Protect MarlinSerial against interrupts

Protect MarlinSerial against interrupts
by shielding the CRITICAL_SECTIONs

Now with a test if RX_BUFFER_SIZE is in the required range.
Code in peek() and read() is now optimized for readability and showing the similarity between the two.

Maybe a bit overprotected in checkRx() and store_char(), but now some days without detected errors from this source.

Marlin/MarlinSerial.cpp

@@ -33,16 +33,19 @@
 #endif
 
 FORCE_INLINE void store_char(unsigned char c) {
-  uint8_t i = (uint8_t)(rx_buffer.head + 1) % RX_BUFFER_SIZE;
+  CRITICAL_SECTION_START;
+    uint8_t h = rx_buffer.head;
+    uint8_t i = (uint8_t)(h + 1)  & (RX_BUFFER_SIZE - 1);
 
     // if we should be storing the received character into the location
     // just before the tail (meaning that the head would advance to the
     // current location of the tail), we're about to overflow the buffer
     // and so we don't write the character or advance the head.
     if (i != rx_buffer.tail) {
-    rx_buffer.buffer[rx_buffer.head] = c;
+      rx_buffer.buffer[h] = c;
       rx_buffer.head = i;
     }
+  CRITICAL_SECTION_END;
 }
 
 
@@ -101,24 +104,32 @@ void MarlinSerial::end() {
 
 
 int MarlinSerial::peek(void) {
-  if (rx_buffer.head == rx_buffer.tail) {
-    return -1;
+  int v;
+  CRITICAL_SECTION_START;
+  uint8_t t = rx_buffer.tail;
+  if (rx_buffer.head == t) {
+    v = -1;
   }
   else {
-    return rx_buffer.buffer[rx_buffer.tail];
+    v = rx_buffer.buffer[t];
   }
+  CRITICAL_SECTION_END;
+  return v;
 }
 
 int MarlinSerial::read(void) {
-  // if the head isn't ahead of the tail, we don't have any characters
-  if (rx_buffer.head == rx_buffer.tail) {
-    return -1;
+  int v;
+  CRITICAL_SECTION_START;
+  uint8_t t = rx_buffer.tail;
+  if (rx_buffer.head == t) {
+    v = -1;
   }
   else {
-    unsigned char c = rx_buffer.buffer[rx_buffer.tail];
-    rx_buffer.tail = (uint8_t)(rx_buffer.tail + 1) % RX_BUFFER_SIZE;
-    return c;
+    v = rx_buffer.buffer[t];
+    rx_buffer.tail = (uint8_t)(t + 1) & (RX_BUFFER_SIZE - 1);
   }
+  CRITICAL_SECTION_END;
+  return v;
 }
 
 void MarlinSerial::flush() {
@@ -131,7 +142,9 @@ void MarlinSerial::flush() {
   // the value to rx_buffer_tail; the previous value of rx_buffer_head
   // may be written to rx_buffer_tail, making it appear as if the buffer
   // were full, not empty.
+  CRITICAL_SECTION_START;
     rx_buffer.head = rx_buffer.tail;
+  CRITICAL_SECTION_END;
 }
 
 

Marlin/MarlinSerial.h

@@ -23,6 +23,12 @@
 #define MarlinSerial_h
 #include "Marlin.h"
 
+#ifndef CRITICAL_SECTION_START
+  #define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
+  #define CRITICAL_SECTION_END    SREG = _sreg;
+#endif
+
+
 #ifndef SERIAL_PORT
   #define SERIAL_PORT 0
 #endif
@@ -69,9 +75,13 @@
 // using a ring buffer (I think), in which rx_buffer_head is the index of the
 // location to which to write the next incoming character and rx_buffer_tail
 // is the index of the location from which to read.
-// 256 is the max limit due to uint8_t head and tail. Thats needed to make them atomic.
+// 256 is the max limit due to uint8_t head and tail. Use only powers of 2. (...,16,32,64,128,256)
+#ifndef RX_BUFFER_SIZE
   #define RX_BUFFER_SIZE 128
-
+#endif
+#if !((RX_BUFFER_SIZE == 256) ||(RX_BUFFER_SIZE == 128) ||(RX_BUFFER_SIZE == 64) ||(RX_BUFFER_SIZE == 32) ||(RX_BUFFER_SIZE == 16) ||(RX_BUFFER_SIZE == 8) ||(RX_BUFFER_SIZE == 4) ||(RX_BUFFER_SIZE == 2))
+  #error RX_BUFFER_SIZE has to be a power of 2 and >= 2
+#endif
 
 struct ring_buffer {
   unsigned char buffer[RX_BUFFER_SIZE];
@@ -94,7 +104,11 @@ class MarlinSerial { //: public Stream
     void flush(void);
 
     FORCE_INLINE uint8_t available(void) {
-      return (uint8_t)(RX_BUFFER_SIZE + rx_buffer.head - rx_buffer.tail) % RX_BUFFER_SIZE;
+      CRITICAL_SECTION_START;
+        uint8_t h = rx_buffer.head;
+        uint8_t t = rx_buffer.tail;
+      CRITICAL_SECTION_END;
+      return (uint8_t)(RX_BUFFER_SIZE + h - t) & (RX_BUFFER_SIZE - 1);
     }
 
     FORCE_INLINE void write(uint8_t c) {
@@ -106,16 +120,19 @@ class MarlinSerial { //: public Stream
     FORCE_INLINE void checkRx(void) {
       if (TEST(M_UCSRxA, M_RXCx)) {
         unsigned char c  =  M_UDRx;
-        uint8_t i = (uint8_t)(rx_buffer.head + 1) % RX_BUFFER_SIZE;
+        CRITICAL_SECTION_START;
+          uint8_t h = rx_buffer.head;
+          uint8_t i = (uint8_t)(h + 1) & (RX_BUFFER_SIZE - 1);
 
           // if we should be storing the received character into the location
           // just before the tail (meaning that the head would advance to the
           // current location of the tail), we're about to overflow the buffer
           // and so we don't write the character or advance the head.
           if (i != rx_buffer.tail) {
-          rx_buffer.buffer[rx_buffer.head] = c;
+            rx_buffer.buffer[h] = c;
             rx_buffer.head = i;
           }
+        CRITICAL_SECTION_END;
       }
     }