clocksource: add enable() and disable() callbacks

Add enable() and disable() callbacks for clocksources.

This allows us to put unused clocksources in power save mode.  The
functions clocksource_enable() and clocksource_disable() wrap the
callbacks and are inserted in the timekeeping code to enable before use
and disable after switching to a new clocksource.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 900f1b6598d1..687dff49f6e7 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -182,7 +182,7 @@ EXPORT_SYMBOL(do_settimeofday);
  */
 static void change_clocksource(void)
 {
-	struct clocksource *new;
+	struct clocksource *new, *old;
 
 	new = clocksource_get_next();
 
@@ -191,11 +191,16 @@ static void change_clocksource(void)
 
 	clocksource_forward_now();
 
-	new->raw_time = clock->raw_time;
+	if (clocksource_enable(new))
+		return;
 
+	new->raw_time = clock->raw_time;
+	old = clock;
 	clock = new;
+	clocksource_disable(old);
+
 	clock->cycle_last = 0;
-	clock->cycle_last = clocksource_read(new);
+	clock->cycle_last = clocksource_read(clock);
 	clock->error = 0;
 	clock->xtime_nsec = 0;
 	clocksource_calculate_interval(clock, NTP_INTERVAL_LENGTH);
@@ -292,6 +297,7 @@ void __init timekeeping_init(void)
 	ntp_init();
 
 	clock = clocksource_get_next();
+	clocksource_enable(clock);
 	clocksource_calculate_interval(clock, NTP_INTERVAL_LENGTH);
 	clock->cycle_last = clocksource_read(clock);