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

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(); }