
-        private const val OTA_STATUS_ERROR: Byte = 0x11
         private const val OTA_START_TIMEOUT_MS = 10000L
         private const val OTA_CHUNK_TIMEOUT_MS = 2000L
         private const val OTA_FINAL_TIMEOUT_MS = 10000L
         private const val MAX_RETRIES = 3
+        private const val OTA_ESP_MAX_CHUNK = 500
     }
 
     interface OtaCallback {
@@ -36,10 +33,16 @@ class OtaManager(
     }
 
     private var firmwareData: ByteArray? = null
+    private var chunkDataSize: Int = 248
+
+    private fun calculateChunkSize(mtu: Int): Int {
+        return minOf(maxOf(mtu - 3 - 4, 20), OTA_ESP_MAX_CHUNK)
+    }
 
     fun loadFirmware(): Boolean {
         return try {
             firmwareData = context.assets.open("firmware/firmware.bin").readBytes()
+            chunkDataSize = calculateChunkSize(relayManager.getNegotiatedMtu())
             true
         } catch (e: Exception) {
             Log.e(TAG, "Failed to load firmware", e)
@@ -85,13 +88,13 @@ class OtaManager(
                 }
             } catch (_: kotlinx.coroutines.TimeoutCancellationException) {}
 
-            val totalChunks = (fw.size + OTA_CHUNK_DATA_SIZE - 1) / OTA_CHUNK_DATA_SIZE
+            val totalChunks = (fw.size + chunkDataSize - 1) / chunkDataSize
             var seq = 0
             var offset = 0
 
             while (offset < fw.size) {
                 val remaining = fw.size - offset
-                val chunkSize = minOf(remaining, OTA_CHUNK_DATA_SIZE)
+                val chunkSize = minOf(remaining, chunkDataSize)
 
                 val packet = ByteBuffer.allocate(4 + chunkSize).order(ByteOrder.BIG_ENDIAN)
                 packet.putInt(seq)

src/main.cpp

@@ -338,6 +338,7 @@ void setup() {
     }
     Serial.println("LED blink done, starting BLE...");
 
+    NimBLEDevice::setMTU(512);
     NimBLEDevice::init("SylDa_Server");
 
     NimBLEServer *pServer = NimBLEDevice::createServer();