aboutsummaryrefslogtreecommitdiff
path: root/include/linux/clocksource.h
diff options
context:
space:
mode:
Diffstat (limited to 'include/linux/clocksource.h')
-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);