stmhal: Fix issues with USB CDC init and receive.

Late USB enumeration could clear settings after they had been set.
Now fixed by not clearing some settings on init.

RX was blocking if received characters were not being processed, so
CTRL-C would not be picked up.  Now "fixed" by not blocking, but
instead discarding incoming characters if they overflow the buffer.

stmhal/usbd_cdc_interface.c

@@ -46,7 +46,7 @@
 
 /* Private typedef -----------------------------------------------------------*/
 /* Private define ------------------------------------------------------------*/
-#define APP_RX_DATA_SIZE  1024 // I think this must be at least CDC_DATA_FS_OUT_PACKET_SIZE (was 2048)
+#define APP_RX_DATA_SIZE  1024 // I think this must be at least CDC_DATA_FS_OUT_PACKET_SIZE=64 (APP_RX_DATA_SIZE was 2048)
 #define APP_TX_DATA_SIZE  1024 // I think this can be any value (was 2048)
 
 /* Private macro -------------------------------------------------------------*/
@@ -138,8 +138,13 @@ static int8_t CDC_Itf_Init(void)
     UserRxBufCur = 0;
     UserRxBufLen = 0;
   
+    /* NOTE: we cannot reset these here, because USBD_CDC_SetInterrupt
+     * may be called before this init function to set these values.
+     * This can happen if the USB enumeration occurs after the call to
+     * USBD_CDC_SetInterrupt.
     user_interrupt_char = VCP_CHAR_NONE;
     user_interrupt_data = NULL;
+    */
 
     return (USBD_OK);
 }
@@ -252,7 +257,7 @@ void USBD_CDC_HAL_TIM_PeriodElapsedCallback(void) {
   {
     if(UserTxBufPtrOut > UserTxBufPtrIn) /* rollback */
     {
-      buffsize = APP_RX_DATA_SIZE - UserTxBufPtrOut;
+      buffsize = APP_TX_DATA_SIZE - UserTxBufPtrOut;
     }
     else 
     {
@@ -266,7 +271,7 @@ void USBD_CDC_HAL_TIM_PeriodElapsedCallback(void) {
     if(USBD_CDC_TransmitPacket(&hUSBDDevice) == USBD_OK)
     {
       UserTxBufPtrOut += buffsize;
-      if (UserTxBufPtrOut == APP_RX_DATA_SIZE)
+      if (UserTxBufPtrOut == APP_TX_DATA_SIZE)
       {
         UserTxBufPtrOut = 0;
       }
@@ -289,9 +294,20 @@ static int8_t CDC_Itf_Receive(uint8_t* Buf, uint32_t *Len) {
     HAL_UART_Transmit_DMA(&UartHandle, Buf, *Len);
 #endif
 
+    // TODO improve this function to implement a circular buffer
+
+    // if we have processed all the characters, reset the buffer counters
+    if (UserRxBufCur > 0 && UserRxBufCur >= UserRxBufLen) {
+        memmove(UserRxBuffer, UserRxBuffer + UserRxBufLen, *Len);
+        UserRxBufCur = 0;
+        UserRxBufLen = 0;
+    }
+
+    uint32_t delta_len;
+
     if (user_interrupt_char == VCP_CHAR_NONE) {
         // no special interrupt character
-        UserRxBufLen = *Len;
+        delta_len = *Len;
 
     } else {
         // filter out sepcial interrupt character from the buffer
@@ -310,25 +326,29 @@ static int8_t CDC_Itf_Receive(uint8_t* Buf, uint32_t *Len) {
             }
         }
 
-        // set length of remaining characters
-        UserRxBufLen = dest - Buf;
-
         if (char_found) {
             // raise exception when interrupts are finished
             user_interrupt_char = VCP_CHAR_NONE;
             pendsv_nlr_jump(user_interrupt_data);
         }
+
+        // length of remaining characters
+        delta_len = dest - Buf;
     }
 
-    // there are new characters at the start of the buffer, so point there
-    UserRxBufCur = 0;
-
-    if (UserRxBufLen == 0) {
-        // initiate next USB packet transfer now that UserRxBuffer has been drained
-        USBD_CDC_ReceivePacket(&hUSBDDevice);
+    if (UserRxBufLen + delta_len + CDC_DATA_FS_MAX_PACKET_SIZE > APP_RX_DATA_SIZE) {
+        // if we keep this data then the buffer can overflow on the next USB rx
+        // so we don't increment the length, and throw this data away
+    } else {
+        // data fits, leaving room for another CDC_DATA_FS_OUT_PACKET_SIZE
+        UserRxBufLen += delta_len;
     }
 
-    return (USBD_OK);
+    // initiate next USB packet transfer, to append to existing data in buffer
+    USBD_CDC_SetRxBuffer(&hUSBDDevice, UserRxBuffer + UserRxBufLen);
+    USBD_CDC_ReceivePacket(&hUSBDDevice);
+
+    return USBD_OK;
 }
 
 int USBD_CDC_IsConnected(void) {
@@ -366,9 +386,6 @@ int USBD_CDC_RxGet(void) {
 
     // get next character
     int c = UserRxBuffer[UserRxBufCur++];
-    if (UserRxBufCur >= UserRxBufLen) {
-        // initiate next USB packet transfer now that UserRxBuffer has been drained
-        USBD_CDC_ReceivePacket(&hUSBDDevice);
-    }
+
     return c;
 }