[esp32_ble] Reduce GATT event latency from 8ms to 12μs with notification socket (#11663)

esphome/components/esp32_ble/__init__.py

@@ -7,6 +7,7 @@ from typing import Any
 
 from esphome import automation
 import esphome.codegen as cg
+from esphome.components import socket
 from esphome.components.esp32 import add_idf_sdkconfig_option, const, get_esp32_variant
 import esphome.config_validation as cv
 from esphome.const import (
@@ -481,6 +482,14 @@ async def to_code(config):
         cg.add(var.set_name(name))
     await cg.register_component(var, config)
 
+    # BLE uses 1 UDP socket for event notification to wake up main loop from select()
+    # This enables low-latency (~12μs) BLE event processing instead of waiting for
+    # select() timeout (0-16ms). The socket is created in ble_setup_() and used to
+    # wake lwip_select() when BLE events arrive from the BLE thread.
+    # Note: Called during config generation, socket is created at runtime. In practice,
+    # always used since esp32_ble only runs on ESP32 which always has USE_SOCKET_SELECT_SUPPORT.
+    socket.consume_sockets(1, "esp32_ble")(config)
+
     # Define max connections for use in C++ code (e.g., ble_server.h)
     max_connections = config.get(CONF_MAX_CONNECTIONS, DEFAULT_MAX_CONNECTIONS)
     cg.add_define("USE_ESP32_BLE_MAX_CONNECTIONS", max_connections)

esphome/components/esp32_ble/ble.cpp

@@ -27,6 +27,10 @@ extern "C" {
 #include <esp32-hal-bt.h>
 #endif
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+#include <lwip/sockets.h>
+#endif
+
 namespace esphome::esp32_ble {
 
 static const char *const TAG = "esp32_ble";
@@ -293,10 +297,21 @@ bool ESP32BLE::ble_setup_() {
   // BLE takes some time to be fully set up, 200ms should be more than enough
   delay(200);  // NOLINT
 
+  // Set up notification socket to wake main loop for BLE events
+  // This enables low-latency (~12μs) event processing instead of waiting for select() timeout
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  this->setup_event_notification_();
+#endif
+
   return true;
 }
 
 bool ESP32BLE::ble_dismantle_() {
+  // Clean up notification socket first before dismantling BLE stack
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  this->cleanup_event_notification_();
+#endif
+
   esp_err_t err = esp_bluedroid_disable();
   if (err != ESP_OK) {
     ESP_LOGE(TAG, "esp_bluedroid_disable failed: %d", err);
@@ -394,6 +409,12 @@ void ESP32BLE::loop() {
       break;
   }
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  // Drain any notification socket events first
+  // This clears the socket so it doesn't stay "ready" in subsequent select() calls
+  this->drain_event_notifications_();
+#endif
+
   BLEEvent *ble_event = this->ble_events_.pop();
   while (ble_event != nullptr) {
     switch (ble_event->type_) {
@@ -582,6 +603,10 @@ void ESP32BLE::gap_event_handler(esp_gap_ble_cb_event_t event, esp_ble_gap_cb_pa
 void ESP32BLE::gatts_event_handler(esp_gatts_cb_event_t event, esp_gatt_if_t gatts_if,
                                    esp_ble_gatts_cb_param_t *param) {
   enqueue_ble_event(event, gatts_if, param);
+  // Wake up main loop to process GATT event immediately
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  global_ble->notify_main_loop_();
+#endif
 }
 #endif
 
@@ -589,6 +614,10 @@ void ESP32BLE::gatts_event_handler(esp_gatts_cb_event_t event, esp_gatt_if_t gat
 void ESP32BLE::gattc_event_handler(esp_gattc_cb_event_t event, esp_gatt_if_t gattc_if,
                                    esp_ble_gattc_cb_param_t *param) {
   enqueue_ble_event(event, gattc_if, param);
+  // Wake up main loop to process GATT event immediately
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  global_ble->notify_main_loop_();
+#endif
 }
 #endif
 
