clocksource: add enable() and disable() callbacks

@@ -43,6 +43,8 @@ struct clocksource;
  *				The ideal clocksource. A must-use where
  *				available.
  * @read:		returns a cycle value, passes clocksource as argument
+ * @enable:		optional function to enable the clocksource
+ * @disable:		optional function to disable the clocksource
  * @mask:		bitmask for two's complement
  *			subtraction of non 64 bit counters
  * @mult:		cycle to nanosecond multiplier (adjusted by NTP)

@@ -62,6 +64,8 @@ struct clocksource {
 	struct list_head list;
 	int rating;
 	cycle_t (*read)(struct clocksource *cs);
+	int (*enable)(struct clocksource *cs);
+	void (*disable)(struct clocksource *cs);
 	cycle_t mask;
 	u32 mult;
 	u32 mult_orig;

@@ -174,6 +178,33 @@ static inline cycle_t clocksource_read(s
 }
 
 /**
+ * clocksource_enable: - enable clocksource
+ * @cs:		pointer to clocksource
+ *
+ * Enables the specified clocksource. The clocksource callback
+ * function should start up the hardware and setup mult and field
+ * members of struct clocksource to reflect hardware capabilities.
+ */
+static inline int clocksource_enable(struct clocksource *cs)
+{
+	return cs->enable ? cs->enable(cs) : 0;
+}
+
+/**
+ * clocksource_disable: - disable clocksource
+ * @cs:		pointer to clocksource
+ *
+ * Disables the specified clocksource. The clocksource callback
+ * function should power down the now unused hardware block to
+ * save power.
+ */
+static inline void clocksource_disable(struct clocksource *cs)
+{
+	if (cs->disable)
+		cs->disable(cs);
+}
+
+/**
  * cyc2ns - converts clocksource cycles to nanoseconds
  * @cs:		Pointer to clocksource
  * @cycles:	Cycles

@@ -184,7 +184,7 @@ EXPORT_SYMBOL(do_settimeofday);
  */
 static void change_clocksource(void)
 {
-	struct clocksource *new;
+	struct clocksource *new, *old;
 
 	new = clocksource_get_next();
 

@@ -193,11 +193,16 @@ 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);

@@ -293,6 +298,7 @@ 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);