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authorjohn stultz <johnstul@us.ibm.com>2006-06-26 00:25:07 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2006-06-26 09:58:20 -0700
commit5eb6d20533d14a432df714520939a6181e28f099 (patch)
treeb032147620a8e213356658783f8037d2f5623b57 /include
parent260a42309b31cbc54eb4b6b85649e412bcad053f (diff)
[PATCH] Time: Use clocksource abstraction for NTP adjustments
Instead of incrementing xtime by tick_nsec + ntp adjustments, use the clocksource abstraction to increment and scale time. Using the clocksource abstraction allows other clocksources to be used consistently in the face of late or lost ticks, while preserving the existing behavior via the jiffies clocksource. This removes the need to keep time_phase adjustments as we just use the current_tick_length() function as the NTP interface and accumulate time using shifted nanoseconds. The basics of this design was by Roman Zippel, however it is my own interpretation and implementation, so the credit should go to him and the blame to me. Signed-off-by: John Stultz <johnstul@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/clocksource.h97
1 files changed, 97 insertions, 0 deletions
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index c4187cda0ee..c4739c4e303 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -173,6 +173,103 @@ static inline void calculate_clocksource_interval(struct clocksource *c,
c->interval_snsecs = (u64)c->interval_cycles * c->mult;
}
+
+/**
+ * error_aproximation - calculates an error adjustment for a given error
+ *
+ * @error: Error value (unsigned)
+ * @unit: Adjustment unit
+ *
+ * For a given error value, this function takes the adjustment unit
+ * and uses binary approximation to return a power of two adjustment value.
+ *
+ * This function is only for use by the the make_ntp_adj() function
+ * and you must hold a write on the xtime_lock when calling.
+ */
+static inline int error_aproximation(u64 error, u64 unit)
+{
+ static int saved_adj = 0;
+ u64 adjusted_unit = unit << saved_adj;
+
+ if (error > (adjusted_unit * 2)) {
+ /* large error, so increment the adjustment factor */
+ saved_adj++;
+ } else if (error > adjusted_unit) {
+ /* just right, don't touch it */
+ } else if (saved_adj) {
+ /* small error, so drop the adjustment factor */
+ saved_adj--;
+ return 0;
+ }
+
+ return saved_adj;
+}
+
+
+/**
+ * make_ntp_adj - Adjusts the specified clocksource for a given error
+ *
+ * @clock: Pointer to clock to be adjusted
+ * @cycles_delta: Current unacounted cycle delta
+ * @error: Pointer to current error value
+ *
+ * Returns clock shifted nanosecond adjustment to be applied against
+ * the accumulated time value (ie: xtime).
+ *
+ * If the error value is large enough, this function calulates the
+ * (power of two) adjustment value, and adjusts the clock's mult and
+ * interval_snsecs values accordingly.
+ *
+ * However, since there may be some unaccumulated cycles, to avoid
+ * time inconsistencies we must adjust the accumulation value
+ * accordingly.
+ *
+ * This is not very intuitive, so the following proof should help:
+ * The basic timeofday algorithm: base + cycle * mult
+ * Thus:
+ * new_base + cycle * new_mult = old_base + cycle * old_mult
+ * new_base = old_base + cycle * old_mult - cycle * new_mult
+ * new_base = old_base + cycle * (old_mult - new_mult)
+ * new_base - old_base = cycle * (old_mult - new_mult)
+ * base_delta = cycle * (old_mult - new_mult)
+ * base_delta = cycle * (mult_delta)
+ *
+ * Where mult_delta is the adjustment value made to mult
+ *
+ */
+static inline s64 make_ntp_adj(struct clocksource *clock,
+ cycles_t cycles_delta, s64* error)
+{
+ s64 ret = 0;
+ if (*error > ((s64)clock->interval_cycles+1)/2) {
+ /* calculate adjustment value */
+ int adjustment = error_aproximation(*error,
+ clock->interval_cycles);
+ /* adjust clock */
+ clock->mult += 1 << adjustment;
+ clock->interval_snsecs += clock->interval_cycles << adjustment;
+
+ /* adjust the base and error for the adjustment */
+ ret = -(cycles_delta << adjustment);
+ *error -= clock->interval_cycles << adjustment;
+ /* XXX adj error for cycle_delta offset? */
+ } else if ((-(*error)) > ((s64)clock->interval_cycles+1)/2) {
+ /* calculate adjustment value */
+ int adjustment = error_aproximation(-(*error),
+ clock->interval_cycles);
+ /* adjust clock */
+ clock->mult -= 1 << adjustment;
+ clock->interval_snsecs -= clock->interval_cycles << adjustment;
+
+ /* adjust the base and error for the adjustment */
+ ret = cycles_delta << adjustment;
+ *error += clock->interval_cycles << adjustment;
+ /* XXX adj error for cycle_delta offset? */
+ }
+ return ret;
+}
+
+
/* used to install a new clocksource */
int register_clocksource(struct clocksource*);
void reselect_clocksource(void);