clocksource: add enable() and disable() callbacks

From: Magnus Damm <[email protected]>

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 <[email protected]>

---

 include/linux/clocksource.h |   31 +++++++++++++++++++++++++++++++
 kernel/time/timekeeping.c   |   12 +++++++++---
 2 files changed, 40 insertions(+), 3 deletions(-)

include/linux/clocksource.h +31 -0

@@ -44,6 +44,8 @@ struct clocksource;
  *				available.
  * @read:		returns a cycle value
  * @read2:		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)

@@ -64,6 +66,8 @@ struct clocksource {
 	int rating;
 	cycle_t (*read)(void);
 	cycle_t (*read2)(struct clocksource *cs);
+	int (*enable)(struct clocksource *cs);
+	void (*disable)(struct clocksource *cs);
 	cycle_t mask;
 	u32 mult;
 	u32 mult_orig;

@@ -176,6 +180,33 @@ 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

kernel/time/timekeeping.c +9 -3

@@ -177,7 +177,7 @@ EXPORT_SYMBOL(do_settimeofday);
  */
 static void change_clocksource(void)
 {
-	struct clocksource *new;
+	struct clocksource *new, *old;
 
 	new = clocksource_get_next();
 

@@ -186,11 +186,16 @@ 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);

@@ -287,6 +292,7 @@ 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);
 

Quoted 9 lines · Magnus Damm

From: Magnus Damm <[email protected]>

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 <[email protected]>

I like this! Its a nice addition to the clocksource structure to allow for improved power management.

I've got some worries that if clocksource_enable() is allowed to fail it might give us some grief in early boot (you don't check for errors there, but understandably, as there isn't much we can do).

However, maybe if a clocksource fails to be enabled, we need to eject it from the list of usable clocksources, as if clocksource with a high rating value, were to fail to be enabled, every tick we'd try to swap the bad clocksource in and fail, never to move to a good clocksource with slightly lower rating.

But other then that nit, I think this is a good change.