Helix-serial.c configuration improvement (#4370)

The new simple API can be selected.

Previous version, can select two way.
  * use old API (compatible with let's split serial.c)
  * use new API (multi-type transaction)

This version, can select three way.
  * use old API (compatible with let's split serial.c)
  * use new API (single-type transaction)
  * use new API (multi-type transaction)

There is no change in the code generated by this change.

keyboards/helix/serial.c

@@ -71,8 +71,8 @@
 #endif
 
 //////////////// for backward compatibility ////////////////////////////////
-#ifndef SERIAL_USE_MULTI_TRANSACTION
-/* --- USE Simple API (OLD API, compatible with let's split serial.c) */
+#if !defined(SERIAL_USE_SINGLE_TRANSACTION) && !defined(SERIAL_USE_MULTI_TRANSACTION)
+/* --- USE OLD API (compatible with let's split serial.c) */
   #if SERIAL_SLAVE_BUFFER_LENGTH > 0
   uint8_t volatile serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH] = {0};
   #endif
@@ -112,7 +112,7 @@ int serial_update_buffers()
     return result;
 }
 
-#endif // end of Simple API (OLD API, compatible with let's split serial.c)
+#endif // end of OLD API (compatible with let's split serial.c)
 ////////////////////////////////////////////////////////////////////////////
 
 #define ALWAYS_INLINE __attribute__((always_inline))

keyboards/helix/serial.h

@@ -15,19 +15,23 @@
 //                                               //  4: about 26kbps
 //                                               //  5: about 20kbps
 //
-// //// USE Simple API (OLD API, compatible with let's split serial.c)
+// //// USE OLD API (compatible with let's split serial.c)
 // ex.
 //  #define SERIAL_SLAVE_BUFFER_LENGTH MATRIX_ROWS/2
 //  #define SERIAL_MASTER_BUFFER_LENGTH 1
 //
+// //// USE NEW API
+//    //// USE simple API (using signle-type transaction function)
+//      #define SERIAL_USE_SINGLE_TRANSACTION
 //    //// USE flexible API (using multi-type transaction function)
 //      #define SERIAL_USE_MULTI_TRANSACTION
 //
 // /////////////////////////////////////////////////////////////////
 
 
-#ifndef SERIAL_USE_MULTI_TRANSACTION
-/* --- USE Simple API (OLD API, compatible with let's split serial.c) */
+//////////////// for backward compatibility ////////////////////////////////
+#if !defined(SERIAL_USE_SINGLE_TRANSACTION) && !defined(SERIAL_USE_MULTI_TRANSACTION)
+/* --- USE OLD API (compatible with let's split serial.c) */
  #if SERIAL_SLAVE_BUFFER_LENGTH > 0
  extern volatile uint8_t serial_slave_buffer[SERIAL_SLAVE_BUFFER_LENGTH];
  #endif
@@ -39,7 +43,8 @@ void serial_master_init(void);
  void serial_slave_init(void);
  int serial_update_buffers(void);
 
-#endif // USE Simple API
+#endif // end of USE OLD API
+////////////////////////////////////////////////////////////////////////////
 
 // Soft Serial Transaction Descriptor
 typedef struct _SSTD_t  {