[POWERPC] print backtrace when entering xmon

xmon does not print a backtrace per default. This is bad on systems with
USB keyboard, the most needed info about the crash is lost.
print a backtrace during the very first xmon entry.

Booting with xmon=nobt disables the autobacktrace functionality.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 179b10c..8adad14 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -137,10 +137,14 @@
 static void proccall(void);
 void dump_segments(void);
 static void symbol_lookup(void);
+static void xmon_show_stack(unsigned long sp, unsigned long lr,
+			    unsigned long pc);
 static void xmon_print_symbol(unsigned long address, const char *mid,
 			      const char *after);
 static const char *getvecname(unsigned long vec);
 
+int xmon_no_auto_backtrace;
+
 extern int print_insn_powerpc(unsigned long, unsigned long, int);
 
 extern void xmon_enter(void);
@@ -736,6 +740,12 @@
 
 	last_cmd = NULL;
 	xmon_regs = excp;
+
+	if (!xmon_no_auto_backtrace) {
+		xmon_no_auto_backtrace = 1;
+		xmon_show_stack(excp->gpr[1], excp->link, excp->nip);
+	}
+
 	for(;;) {
 #ifdef CONFIG_SMP
 		printf("%x:", smp_processor_id());