Segger RTT: support CONFIG_SEGGER_RTT_INIT_MODE and optimizations

Support Zephyr "RTT Initialization mode" config choice, allowing
Control Block initialization to be retained from another program
(e.g. bootloader).
It includes conditional CB init with full check on Control Block ID or
partial one that checks only the first byte, yet improving its logic.
Minor optimizations with static const added.

Signed-off-by: Giancarlo Stasi <[email protected]>

Author: giansta

SEGGER/SEGGER_RTT.c

@@ -254,7 +254,9 @@ Additional information:
 **********************************************************************
 */
 
-static unsigned char _aTerminalId[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
+static const unsigned char _aTerminalId[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
+
+static const char _aInitStr[] = "\0\0\0\0\0\0TTR REGGES";  // Init complete ID string to make sure that things also work if RTT is linked to a no-init memory area
 
 /*********************************************************************
 *
@@ -309,15 +311,14 @@ static unsigned char _ActiveTerminal;
                   volatile SEGGER_RTT_CB* pRTTCBInit;                                                \
                   pRTTCBInit = (volatile SEGGER_RTT_CB*)((char*)&_SEGGER_RTT + SEGGER_RTT_UNCACHED_OFF); \
                   do {                                                                               \
-                    if (pRTTCBInit->acID[0] == '\0') {                                               \
+                    if (pRTTCBInit->acID[0] != 'S') {                                                \
                       _DoInit();                                                                     \
                     }                                                                                \
                   } while (0);                                                                       \
                 }
 
 static void _DoInit(void) {
   volatile SEGGER_RTT_CB* p;   // Volatile to make sure that compiler cannot change the order of accesses to the control block
-  static const char _aInitStr[] = "\0\0\0\0\0\0TTR REGGES";  // Init complete ID string to make sure that things also work if RTT is linked to a no-init memory area
   unsigned i;
   //
   // Initialize control block
@@ -1892,6 +1893,28 @@ int SEGGER_RTT_SetFlagsDownBuffer(unsigned BufferIndex, unsigned Flags) {
   return r;
 }
 
+#if defined(CONFIG_SEGGER_RTT_INIT_MODE_STRONG_CHECK)
+static void SEGGER_RTT_StrongCheckInit(void) {
+  volatile SEGGER_RTT_CB* p;
+  p = (volatile SEGGER_RTT_CB*)((char*)&_SEGGER_RTT + SEGGER_RTT_UNCACHED_OFF);
+  unsigned i;
+  int DoInit = 0;
+
+  RTT__DMB();                       // Force order of memory accesses for cores that may perform out-of-order memory accesses
+  for (i = 0; i < sizeof(_aInitStr) - 1; ++i) {
+    if (p->acID[i] != _aInitStr[sizeof(_aInitStr) - 2 - i]) { // Skip terminating \0 at the end of the array
+        DoInit = 1;
+        break;
+    }
+  }
+  RTT__DMB();                       // Force order of memory accesses for cores that may perform out-of-order memory accesses
+
+  if (DoInit) {
+    _DoInit();
+  }
+}
+#endif
+
 /*********************************************************************
 *
 *       SEGGER_RTT_Init
@@ -1902,7 +1925,22 @@ int SEGGER_RTT_SetFlagsDownBuffer(unsigned BufferIndex, unsigned Flags) {
 *
 */
 void SEGGER_RTT_Init (void) {
+  /*
+   * The standard Segger code calls only _DoInit() here. This happens,
+   * for example, in a bootloader or in stand-alone applications.
+   * For applications started by a bootloader, Segger recommends not
+   * calling this function, as all the recommended APIs use INIT().
+   * Unfortunately Zephyr log backend uses SEGGER_RTT_WriteSkipNoLock()
+   * that doesn't call INIT(), so the additional two following variants
+   * have been added here for a safe API usage.
+   */
+#if defined(CONFIG_SEGGER_RTT_INIT_MODE_STRONG_CHECK)
+  SEGGER_RTT_StrongCheckInit();
+#elif defined(CONFIG_SEGGER_RTT_INIT_MODE_WEAK_CHECK)
+  INIT();
+#else
   _DoInit();
+#endif
 }
 
 /*********************************************************************