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Use negotiated BLE MTU for chunking

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Sybren A. Stüvel 2026-03-14 21:23:45 +01:00
parent 7f48cb0485
commit a705429bdb
1 changed files with 22 additions and 8 deletions
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30
src/helpers/esp32/BLELogInterface.h
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@ -25,23 +25,29 @@ class BLELogInterface : public Print, BLEServerCallbacks {
BLEServer *_server; BLEServer *_server;
BLECharacteristic *_tx_char; BLECharacteristic *_tx_char;
bool _connected; bool _connected;
uint16_t _conn_id;
unsigned long _adv_restart_time; unsigned long _adv_restart_time;
char _line_buf[256]; char _line_buf[256];
int _line_len; int _line_len;
// Returns the max bytes per notification for the current connection.
// BLE notifications carry ATT_MTU-3 bytes of payload. Falls back to 20
// (the minimum guaranteed by the spec) if MTU has not been negotiated yet.
int notifyPayloadSize() const {
const uint16_t mtu = _server->getPeerMTU(_conn_id);
return (mtu > 3) ? mtu - 3 : 20;
}
void flushLine() { void flushLine() {
if (_line_len == 0 || !_connected) { if (_line_len == 0 || !_connected) {
_line_len = 0; _line_len = 0;
return; return;
} }
// BLE notifications are capped at ATT_MTU-3 bytes; MTU negotiation is not const int chunk = notifyPayloadSize();
// guaranteed, so always chunk at 20 bytes (the safe minimum) to ensure the
// full line is delivered regardless of the negotiated MTU.
const int CHUNK = 20;
int offset = 0; int offset = 0;
while (offset < _line_len) { while (offset < _line_len) {
int n = _line_len - offset; int n = _line_len - offset;
if (n > CHUNK) n = CHUNK; if (n > chunk) n = chunk;
_tx_char->setValue((uint8_t *)_line_buf + offset, n); _tx_char->setValue((uint8_t *)_line_buf + offset, n);
_tx_char->notify(); _tx_char->notify();
offset += n; offset += n;
@ -49,7 +55,14 @@ class BLELogInterface : public Print, BLEServerCallbacks {
_line_len = 0; _line_len = 0;
} }
void onConnect(BLEServer *pServer) override { _connected = true; } void onConnect(BLEServer *pServer) override {
_connected = true;
}
void onConnect(BLEServer *pServer, esp_ble_gatts_cb_param_t *param) override {
_connected = true;
_conn_id = param->connect.conn_id;
}
void onDisconnect(BLEServer *pServer) override { void onDisconnect(BLEServer *pServer) override {
_connected = false; _connected = false;
@ -59,11 +72,12 @@ class BLELogInterface : public Print, BLEServerCallbacks {
public: public:
BLELogInterface() BLELogInterface()
: _server(nullptr), _tx_char(nullptr), _connected(false), _adv_restart_time(0), _line_len(0) {} : _server(nullptr), _tx_char(nullptr), _connected(false), _conn_id(0), _adv_restart_time(0),
_line_len(0) {}
void begin(const char *device_name) { void begin(const char *device_name) {
BLEDevice::init(device_name); BLEDevice::init(device_name);
BLEDevice::setMTU(256); // default 23-byte MTU caps notifications at 20 bytes; request more BLEDevice::setMTU(256); // raise the ceiling; client may negotiate a larger MTU
// Explicitly disable bonding so the ESP32 does not send security requests. // Explicitly disable bonding so the ESP32 does not send security requests.
// Without this the BLE stack initiates Just Works pairing by default, which // Without this the BLE stack initiates Just Works pairing by default, which