Optionally detect reset and print a stack trace

The new configuation option CONFIG_CHECK_RESET adds a function to dump
the stack (to the serial port) if a reset is detected. This happens
when a null pointer is followed or when the program is reloaded
(spec-sw/tools/spec-cl, for example).  For this reason, the feature is
disabled by default (actually, spec-cl may clear the RAM first, but it
currently doesn't do that and the check is triggered).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

Kconfig

@@ -27,6 +27,14 @@ config PPSI
 
 endchoice
 
+config CHECK_RESET
+	bool "Print a stack trace if reset happens"
+	help
+	  If the CPU is following a NULL pointer, the program will
+	  be restarted. If this option is activated, the main function
+	  detects that it is re-executed and dumps a stack trace; it
+	  then clears the stack (for next time) and restarts again.
+
 config PPSI_VERBOSITY
 	depends on PPSI
 	int "Default verbosity for PPSI"

wrc_main.c

@@ -151,6 +151,7 @@ static void ui_update()
 }
 
 extern uint32_t _endram;
+extern uint32_t _fstack;
 #define ENDRAM_MAGIC 0xbadc0ffe
 
 static void check_stack(void)
@@ -161,8 +162,63 @@ static void check_stack(void)
 	}
 }
 
+#ifdef CONFIG_CHECK_RESET
+
+static void check_reset(void)
+{
+	extern void _reset_handler(void); /* user to reset again */
+	/* static variables to preserve stack (for dumping it) */
+	static uint32_t *p, *save;
+
+	/* _endram is set to ENDRAM_MAGIC after calling this function */
+	if (_endram != ENDRAM_MAGIC)
+		return;
+
+	/* Before calling anything, find the beginning of the stack */
+	p = &_endram + 1;
+	while (!*p)
+		p++;
+	p = (void *)((unsigned long)p & ~0xf); /* align */
+
+	/* Copy it to the beginning of the stack, then reset pointers */
+	save = &_endram;
+	while (p <= &_fstack)
+		*save++ = *p++;
+	p -= (save - &_endram);
+	save = &_endram;
+
+	/* Ok, now init the devices so we can printf and delay */
+	sdb_find_devices();
+	uart_init();
+	timer_init(1);
+
+	pp_printf("\nWarning: the CPU was reset\nStack trace:\n");
+	while (p < &_fstack) {
+		pp_printf("%08x: %08x %08x %08x %08x\n",
+			  (int)p, save[0], save[1], save[2], save[3]);
+		p += 4;
+		save += 4;
+	}
+	pp_printf("Rebooting in 1 second\n\n\n");
+	timer_delay(1000);
+
+	/* Zero the stack and start over (so we dump correctly next time) */
+	for (p = &_endram; p < &_fstack; p++)
+		*p = 0;
+	_endram = 0;
+	_reset_handler();
+}
+
+# else /* no CONFIG_CHECK_RESET */
+
+static void check_reset(void) {}
+
+#endif
+
+
 int main(void)
 {
+	check_reset();
 	wrc_extra_debug = 1;
 	wrc_ui_mode = UI_SHELL_MODE;
 	_endram = ENDRAM_MAGIC;