add framework for properly receiving/handling incoming data..

The old method would introduce a race condition where the buffer
could be overwritten before the data was consumed.  For slow, single
character typing this was fine, but w/ packets being generated by
a computer, this would cause corruption of the incoming data..

stm32/usb/usbd_cdc_if.c

@@ -24,6 +24,7 @@
 
 /* USER CODE BEGIN INCLUDE */
 uint8_t* CDC_RX_BUFFER = NULL ;
+uint16_t CDC_RX_LEN = 0;
 /* USER CODE END INCLUDE */
 
 /* Private typedef -----------------------------------------------------------*/
@@ -264,8 +265,9 @@ static int8_t CDC_Receive_FS(uint8_t* Buf, uint32_t *Len)
 {
   /* USER CODE BEGIN 6 */
   USBD_CDC_SetRxBuffer(&hUsbDeviceFS, &Buf[0]);
-  USBD_CDC_ReceivePacket(&hUsbDeviceFS);
+
   CDC_RX_BUFFER = Buf;
+  CDC_RX_LEN = *Len;
   return (USBD_OK);
   /* USER CODE END 6 */
 }

stm32/usb/usbd_cdc_if.h

@@ -91,6 +91,16 @@
 extern USBD_CDC_ItfTypeDef USBD_Interface_fops_FS;
 
 /* USER CODE BEGIN EXPORTED_VARIABLES */
+extern USBD_HandleTypeDef hUsbDeviceFS;
+extern uint8_t* CDC_RX_BUFFER;
+extern uint16_t CDC_RX_LEN;
+/*
+ * when CDC_RX_LEN != 0, CDC_RX_LEN bytes are available in CDC_RX_BUFFER.
+ * Once all bytes have been consumed from the buffer, set CDC_RX_LEN to
+ * zero and call:
+ *     USBD_CDC_ReceivePacket(&hUsbDeviceFS);
+ * to receive more data.
+ */
 
 /* USER CODE END EXPORTED_VARIABLES */