clocksource: Don't walk the clocksource list for empty override

If the override clocksource name is empty there is no point in walking the
clocksource list for a match.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: sboyd@codeaurora.org
Cc: broonie@kernel.org
Cc: john.stultz@linaro.org
Link: https://lkml.kernel.org/r/069ce2a605546bcad6552968cff755f0a03f9f10.1516167691.git.baolin.wang@linaro.org

kernel/time/clocksource.c

@@ -594,6 +594,9 @@ static void __clocksource_select(bool skipcur)
 	if (!best)
 		return;
 
+	if (!strlen(override_name))
+		goto found;
+
 	/* Check for the override clocksource. */
 	list_for_each_entry(cs, &clocksource_list, list) {
 		if (skipcur && cs == curr_clocksource)
@@ -625,6 +628,7 @@ static void __clocksource_select(bool skipcur)
 		break;
 	}
 
+found:
 	if (curr_clocksource != best && !timekeeping_notify(best)) {
 		pr_info("Switched to clocksource %s\n", best->name);
 		curr_clocksource = best;