feat(core): Optionally disable endpoint fallback

app/Kconfig

@@ -145,6 +145,9 @@ config USB_HID_POLL_INTERVAL_MS
 #ZMK_USB
 endif
 
+config ZMK_ENDPOINT_DISABLE_FALLBACK
+    bool "Disable automatic endpoint fallback when preferred endpoint is unavailable"
+
 menuconfig ZMK_BLE
     bool "BLE (HID over GATT)"
     select BT

app/include/zmk/endpoints.h

@@ -33,7 +33,7 @@
  * Note that this value may change between firmware versions, so it should not
  * be used in any persistent storage.
  */
-#define ZMK_ENDPOINT_COUNT (ZMK_ENDPOINT_USB_COUNT + ZMK_ENDPOINT_BLE_COUNT)
+#define ZMK_ENDPOINT_COUNT (ZMK_ENDPOINT_USB_COUNT + ZMK_ENDPOINT_BLE_COUNT + 1)
 
 bool zmk_endpoint_instance_eq(struct zmk_endpoint_instance a, struct zmk_endpoint_instance b);
 
@@ -64,10 +64,15 @@ int zmk_endpoints_select_transport(enum zmk_transport transport);
 int zmk_endpoints_toggle_transport(void);
 
 /**
- * Gets the currently-selected endpoint.
+ * Gets the currently in use endpoint.
  */
 struct zmk_endpoint_instance zmk_endpoints_selected(void);
 
+/**
+ * Gets the preferred endpoint.
+ */
+struct zmk_endpoint_instance zmk_endpoints_preferred(void);
+
 int zmk_endpoints_send_report(uint16_t usage_page);
 
 #if IS_ENABLED(CONFIG_ZMK_MOUSE)

app/include/zmk/endpoints_types.h

@@ -10,6 +10,7 @@
  * The method by which data is sent.
  */
 enum zmk_transport {
+    ZMK_TRANSPORT_NONE,
     ZMK_TRANSPORT_USB,
     ZMK_TRANSPORT_BLE,
 };

app/src/endpoints.c

@@ -23,8 +23,12 @@
 #include <zephyr/logging/log.h>
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
+#if IS_ENABLED(CONFIG_ZMK_ENDPOINT_DISABLE_FALLBACK)
+#define DEFAULT_TRANSPORT ZMK_TRANSPORT_NONE
+#else
 #define DEFAULT_TRANSPORT                                                                          \
     COND_CODE_1(IS_ENABLED(CONFIG_ZMK_BLE), (ZMK_TRANSPORT_BLE), (ZMK_TRANSPORT_USB))
+#endif
 
 static struct zmk_endpoint_instance current_instance = {};
 static enum zmk_transport preferred_transport =
@@ -59,6 +63,9 @@ bool zmk_endpoint_instance_eq(struct zmk_endpoint_instance a, struct zmk_endpoin
 
     case ZMK_TRANSPORT_BLE:
         return a.ble.profile_index == b.ble.profile_index;
+
+    case ZMK_TRANSPORT_NONE:
+        return true;
     }
 
     LOG_ERR("Invalid transport %d", a.transport);
@@ -73,13 +80,17 @@ int zmk_endpoint_instance_to_str(struct zmk_endpoint_instance endpoint, char *st
     case ZMK_TRANSPORT_BLE:
         return snprintf(str, len, "BLE:%d", endpoint.ble.profile_index);
 
+    case ZMK_TRANSPORT_NONE:
+        return snprintf(str, len, "None");
+
     default:
         return snprintf(str, len, "Invalid");
     }
 }
 
 #define INSTANCE_INDEX_OFFSET_USB 0
 #define INSTANCE_INDEX_OFFSET_BLE ZMK_ENDPOINT_USB_COUNT
+#define INSTANCE_INDEX_OFFSET_NONE ZMK_ENDPOINT_USB_COUNT + ZMK_ENDPOINT_BLE_COUNT
 
 int zmk_endpoint_instance_to_index(struct zmk_endpoint_instance endpoint) {
     switch (endpoint.transport) {
@@ -88,6 +99,9 @@ int zmk_endpoint_instance_to_index(struct zmk_endpoint_instance endpoint) {
 
     case ZMK_TRANSPORT_BLE:
         return INSTANCE_INDEX_OFFSET_BLE + endpoint.ble.profile_index;
+
+    case ZMK_TRANSPORT_NONE:
+        return INSTANCE_INDEX_OFFSET_NONE;
     }
 
     LOG_ERR("Invalid transport %d", endpoint.transport);
@@ -118,6 +132,24 @@ int zmk_endpoints_toggle_transport(void) {
 
 struct zmk_endpoint_instance zmk_endpoints_selected(void) { return current_instance; }
 
+struct zmk_endpoint_instance zmk_endpoints_preferred(void) {
+    struct zmk_endpoint_instance instance = {.transport = preferred_transport};
+
+    switch (instance.transport) {
+#if IS_ENABLED(CONFIG_ZMK_BLE)
+    case ZMK_TRANSPORT_BLE:
+        instance.ble.profile_index = zmk_ble_active_profile_index();
+        break;
+#endif // IS_ENABLED(CONFIG_ZMK_BLE)
+
+    default:
+        // No extra data for this transport.
+        break;
+    }
+
+    return instance;
+}
+
 static int send_keyboard_report(void) {
     switch (current_instance.transport) {
     case ZMK_TRANSPORT_USB: {
@@ -146,6 +178,8 @@ static int send_keyboard_report(void) {
         return -ENOTSUP;
 #endif /* IS_ENABLED(CONFIG_ZMK_BLE) */
     }
+    case ZMK_TRANSPORT_NONE:
+        return 0;
     }
 
     LOG_ERR("Unhandled endpoint transport %d", current_instance.transport);
@@ -180,6 +214,8 @@ static int send_consumer_report(void) {
         return -ENOTSUP;
 #endif /* IS_ENABLED(CONFIG_ZMK_BLE) */
     }
+    case ZMK_TRANSPORT_NONE:
+        return 0;
     }
 
     LOG_ERR("Unhandled endpoint transport %d", current_instance.transport);
@@ -230,6 +266,8 @@ int zmk_endpoints_send_mouse_report() {
         return -ENOTSUP;
 #endif /* IS_ENABLED(CONFIG_ZMK_BLE) */
     }
+    case ZMK_TRANSPORT_NONE:
+        return 0;
     }
 
     LOG_ERR("Unhandled endpoint transport %d", current_instance.transport);
@@ -282,6 +320,17 @@ static bool is_ble_ready(void) {
 }
 
 static enum zmk_transport get_selected_transport(void) {
+#if IS_ENABLED(CONFIG_ZMK_ENDPOINT_DISABLE_FALLBACK)
+    switch (preferred_transport) {
+    case ZMK_TRANSPORT_BLE:
+        return is_ble_ready() ? ZMK_TRANSPORT_BLE : ZMK_TRANSPORT_NONE;
+    case ZMK_TRANSPORT_USB:
+        return is_usb_ready() ? ZMK_TRANSPORT_USB : ZMK_TRANSPORT_NONE;
+
+    default:
+        break;
+    }
+#else
     if (is_ble_ready()) {
         if (is_usb_ready()) {
             LOG_DBG("Both endpoint transports are ready. Using %d", preferred_transport);
@@ -296,6 +345,7 @@ static enum zmk_transport get_selected_transport(void) {
         LOG_DBG("Only USB is ready.");
         return ZMK_TRANSPORT_USB;
     }
+#endif
 
     LOG_DBG("No endpoint transports are ready.");
     return DEFAULT_TRANSPORT;