Fix USB VCP lockup on disconnect (issue #11348)

src/main/drivers/serial.c

@@ -111,7 +111,7 @@ void serialEndWrite(serialPort_t *instance)
 bool serialIsConnected(const serialPort_t *instance)
 {
     if (instance->vTable->isConnected)
-        instance->vTable->isConnected(instance);
+        return(instance->vTable->isConnected(instance));
 
     // If API is not defined - assume connected
     return true;

src/main/drivers/serial_usb_vcp.c

@@ -51,6 +51,17 @@ USBD_HandleTypeDef USBD_Device;
 
 static vcpPort_t vcpPort;
 
+// Track DTR (Data Terminal Ready) state - indicates if host has COM port open
+// Default to true - assume connected until host explicitly clears DTR
+static volatile bool cdcPortOpened = true;
+
+static void cdcCtrlLineStateCallback(void *context, uint16_t ctrlLineState)
+{
+    UNUSED(context);
+    // DTR is bit 0 of control line state
+    cdcPortOpened = (ctrlLineState & 0x01) != 0;
+}
+
 static void usbVcpSetBaudRate(serialPort_t *instance, uint32_t baudRate)
 {
     UNUSED(instance);
@@ -103,7 +114,8 @@ static uint8_t usbVcpRead(serialPort_t *instance)
 static bool usbVcpIsConnected(const serialPort_t *instance)
 {
     (void)instance;
-    return usbIsConnected() && usbIsConfigured();
+    // Check USB hardware state AND whether host has opened the COM port (DTR)
+    return usbIsConnected() && usbIsConfigured() && cdcPortOpened;
 }
 
 static void usbVcpWriteBuf(serialPort_t *instance, const void *data, int count)
@@ -209,6 +221,9 @@ void usbVcpInitHardware(void)
     IOInit(IOGetByTag(IO_TAG(PA11)), OWNER_USB, RESOURCE_INPUT, 0);
     IOInit(IOGetByTag(IO_TAG(PA12)), OWNER_USB, RESOURCE_OUTPUT, 0);
     USBD_Init(&USB_OTG_dev, USB_OTG_FS_CORE_ID, &USR_desc, &USBD_CDC_cb, &USR_cb);
+
+    // Register callback for DTR state changes
+    CDC_SetCtrlLineStateCb(cdcCtrlLineStateCallback, NULL);
 #elif defined(STM32F7) || defined(STM32H7)
     usbGenerateDisconnectPulse();
 
@@ -225,7 +240,10 @@ void usbVcpInitHardware(void)
 
     /* Start Device Process */
     USBD_Start(&USBD_Device);
-
+
+    // Register callback for DTR state changes
+    CDC_SetCtrlLineStateCb(cdcCtrlLineStateCallback, NULL);
+
 #ifdef STM32H7
     HAL_PWREx_EnableUSBVoltageDetector();
     delay(100); // Cold boot failures observed without this, even when USB cable is not connected

src/main/vcp_hal/usbd_cdc_interface.c

@@ -415,6 +415,8 @@ uint32_t CDC_Send_FreeBytes(void)
  * @param  sendLength: Number of data to be sent (in bytes)
  * @retval Bytes sent
  */
+#define VCP_WRITE_TIMEOUT_MS 50
+
 uint32_t CDC_Send_DATA(const uint8_t *ptrBuffer, uint32_t sendLength)
 {
 #if defined(STM32H7)
@@ -423,13 +425,22 @@ uint32_t CDC_Send_DATA(const uint8_t *ptrBuffer, uint32_t sendLength)
     USBD_CDC_HandleTypeDef *hcdc = (USBD_CDC_HandleTypeDef*)USBD_Device.pCDC_ClassData;
 #endif
 
