Add hidden Serial overflow debug options

Marlin/src/HAL/HAL_AVR/MarlinSerial.cpp

     uint8_t rx_dropped_bytes = 0;
   #endif
 
+  #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+    uint8_t rx_buffer_overruns = 0;
+  #endif
+
+  #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+    uint8_t rx_framing_errors = 0;
+  #endif
+
   #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
     ring_buffer_pos_t rx_max_enqueued = 0;
   #endif
     // Get the next element
     ring_buffer_pos_t i = (ring_buffer_pos_t)(h + 1) & (ring_buffer_pos_t)(RX_BUFFER_SIZE - 1);
 
+    // This must read the M_UCSRxA register before reading the received byte to detect error causes
+    #if ENABLED(SERIAL_STATS_DROPPED_RX)
+      if (TEST(M_UCSRxA, M_DORx) && !++rx_dropped_bytes) --rx_dropped_bytes;
+    #endif
+
+    #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+      if (TEST(M_UCSRxA, M_DORx) && !++rx_buffer_overruns) --rx_buffer_overruns;
+    #endif
+
+    #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+      if (TEST(M_UCSRxA, M_FEx) && !++rx_framing_errors) --rx_framing_errors;
+    #endif
+
     // Read the character from the USART
     uint8_t c = M_UDRx;
 

Marlin/src/HAL/HAL_AVR/MarlinSerial.h

 #define M_TXCx             SERIAL_REGNAME(TXC,SERIAL_PORT,)
 #define M_RXCIEx           SERIAL_REGNAME(RXCIE,SERIAL_PORT,)
 #define M_UDREx            SERIAL_REGNAME(UDRE,SERIAL_PORT,)
+#define M_FEx              SERIAL_REGNAME(FE,SERIAL_PORT,)
+#define M_DORx             SERIAL_REGNAME(DOR,SERIAL_PORT,)
+#define M_UPEx             SERIAL_REGNAME(UPE,SERIAL_PORT,)
 #define M_UDRIEx           SERIAL_REGNAME(UDRIE,SERIAL_PORT,)
 #define M_UDRx             SERIAL_REGNAME(UDR,SERIAL_PORT,)
 #define M_UBRRxH           SERIAL_REGNAME(UBRR,SERIAL_PORT,H)
     extern uint8_t rx_dropped_bytes;
   #endif
 
+  #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+    extern uint8_t rx_buffer_overruns;
+  #endif
+
+  #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+    extern uint8_t rx_framing_errors;
+  #endif
+
   #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
     extern ring_buffer_pos_t rx_max_enqueued;
   #endif
         FORCE_INLINE static uint32_t dropped() { return rx_dropped_bytes; }
       #endif
 
+      #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+        FORCE_INLINE static uint32_t buffer_overruns() { return rx_buffer_overruns; }
+      #endif
+
+      #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+        FORCE_INLINE static uint32_t framing_errors() { return rx_framing_errors; }
+      #endif
+
       #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
         FORCE_INLINE static ring_buffer_pos_t rxMaxEnqueued() { return rx_max_enqueued; }
       #endif

Marlin/src/HAL/HAL_DUE/MarlinSerial_Due.cpp

     uint8_t rx_dropped_bytes = 0;
   #endif
 
+  #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+    uint8_t rx_buffer_overruns = 0;
+  #endif
+
+  #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+    uint8_t rx_framing_errors = 0;
+  #endif
+
   #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
     ring_buffer_pos_t rx_max_enqueued = 0;
   #endif
 
     // Acknowledge errors
     if ((status & UART_SR_OVRE) || (status & UART_SR_FRAME)) {
+
+      #if ENABLED(SERIAL_STATS_DROPPED_RX)
+        if (status & UART_SR_OVRE && !++rx_dropped_bytes) --rx_dropped_bytes;
+      #endif
+
+      #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+        if (status & UART_SR_OVRE && !++rx_buffer_overruns) --rx_buffer_overruns;
+      #endif
+
+      #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+        if (status & UART_SR_FRAME && !++rx_framing_errors) --rx_framing_errors;
+      #endif
+
       // TODO: error reporting outside ISR
       HWUART->UART_CR = UART_CR_RSTSTA;
     }

Marlin/src/HAL/HAL_DUE/MarlinSerial_Due.h

   extern uint8_t rx_dropped_bytes;
 #endif
 
+#if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+  extern uint8_t rx_buffer_overruns;
+#endif
+
+#if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+  extern uint8_t rx_framing_errors;
+#endif
+
 #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
   extern ring_buffer_pos_t rx_max_enqueued;
 #endif
     FORCE_INLINE static uint32_t dropped() { return rx_dropped_bytes; }
   #endif
 
+  #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+    FORCE_INLINE static uint32_t buffer_overruns() { return rx_buffer_overruns; }
+  #endif
+
+  #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+    FORCE_INLINE static uint32_t framing_errors() { return rx_framing_errors; }
+  #endif
+
   #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
     FORCE_INLINE static ring_buffer_pos_t rxMaxEnqueued() { return rx_max_enqueued; }
   #endif

Marlin/src/gcode/control/M111.cpp

   }
   else {
     SERIAL_ECHOPGM(MSG_DEBUG_OFF);
+    #if !defined(__AVR__) || !defined(USBCON)
+      #if ENABLED(SERIAL_STATS_RX_BUFFER_OVERRUNS)
+        SERIAL_ECHOLNPAIR("Buffer Overruns: ", customizedSerial.buffer_overruns());
+      #endif
+
+      #if ENABLED(SERIAL_STATS_RX_FRAMING_ERRORS)
+        SERIAL_ECHOLNPAIR("Framing Errors: ", customizedSerial.framing_errors());
+      #endif
+
+      #if ENABLED(SERIAL_STATS_DROPPED_RX)
+        SERIAL_ECHOLNPAIR("Dropped bytes: ", customizedSerial.dropped());
+      #endif
+
+      #if ENABLED(SERIAL_STATS_MAX_RX_QUEUED)
+        SERIAL_ECHOLNPAIR("Max RX Queue Size: ", customizedSerial.rxMaxEnqueued());
+      #endif
+    #endif //  !defined(__AVR__) || !defined(USBCON)
   }
   SERIAL_EOL();
 }