panic: Distinguish exceptions from other panics

On exceptions and panics, the code will reach panic_rewind. That
function will, among the other things it needs to do to dump the info,
will also dump the exception cause (exccause register). This means that
normally coredumps from panics and those from exceptions are
indistinguishable.

In this commit I reserve the maximum value for exccause (63) to signify
that the dump actually came from a panic instead of a hardware
exception. The easiest way I could see that would not duplicate code is
to simply set the exccause register to this reserved value (otherwise it
could be undefined, as it's not initialized at boot).

I also update the core dumper tool to use 63 as the value reserved for
panics instead of 0 (which is IllegalInstructionCause, a valid hardware
exception).

Signed-off-by: Paul Olaru <paul.olaru@nxp.com>

src/debug/panic.c

@@ -88,5 +88,14 @@ void __panic(uint32_t p, char *filename, uint32_t linenum)
 				 filename, strlen + 1));
 	}
 
+	/* To distinguish regular panic() calls from exceptions, we will
+	 * set a reserved value for the exception cause (63) so the
+	 * coredumper tool could distinguish between the situations.
+	 */
+	__asm__ __volatile__("movi a3, 63\n\t"
+			     "wsr.exccause a3\n\t"
+			     "esync" : : :
+			     "a3", "memory");
+
 	panic_rewind(p, 0, &panicinfo, NULL);
 }

tools/coredumper/sof-coredump-reader.py

@@ -729,7 +729,9 @@ class CoreDumpReader(object):
 			stdoutPrint("set *0x{:08x}=0x{:08x}\n"
 				.format(addr, dw))
 
-		if self.core_dump.exccause:
+                # Exccause 63 is reserved for panics; the other causes come
+                # from actual exceptions
+		if self.core_dump.exccause != 63:
 			verbosePrint("\n# *EXCEPTION*\n")
 			verbosePrint("# exccause: " + EXCCAUSE_CODE[self.core_dump.exccause][0])
 			if EXCCAUSE_CODE[self.core_dump.exccause][1]: