atmel-samd: Add Micro Trace Buffer support to the debug build.

The Micro Trace Buffer records the history of the pc and can be used
to debug hard faults even when the stack trace is useless.

Author: tannewt

atmel-samd/Makefile

@@ -92,8 +92,10 @@ CFLAGS_CORTEX_M0 = \
 CFLAGS = $(INC) -Wall -Werror -ansi -std=gnu99 -nostdlib $(CFLAGS_CORTEX_M0) $(COPT)
 
 #Debugging/Optimization
+# TODO(tannewt): Figure out what NDEBUG does. Adding it to the debug build
+# reduces code size pretty dramatically.
 ifeq ($(DEBUG), 1)
-CFLAGS += -Os -ggdb
+CFLAGS += -Os -ggdb -DNDEBUG -DENABLE_MICRO_TRACE_BUFFER
 else
 CFLAGS += -Os -DNDEBUG
 endif

atmel-samd/asf/sam0/utils/cmsis/samd21/source/gcc/startup_samd21.c

@@ -41,6 +41,8 @@
  *
  */
 
+#include <stdbool.h>
+
 #include "samd21.h"
 
 /* Initialize segments */
@@ -65,7 +67,7 @@ void Dummy_Handler(void);
 
 /* Cortex-M0+ core handlers */
 void NMI_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
-void HardFault_Handler       ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
+void HardFault_Handler( void ) __attribute__( ( naked ) );
 void SVC_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
 void PendSV_Handler          ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
 void SysTick_Handler         ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
@@ -269,14 +271,24 @@ void Reset_Handler(void)
         main();
 
         /* Infinite loop */
-        while (1);
+        while (true);
+}
+
+void HardFault_Handler(void)
+{
+#ifdef ENABLE_MICRO_TRACE_BUFFER
+    // Turn off the micro trace buffer so we don't fill it up in the infinite
+    // loop below.
+    REG_MTB_MASTER = 0x00000000 + 6;
+#endif
+    while(true) {}
 }
 
 /**
  * \brief Default interrupt handler for unused IRQs.
  */
 void Dummy_Handler(void)
 {
-        while (1) {
+        while (true) {
         }
 }

atmel-samd/main.c

@@ -270,7 +270,19 @@ void MP_WEAK __assert_func(const char *file, int line, const char *func, const c
 
 struct usart_module usart_instance;
 
+#ifdef ENABLE_MICRO_TRACE_BUFFER
+#define TRACE_BUFFER_SIZE 256
+__attribute__((__aligned__(TRACE_BUFFER_SIZE * sizeof(uint32_t)))) uint32_t mtb[TRACE_BUFFER_SIZE];
+#endif
+
 void samd21_init(void) {
+#ifdef ENABLE_MICRO_TRACE_BUFFER
+    memset(mtb, 0, sizeof(mtb));
+    // AWIDTH is 15.
+    REG_MTB_POSITION = ((uint32_t) (mtb - REG_MTB_BASE)) & 0xFFFFFFF8;
+    REG_MTB_FLOW = ((uint32_t) mtb + TRACE_BUFFER_SIZE * sizeof(uint32_t)) & 0xFFFFFFF8;
+    REG_MTB_MASTER = 0x80000000 + 6;
+#endif
     irq_initialize_vectors();
     cpu_irq_enable();