Mi Band 4: Support MTU that gets reported from the Band

In my case that boosts the MTU from 23 to 247, firmware updates and watchface
istallations are extremely fast. This also affects notifications, weather etc.

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/huami/HuamiDeviceEvent.java

@@ -30,5 +30,6 @@ public class HuamiDeviceEvent {
     public static final byte BUTTON_PRESSED_LONG = 0x0b;
     public static final byte TICK_30MIN = 0x0e; // unsure
     public static final byte FIND_PHONE_STOP = 0x0f;
+    public static final byte MTU_REQUEST = 0x16;
     public static final byte MUSIC_CONTROL = (byte) 0xfe;
 }

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/huami/HuamiSupport.java

@@ -164,7 +164,7 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
     };
 
     private BluetoothGattCharacteristic characteristicHRControlPoint;
-    protected BluetoothGattCharacteristic characteristicChunked;
+    private BluetoothGattCharacteristic characteristicChunked;
 
     private boolean needsAuth;
     private volatile boolean telephoneRinging;
@@ -181,6 +181,7 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
     private MusicSpec bufferMusicSpec = null;
     private MusicStateSpec bufferMusicStateSpec = null;
     private boolean heartRateNotifyEnabled;
+    private int mMTU = 23;
 
     public HuamiSupport() {
         this(LOG);
@@ -293,13 +294,13 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
         builder.notify(getCharacteristic(GattService.UUID_SERVICE_CURRENT_TIME), enable);
         // Notify CHARACTERISTIC9 to receive random auth code
         builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_AUTH), enable);
+        builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_DEVICEEVENT), enable);
         return this;
     }
 
     public HuamiSupport enableFurtherNotifications(TransactionBuilder builder, boolean enable) {
         builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_3_CONFIGURATION), enable);
         builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_6_BATTERY_INFO), enable);
-        builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_DEVICEEVENT), enable);
         builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_AUDIO), enable);
         builder.notify(getCharacteristic(HuamiService.UUID_CHARACTERISTIC_AUDIODATA), enable);
 
@@ -1212,11 +1213,34 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
                 }
                 evaluateGBDeviceEvent(deviceEventMusicControl);
                 break;
+            case HuamiDeviceEvent.MTU_REQUEST:
+                int mtu = (value[2] & 0xff) << 8 | value[1] & 0xff;
+                LOG.info("device announced MTU of " + mtu);
+                mMTU = mtu;
+                /*
+                 * not really sure if this would make sense, is this event already a proof of a successful MTU
+                 * negotiation initiated by the Huami device, and acknowledged by the phone? do we really have to
+                 * requestMTU() from our side after receiving this?
+                 * /
+                if (mMTU != mtu) {
+                    requestMTU(mtu);
+                }
+                */
+                break;
             default:
                 LOG.warn("unhandled event " + value[0]);
         }
     }
 
+    private void requestMTU(int mtu) {
+        if (GBApplication.isRunningLollipopOrLater()) {
+            new TransactionBuilder("requestMtu")
+                    .requestMtu(mtu)
+                    .queue(getQueue());
+            mMTU = mtu;
+        }
+    }
+
     private void acknowledgeFindPhone() {
         try {
             TransactionBuilder builder = performInitialized("acknowledge find phone");
@@ -2174,8 +2198,8 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
         return this;
     }
 
-    protected void writeToChunked(TransactionBuilder builder, int type, byte[] data) {
+    private void writeToChunked(TransactionBuilder builder, int type, byte[] data) {
-        final int MAX_CHUNKLENGTH = 17;
+        final int MAX_CHUNKLENGTH = mMTU - 6;
         int remaining = data.length;
         byte count = 0;
         while (remaining > 0) {
@@ -2275,4 +2299,8 @@ public class HuamiSupport extends AbstractBTLEDeviceSupport {
     public UpdateFirmwareOperation createUpdateFirmwareOperation(Uri uri) {
         return new UpdateFirmwareOperation(uri, this);
     }
+
+    public int getMTU() {
+        return mMTU;
+    }
 }

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/huami/operations/UpdateFirmwareOperation.java

@@ -232,7 +232,7 @@ public class UpdateFirmwareOperation extends AbstractHuamiOperation {
     private boolean sendFirmwareData(HuamiFirmwareInfo info) {
         byte[] fwbytes = info.getBytes();
         int len = fwbytes.length;
-        final int packetLength = 20;
+        final int packetLength = getSupport().getMTU() - 3;
         int packets = len / packetLength;
 
         try {