powerpc/xmon: Add option to show uptime information

It might be useful to quickly get the uptime of a running system on
xmon, without needing to grab data from memory and doing math on
struct addresses.

For example, it'd be useful to check for how long after a crash a
system is on xmon shell or if some test was started after the first
test crashed (and this 2nd test crashed too into xmon).

This small patch adds the 'U' command, to accomplish this.
Suggested-by: Murilo Fossa Vicentini <muvic@linux.vnet.ibm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
[mpe: Display units (seconds), add sync()/__delay() sequence]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/xmon/xmon.c

@@ -278,6 +278,7 @@ Commands:\n\
 #elif defined(CONFIG_44x) || defined(CONFIG_PPC_BOOK3E)
 "  u	dump TLB\n"
 #endif
+"  U	show uptime information\n"
 "  ?	help\n"
 "  # n	limit output to n lines per page (for dp, dpa, dl)\n"
 "  zr	reboot\n\
@@ -905,6 +906,26 @@ static void remove_cpu_bpts(void)
 	write_ciabr(0);
 }
 
+/* Based on uptime_proc_show(). */
+static void
+show_uptime(void)
+{
+	struct timespec uptime;
+
+	if (setjmp(bus_error_jmp) == 0) {
+		catch_memory_errors = 1;
+		sync();
+
+		get_monotonic_boottime(&uptime);
+		printf("Uptime: %lu.%.2lu seconds\n", (unsigned long)uptime.tv_sec,
+			((unsigned long)uptime.tv_nsec / (NSEC_PER_SEC/100)));
+
+		sync();
+		__delay(200);						\
+	}
+	catch_memory_errors = 0;
+}
+
 static void set_lpp_cmd(void)
 {
 	unsigned long lpp;
@@ -1040,6 +1061,9 @@ cmds(struct pt_regs *excp)
 			dump_tlb_book3e();
 			break;
 #endif
+		case 'U':
+			show_uptime();
+			break;
 		default:
 			printf("Unrecognized command: ");
 			do {