Merge pull request #3058 from KentLee86/fix-cdc-null-rx-buffer

fix(cdc): stop arming OUT endpoint with a NULL buffer

Author: fpistm

libraries/USBDevice/inc/usbd_cdc.h

@@ -148,12 +148,10 @@ uint8_t USBD_CDC_RegisterInterface(USBD_HandleTypeDef *pdev,
 uint8_t USBD_CDC_SetTxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff,
                              uint32_t length, uint8_t ClassId);
 uint8_t USBD_CDC_TransmitPacket(USBD_HandleTypeDef *pdev, uint8_t ClassId);
-uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev, uint8_t ClassId);
 #else
 uint8_t USBD_CDC_SetTxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff,
                              uint32_t length);
 uint8_t USBD_CDC_TransmitPacket(USBD_HandleTypeDef *pdev);
-uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev);
 #endif /* USE_USBD_COMPOSITE */
 uint8_t USBD_CDC_SetRxBuffer(USBD_HandleTypeDef *pdev, uint8_t *pbuff);
 uint8_t USBD_CDC_ReceivePacket(USBD_HandleTypeDef *pdev);

libraries/USBDevice/src/cdc/usbd_cdc.c

@@ -1029,24 +1029,6 @@ uint8_t USBD_CDC_ReceivePacket(USBD_HandleTypeDef *pdev)
 
   return (uint8_t)USBD_OK;
 }
-#ifdef USE_USBD_COMPOSITE
-uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev, uint8_t ClassId)
-{
-  /* Suspend or Resume USB Out process */
-  if (pdev->pClassDataCmsit[classId] != NULL) {
-#else
-uint8_t USBD_CDC_ClearBuffer(USBD_HandleTypeDef *pdev)
-{
-  /* Suspend or Resume USB Out process */
-  if (pdev->pClassDataCmsit[pdev->classId] != NULL) {
-#endif /* USE_USBD_COMPOSITE */
-    /* Prepare Out endpoint to receive next packet */
-    USBD_LL_PrepareReceive(pdev, CDC_OUT_EP, 0, 0);
-    return (uint8_t)USBD_OK;
-  } else {
-    return (uint8_t)USBD_FAIL;
-  }
-}
 
 #endif /* USBD_USE_CDC */
 #endif /* USBCON */

libraries/USBDevice/src/cdc/usbd_cdc_if.c

@@ -240,10 +240,15 @@ static int8_t USBD_CDC_Receive(uint8_t *Buf, uint32_t *Len)
   /* It always contains required amount of free space for writing */
   CDC_ReceiveQueue_CommitBlock(&ReceiveQueue, (uint16_t)(*Len));
   receivePended = false;
-  /* If enough space in the queue for a full buffer then continue receive */
-  if (!CDC_resume_receive()) {
-    USBD_CDC_ClearBuffer(&hUSBD_Device_CDC);
-  }
+  /*
+   * If there is enough space in the queue for a full packet, continue receive.
+   * Else leave the OUT endpoint unarmed: it then NAKs and the host retries.
+   * USBSerial::read() and the readBytes() family call CDC_resume_receive()
+   * after dequeuing, so the endpoint is armed again with a valid block as soon
+   * as the sketch drains data. Arming it with a NULL buffer instead makes the
+   * low level driver copy the retried packet to address 0.
+   */
+  (void)CDC_resume_receive();
   return ((int8_t)USBD_OK);
 }