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authorJohn Stultz <johnstul@us.ibm.com>2010-05-07 21:07:38 -0400
committerThomas Gleixner <tglx@linutronix.de>2010-05-10 08:24:26 -0400
commitd7e81c269db899b800e0963dc4aceece1f82a680 (patch)
treebf30c8c5ed86dbf3c71a25e0f3ab1093c19e516f
parent29f87b793da421a6ab816d991dc8dbf909dfb66a (diff)
clocksource: Add clocksource_register_hz/khz interface
How to pick good mult/shift pairs has always been difficult to describe to folks writing clocksource drivers, since it requires careful tradeoffs in adjustment accuracy vs overflow limits. Now, with the clocks_calc_mult_shift function, its much easier. However, not many clocksources have converted to using that function, and there is still the issue of the max interval length assumption being made by each clocksource driver independently. So this patch simplifies the registration process by having clocksources be registered with a hz/khz value and the registration function taking care of setting mult/shift. This should take most of the confusion out of writing a clocksource driver. Additionally it also keeps the shift size tradeoff (more accuracy vs longer possible nohz times) centralized so the timekeeping core can keep track of the assumptions being made. [ tglx: Coding style and comments fixed ] Signed-off-by: John Stultz <johnstul@us.ibm.com> LKML-Reference: <1273280858-30143-1-git-send-email-johnstul@us.ibm.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
-rw-r--r--include/linux/clocksource.h19
-rw-r--r--kernel/time/clocksource.c48
2 files changed, 66 insertions, 1 deletions
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 4bca8b60cdf7..5ea3c60c160c 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -273,7 +273,6 @@ static inline s64 clocksource_cyc2ns(cycle_t cycles, u32 mult, u32 shift)
273} 273}
274 274
275 275
276/* used to install a new clocksource */
277extern int clocksource_register(struct clocksource*); 276extern int clocksource_register(struct clocksource*);
278extern void clocksource_unregister(struct clocksource*); 277extern void clocksource_unregister(struct clocksource*);
279extern void clocksource_touch_watchdog(void); 278extern void clocksource_touch_watchdog(void);
@@ -287,6 +286,24 @@ extern void clocksource_mark_unstable(struct clocksource *cs);
287extern void 286extern void
288clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec); 287clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec);
289 288
289/*
290 * Don't call __clocksource_register_scale directly, use
291 * clocksource_register_hz/khz
292 */
293extern int
294__clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq);
295
296static inline int clocksource_register_hz(struct clocksource *cs, u32 hz)
297{
298 return __clocksource_register_scale(cs, 1, hz);
299}
300
301static inline int clocksource_register_khz(struct clocksource *cs, u32 khz)
302{
303 return __clocksource_register_scale(cs, 1000, khz);
304}
305
306
290static inline void 307static inline void
291clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec) 308clocksource_calc_mult_shift(struct clocksource *cs, u32 freq, u32 minsec)
292{ 309{
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 1f5dde637457..f08e99c1d561 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -625,6 +625,54 @@ static void clocksource_enqueue(struct clocksource *cs)
625 list_add(&cs->list, entry); 625 list_add(&cs->list, entry);
626} 626}
627 627
628
629/*
630 * Maximum time we expect to go between ticks. This includes idle
631 * tickless time. It provides the trade off between selecting a
632 * mult/shift pair that is very precise but can only handle a short
633 * period of time, vs. a mult/shift pair that can handle long periods
634 * of time but isn't as precise.
635 *
636 * This is a subsystem constant, and actual hardware limitations
637 * may override it (ie: clocksources that wrap every 3 seconds).
638 */
639#define MAX_UPDATE_LENGTH 5 /* Seconds */
640
641/**
642 * __clocksource_register_scale - Used to install new clocksources
643 * @t: clocksource to be registered
644 * @scale: Scale factor multiplied against freq to get clocksource hz
645 * @freq: clocksource frequency (cycles per second) divided by scale
646 *
647 * Returns -EBUSY if registration fails, zero otherwise.
648 *
649 * This *SHOULD NOT* be called directly! Please use the
650 * clocksource_register_hz() or clocksource_register_khz helper functions.
651 */
652int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
653{
654
655 /*
656 * Ideally we want to use some of the limits used in
657 * clocksource_max_deferment, to provide a more informed
658 * MAX_UPDATE_LENGTH. But for now this just gets the
659 * register interface working properly.
660 */
661 clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
662 NSEC_PER_SEC/scale,
663 MAX_UPDATE_LENGTH*scale);
664 cs->max_idle_ns = clocksource_max_deferment(cs);
665
666 mutex_lock(&clocksource_mutex);
667 clocksource_enqueue(cs);
668 clocksource_select();
669 clocksource_enqueue_watchdog(cs);
670 mutex_unlock(&clocksource_mutex);
671 return 0;
672}
673EXPORT_SYMBOL_GPL(__clocksource_register_scale);
674
675
628/** 676/**
629 * clocksource_register - Used to install new clocksources 677 * clocksource_register - Used to install new clocksources
630 * @t: clocksource to be registered 678 * @t: clocksource to be registered