bluetooth: esp32: fix VHCI semaphore misuse in HCI send path

The TX flow control semaphore was initialized with a count of 1 and
released again after every bt_esp32_send() call. This broke the VHCI
flow control contract and could allow sending more packets than the
controller advertised.

This prevents host/controller overruns and ensures proper synchronization
with the ESP32 VHCI interface.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>

Author: sylvioalves

drivers/bluetooth/hci/hci_esp32.c

@@ -25,7 +25,8 @@ struct bt_esp32_data {
 	bt_hci_recv_t recv;
 };
 
-static K_SEM_DEFINE(hci_send_sem, 1, 1);
+/* VHCI notifies when exactly one more HCI packet can be sent */
+static K_SEM_DEFINE(hci_send_sem, 0, 1);
 
 static bool is_hci_event_discardable(const uint8_t *evt_data)
 {
@@ -64,7 +65,7 @@ static struct net_buf *bt_esp_evt_recv(uint8_t *data, size_t remaining)
 		return NULL;
 	}
 
-	discardable = is_hci_event_discardable(data);
+	discardable = is_hci_event_discardable(data, remaining);
 
 	memcpy((void *)&hdr, data, sizeof(hdr));
 	data += sizeof(hdr);
@@ -242,22 +243,21 @@ static int bt_esp32_send(const struct device *dev, struct net_buf *buf)
 
 	LOG_HEXDUMP_DBG(buf->data, buf->len, "Final HCI buffer:");
 
-	if (!esp_vhci_host_check_send_available()) {
-		LOG_WRN("Controller not ready to receive packets");
-	}
-
-	if (k_sem_take(&hci_send_sem, HCI_BT_ESP32_TIMEOUT) == 0) {
-		esp_vhci_host_send_packet(buf->data, buf->len);
-	} else {
+	/* Wait for controller credit (callback gives the semaphore) */
+	if (k_sem_take(&hci_send_sem, HCI_BT_ESP32_TIMEOUT) != 0) {
 		LOG_ERR("Send packet timeout error");
 		err = -ETIMEDOUT;
+	} else {
+		if (!esp_vhci_host_check_send_available()) {
+			LOG_WRN("VHCI not available, sending anyway");
+		}
+		esp_vhci_host_send_packet(buf->data, buf->len);
 	}
 
 	if (!err) {
 		net_buf_unref(buf);
 	}
 
-	k_sem_give(&hci_send_sem);
 
 	return err;
 }
@@ -291,6 +291,10 @@ static int bt_esp32_ble_init(void)
 
 	esp_vhci_host_register_callback(&vhci_host_cb);
 
+	if (esp_vhci_host_check_send_available()) {
+		k_sem_give(&hci_send_sem);
+	}
+
 	return 0;
 }
 
@@ -318,6 +322,8 @@ static int bt_esp32_open(const struct device *dev, bt_hci_recv_t recv)
 	struct bt_esp32_data *hci = dev->data;
 	int err;
 
+	k_sem_reset(&hci_send_sem);
+
 	err = bt_esp32_ble_init();
 	if (err) {
 		return err;