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MTU exchange happens before pairing, causing peripherals to disconnect #10915

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MTU exchange happens before pairing, causing peripherals to disconnect
fede2cr Apr 1, 2026
2e4800d
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fede2cr Apr 1, 2026
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54 changes: 42 additions & 12 deletions ports/espressif/common-hal/_bleio/Adapter.c
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,7 @@
#include "nimble/nimble_port.h"
#include "nimble/nimble_port_freertos.h"
#include "host/ble_gap.h"
#include "host/ble_store.h"
#include "host/util/util.h"
#include "services/ans/ble_svc_ans.h"
#include "services/gap/ble_svc_gap.h"
Expand All @@ -50,7 +51,14 @@

// Status variables used while busy-waiting for events.
static volatile bool _nimble_sync;
static volatile int _connection_status;
// Non-static: Connection.c needs to set this when MTU exchange
// is deferred until after encryption completes.
volatile int _connection_status;

// Forward declaration - called from Connection.c after encryption completes.
int _mtu_reply(uint16_t conn_handle,
const struct ble_gatt_error *error,
uint16_t mtu, void *arg);

bleio_connection_internal_t bleio_connections[BLEIO_TOTAL_CONNECTION_COUNT];

Expand Down Expand Up @@ -323,7 +331,8 @@ static void _convert_address(const bleio_address_obj_t *address, ble_addr_t *nim
memcpy(nimble_address->val, (uint8_t *)address_buf_info.buf, NUM_BLEIO_ADDRESS_BYTES);
}

static int _mtu_reply(uint16_t conn_handle,
// Non-static: called from Connection.c after encryption completes.
int _mtu_reply(uint16_t conn_handle,
const struct ble_gatt_error *error,
uint16_t mtu, void *arg) {
bleio_connection_internal_t *connection = (bleio_connection_internal_t *)arg;
Expand All @@ -339,7 +348,7 @@ static int _mtu_reply(uint16_t conn_handle,
return 0;
}

static void _new_connection(uint16_t conn_handle) {
static void _new_connection(uint16_t conn_handle, bool is_central) {
// Set the tx_power for the connection higher than the advertisement.
esp_ble_tx_power_set(conn_handle, ESP_PWR_LVL_N0);

Expand All @@ -360,13 +369,32 @@ static void _new_connection(uint16_t conn_handle) {

connection->conn_handle = conn_handle;
connection->connection_obj = mp_const_none;
connection->pair_status = PAIR_NOT_PAIRED;
connection->is_central = is_central;
connection->pair_status = is_central ? PAIR_WAITING : PAIR_NOT_PAIRED;
connection->mtu = 0;

ble_gattc_exchange_mtu(conn_handle, _mtu_reply, connection);

// Change the callback for the connection.
// Change the callback FIRST so BLE_GAP_EVENT_ENC_CHANGE is handled.
ble_gap_set_event_cb(conn_handle, bleio_connection_event_cb, connection);

if (is_central) {
// Initiate SMP security BEFORE any GATT traffic.
struct ble_gap_conn_desc _desc;
if (ble_gap_conn_find(conn_handle, &_desc) == 0) {
ble_store_util_delete_peer(&_desc.peer_id_addr);
}

int rc = ble_gap_security_initiate(conn_handle);
if (rc != 0) {
// Fallback for peers that do not support immediate security.
ble_gattc_exchange_mtu(conn_handle, _mtu_reply, connection);
}

// Unblock the connect() wait loop now.
_connection_status = conn_handle;
} else {
// Peripheral role keeps stock MTU-first behavior.
ble_gattc_exchange_mtu(conn_handle, _mtu_reply, connection);
}
}

static int _connect_event(struct ble_gap_event *event, void *self_in) {
Expand All @@ -378,8 +406,8 @@ static int _connect_event(struct ble_gap_event *event, void *self_in) {
switch (event->type) {
case BLE_GAP_EVENT_CONNECT:
if (event->connect.status == 0) {
// This triggers an MTU exchange. Its reply will exit the loop waiting for a connection.
_new_connection(event->connect.conn_handle);
// Central-role connection: initiate security before MTU.
_new_connection(event->connect.conn_handle, true);
// Set connections objs back to NULL since we have a new
// connection and need a new tuple.
self->connection_objs = NULL;
Expand Down Expand Up @@ -458,7 +486,7 @@ mp_obj_t common_hal_bleio_adapter_connect(bleio_adapter_obj_t *self, bleio_addre
for (size_t i = 0; i < BLEIO_TOTAL_CONNECTION_COUNT; i++) {
bleio_connection_internal_t *connection = &bleio_connections[i];
if (connection->conn_handle == conn_handle) {
connection->is_central = true;
// is_central was already set in _new_connection().
return bleio_connection_new_from_internal(connection);
}
}
Expand Down Expand Up @@ -498,7 +526,8 @@ static int _advertising_event(struct ble_gap_event *event, void *self_in) {

#if !MYNEWT_VAL(BLE_EXT_ADV)
if (event->connect.status == NIMBLE_OK) {
_new_connection(event->connect.conn_handle);
// Peripheral-role: no auto-pair, normal MTU exchange.
_new_connection(event->connect.conn_handle, false);
// Set connections objs back to NULL since we have a new
// connection and need a new tuple.
self->connection_objs = NULL;
Expand All @@ -511,7 +540,8 @@ static int _advertising_event(struct ble_gap_event *event, void *self_in) {
case BLE_GAP_EVENT_ADV_COMPLETE:
#if MYNEWT_VAL(BLE_EXT_ADV)
if (event->adv_complete.reason == NIMBLE_OK) {
_new_connection(event->adv_complete.conn_handle);
// Peripheral-role: no auto-pair, normal MTU exchange.
_new_connection(event->adv_complete.conn_handle, false);
// Set connections objs back to NULL since we have a new
// connection and need a new tuple.
self->connection_objs = NULL;
Expand Down
14 changes: 14 additions & 0 deletions ports/espressif/common-hal/_bleio/Connection.c
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,13 @@

#include "host/ble_att.h"

// Defined in Adapter.c - we need these to trigger MTU exchange after
// encryption completes and to unblock the connect() wait loop.
extern volatile int _connection_status;
extern int _mtu_reply(uint16_t conn_handle,
const struct ble_gatt_error *error,
uint16_t mtu, void *arg);

// Uncomment to turn on debug logging just in this file.
// #undef CIRCUITPY_VERBOSE_BLE
// #define CIRCUITPY_VERBOSE_BLE (1)
Expand Down Expand Up @@ -76,6 +83,13 @@ int bleio_connection_event_cb(struct ble_gap_event *event, void *connection_in)
if (desc.sec_state.encrypted) {
connection->pair_status = PAIR_PAIRED;
}
// For central-role connections the MTU exchange was deferred
// from _new_connection() so no unencrypted GATT traffic is sent
// before pairing. Do it once encryption is settled.
if (connection->is_central) {
ble_gattc_exchange_mtu(event->enc_change.conn_handle,
_mtu_reply, connection);
}
break;
}
case BLE_GAP_EVENT_MTU: {
Expand Down
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