@@ -628,6 +657,89 @@ void ESP32BLE::dump_config() {
   }
 }
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+void ESP32BLE::setup_event_notification_() {
+  // Create UDP socket for event notifications
+  this->notify_fd_ = lwip_socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+  if (this->notify_fd_ < 0) {
+    ESP_LOGW(TAG, "Event socket create failed: %d", errno);
+    return;
+  }
+
+  // Bind to loopback with auto-assigned port
+  struct sockaddr_in addr = {};
+  addr.sin_family = AF_INET;
+  addr.sin_addr.s_addr = lwip_htonl(INADDR_LOOPBACK);
+  addr.sin_port = 0;  // Auto-assign port
+
+  if (lwip_bind(this->notify_fd_, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
+    ESP_LOGW(TAG, "Event socket bind failed: %d", errno);
+    lwip_close(this->notify_fd_);
+    this->notify_fd_ = -1;
+    return;
+  }
+
+  // Get the assigned address and connect to it
+  // Connecting a UDP socket allows using send() instead of sendto() for better performance
+  struct sockaddr_in notify_addr;
+  socklen_t len = sizeof(notify_addr);
+  if (lwip_getsockname(this->notify_fd_, (struct sockaddr *) &notify_addr, &len) < 0) {
+    ESP_LOGW(TAG, "Event socket address failed: %d", errno);
+    lwip_close(this->notify_fd_);
+    this->notify_fd_ = -1;
+    return;
+  }
+
+  // Connect to self (loopback) - allows using send() instead of sendto()
+  // After connect(), no need to store notify_addr - the socket remembers it
+  if (lwip_connect(this->notify_fd_, (struct sockaddr *) &notify_addr, sizeof(notify_addr)) < 0) {
+    ESP_LOGW(TAG, "Event socket connect failed: %d", errno);
+    lwip_close(this->notify_fd_);
+    this->notify_fd_ = -1;
+    return;
+  }
+
+  // Set non-blocking mode
+  int flags = lwip_fcntl(this->notify_fd_, F_GETFL, 0);
+  lwip_fcntl(this->notify_fd_, F_SETFL, flags | O_NONBLOCK);
+
+  // Register with application's select() loop
+  if (!App.register_socket_fd(this->notify_fd_)) {
+    ESP_LOGW(TAG, "Event socket register failed");
+    lwip_close(this->notify_fd_);
+    this->notify_fd_ = -1;
+    return;
+  }
+
+  ESP_LOGD(TAG, "Event socket ready");
+}
+
+void ESP32BLE::cleanup_event_notification_() {
+  if (this->notify_fd_ >= 0) {
+    App.unregister_socket_fd(this->notify_fd_);
+    lwip_close(this->notify_fd_);
+    this->notify_fd_ = -1;
+    ESP_LOGD(TAG, "Event socket closed");
+  }
+}
+
+void ESP32BLE::drain_event_notifications_() {
+  // Called from main loop to drain any pending notifications
+  // Must check is_socket_ready() to avoid blocking on empty socket
+  if (this->notify_fd_ >= 0 && App.is_socket_ready(this->notify_fd_)) {
+    char buffer[BLE_EVENT_NOTIFY_DRAIN_BUFFER_SIZE];
+    // Drain all pending notifications with non-blocking reads
+    // Multiple BLE events may have triggered multiple writes, so drain until EWOULDBLOCK
+    // We control both ends of this loopback socket (always write 1 byte per event),
+    // so no error checking needed - any errors indicate catastrophic system failure
+    while (lwip_recvfrom(this->notify_fd_, buffer, sizeof(buffer), 0, nullptr, nullptr) > 0) {
+      // Just draining, no action needed - actual BLE events are already queued
+    }
+  }
+}
+
+#endif  // USE_SOCKET_SELECT_SUPPORT
+
 uint64_t ble_addr_to_uint64(const esp_bd_addr_t address) {
   uint64_t u = 0;
   u |= uint64_t(address[0] & 0xFF) << 40;

esphome/components/esp32_ble/ble.h

@@ -25,6 +25,10 @@
 #include <esp_gattc_api.h>
 #include <esp_gatts_api.h>
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+#include <lwip/sockets.h>
+#endif
+
 namespace esphome::esp32_ble {
 
 // Maximum size of the BLE event queue
@@ -162,6 +166,13 @@ class ESP32BLE : public Component {
   void advertising_init_();
 #endif
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  void setup_event_notification_();    // Create notification socket
+  void cleanup_event_notification_();  // Close and unregister socket
+  inline void notify_main_loop_();     // Wake up select() from BLE thread (hot path - inlined)
+  void drain_event_notifications_();   // Read pending notifications in main loop
+#endif
+
  private:
   template<typename... Args> friend void enqueue_ble_event(Args... args);
 
@@ -196,6 +207,13 @@ class ESP32BLE : public Component {
   esp_ble_io_cap_t io_cap_{ESP_IO_CAP_NONE};  // 4 bytes (enum)
   uint32_t advertising_cycle_time_{};         // 4 bytes
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+  // Event notification socket for waking up main loop from BLE thread
+  // Uses connected UDP loopback socket to wake lwip_select() with ~12μs latency vs 0-16ms timeout
+  // Socket is connected during setup, allowing use of send() instead of sendto() for efficiency
+  int notify_fd_{-1};  // 4 bytes (file descriptor)
+#endif
+
   // 2-byte aligned members
   uint16_t appearance_{0};  // 2 bytes
 
@@ -207,6 +225,29 @@ class ESP32BLE : public Component {
 // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
 extern ESP32BLE *global_ble;
 
+#ifdef USE_SOCKET_SELECT_SUPPORT
+// Inline implementations for hot-path functions
+// These are called from BLE thread (notify) and main loop (drain) on every event
+
+// Small buffer for draining notification bytes (1 byte sent per BLE event)
+// Size allows draining multiple notifications per recvfrom() without wasting stack
+static constexpr size_t BLE_EVENT_NOTIFY_DRAIN_BUFFER_SIZE = 16;
+
+inline void ESP32BLE::notify_main_loop_() {
+  // Called from BLE thread context when events are queued
+  // Wakes up lwip_select() in main loop by writing to connected loopback socket
+  if (this->notify_fd_ >= 0) {
+    const char dummy = 1;
+    // Non-blocking send - if it fails (unlikely), select() will wake on timeout anyway
+    // No error checking needed: we control both ends of this loopback socket, and the
+    // BLE event is already queued. Notification is best-effort to reduce latency.
+    // This is safe to call from BLE thread - send() is thread-safe in lwip
+    // Socket is already connected to loopback address, so send() is faster than sendto()
+    lwip_send(this->notify_fd_, &dummy, 1, 0);
+  }
+}
+#endif  // USE_SOCKET_SELECT_SUPPORT
+
 template<typename... Ts> class BLEEnabledCondition : public Condition<Ts...> {
  public:
   bool check(Ts... x) override { return global_ble->is_active(); }