[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>

1 files changed, 97 insertions, 0 deletions

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index c4187cda0ee4..c4739c4e3039 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);