-    while (hcdc->TxState != 0);
+    uint32_t start = millis();
+
+    while (hcdc->TxState != 0) {
+        if (millis() - start > VCP_WRITE_TIMEOUT_MS) {
+            return 0;
+        }
+    }
 
     for (uint32_t i = 0; i < sendLength; i++)
     {
         UserTxBuffer[UserTxBufPtrIn] = ptrBuffer[i];
         UserTxBufPtrIn = (UserTxBufPtrIn + 1) % APP_TX_DATA_SIZE;
         while (CDC_Send_FreeBytes() == 0) {
+            if (millis() - start > VCP_WRITE_TIMEOUT_MS) {
+                return i;  // Return partial count
+            }
             delay(1);
         }
     }

src/main/vcpf4/usbd_cdc_vcp.c

@@ -178,13 +178,24 @@ static uint16_t VCP_Ctrl(uint32_t Cmd, uint8_t* Buf, uint32_t Len)
  *******************************************************************************/
 uint32_t CDC_Send_DATA(const uint8_t *ptrBuffer, uint32_t sendLength)
 {
-    VCP_DataTx(ptrBuffer, sendLength);
+    if (VCP_DataTx(ptrBuffer, sendLength) != USBD_OK) {
+        return 0;
+    }
     return sendLength;
 }
 
 uint32_t CDC_Send_FreeBytes(void)
 {
-    return APP_RX_DATA_SIZE - CDC_Receive_BytesAvailable();
+    // Calculate free space in APP_Rx_Buffer (outbound to host)
+    // Using correct circular buffer math with volatile pointers
+    uint32_t ptr_in = APP_Rx_ptr_in;
+    uint32_t ptr_out = APP_Rx_ptr_out;
+
+    if (ptr_out > ptr_in) {
+        return ptr_out - ptr_in - 1;
+    } else {
+        return APP_RX_DATA_SIZE + ptr_out - ptr_in - 1;
+    }
 }
 
 /**
@@ -194,18 +205,29 @@ uint32_t CDC_Send_FreeBytes(void)
  * @param  Len: Number of data to be sent (in bytes)
  * @retval Result of the operation: USBD_OK if all operations are OK else VCP_FAIL
  */
+#define VCP_WRITE_TIMEOUT_MS 50
+
 static uint16_t VCP_DataTx(const uint8_t* Buf, uint32_t Len)
 {
+    uint32_t start = millis();
+
     /*
         make sure that any paragraph end frame is not in play
         could just check for: USB_CDC_ZLP, but better to be safe
         and wait for any existing transmission to complete.
     */
-    while (USB_Tx_State != 0);
+    while (USB_Tx_State != 0) {
+        if (millis() - start > VCP_WRITE_TIMEOUT_MS) {
+            return USBD_FAIL;
+        }
+    }
 
     for (uint32_t i = 0; i < Len; i++) {
         // Stall if the ring buffer is full
         while (((APP_Rx_ptr_in + 1) % APP_RX_DATA_SIZE) == APP_Rx_ptr_out) {
+            if (millis() - start > VCP_WRITE_TIMEOUT_MS) {
+                return USBD_FAIL;
+            }
             delay(1);
         }
 

src/main/vcpf4/usbd_usr.c

@@ -21,6 +21,7 @@
 
 #include "usbd_usr.h"
 #include "usbd_ioreq.h"
+#include "usbd_cdc_vcp.h"
 
 USBD_Usr_cb_TypeDef USR_cb =
 {
@@ -102,13 +103,17 @@ void USBD_USR_DeviceDisconnected (void)
 
 /**
 * @brief  USBD_USR_DeviceSuspended
-*         Displays the message on LCD on device suspend Event
+*         Handle device suspend event - treat as disconnect for safety.
+*         This helps on boards without VBUS sensing where hardware
+*         disconnect detection may not work.
 * @param  None
 * @retval None
 */
 void USBD_USR_DeviceSuspended(void)
 {
-  /* Users can do their application actions here for the USB-Reset */
+  // Treat suspend as disconnect - prevents blocking on USB writes
+  // when cable is unplugged on boards without VBUS sensing
+  bDeviceState = UNCONNECTED;
 }