[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, 28 insertions, 19 deletions

diff --git a/kernel/timer.c b/kernel/timer.c
index 623f9ea198d8..6811436a031d 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -597,7 +597,6 @@ long time_tolerance = MAXFREQ;		/* frequency tolerance (ppm)	*/
 long time_precision = 1;                /* clock precision (us)         */
 long time_maxerror = NTP_PHASE_LIMIT;   /* maximum error (us)           */
 long time_esterror = NTP_PHASE_LIMIT;   /* estimated error (us)         */
-static long time_phase;                 /* phase offset (scaled us)     */
 long time_freq = (((NSEC_PER_SEC + HZ/2) % HZ - HZ/2) << SHIFT_USEC) / NSEC_PER_USEC;
                                         /* frequency offset (scaled ppm)*/
 static long time_adj;                   /* tick adjust (scaled 1 / HZ)  */
@@ -747,27 +746,14 @@ static long adjtime_adjustment(void)
 }
 
 /* in the NTP reference this is called "hardclock()" */
-static void update_wall_time_one_tick(void)
+static void update_ntp_one_tick(void)
 {
-        long time_adjust_step, delta_nsec;
+        long time_adjust_step;
 
         time_adjust_step = adjtime_adjustment();
         if (time_adjust_step)
                 /* Reduce by this step the amount of time left  */
                 time_adjust -= time_adjust_step;
-        delta_nsec = tick_nsec + time_adjust_step * 1000;
-        /*
-         * Advance the phase, once it gets to one microsecond, then
-         * advance the tick more.
-         */
-        time_phase += time_adj;
-        if ((time_phase >= FINENSEC) || (time_phase <= -FINENSEC)) {
-                long ltemp = shift_right(time_phase, (SHIFT_SCALE - 10));
-                time_phase -= ltemp << (SHIFT_SCALE - 10);
-                delta_nsec += ltemp;
-        }
-        xtime.tv_nsec += delta_nsec;
-        time_interpolator_update(delta_nsec);
 
         /* Changes by adjtime() do not take effect till next tick. */
         if (time_next_adjust != 0) {
@@ -872,8 +858,13 @@ device_initcall(timekeeping_init_device);
  */
 static void update_wall_time(void)
 {
+        static s64 remainder_snsecs, error;
+        s64 snsecs_per_sec;
         cycle_t now, offset;
 
+        snsecs_per_sec = (s64)NSEC_PER_SEC << clock->shift;
+        remainder_snsecs += (s64)xtime.tv_nsec << clock->shift;
+
         now = read_clocksource(clock);
         offset = (now - last_clock_cycle)&clock->mask;
 
@@ -881,17 +872,35 @@ static void update_wall_time(void)
          * case of lost or late ticks, it will accumulate correctly.
          */
         while (offset > clock->interval_cycles) {
+                /* get the ntp interval in clock shifted nanoseconds */
+                s64 ntp_snsecs  = current_tick_length(clock->shift);
+
                 /* accumulate one interval */
+                remainder_snsecs += clock->interval_snsecs;
                 last_clock_cycle += clock->interval_cycles;
                 offset -= clock->interval_cycles;
 
-                update_wall_time_one_tick();
-                if (xtime.tv_nsec >= 1000000000) {
-                        xtime.tv_nsec -= 1000000000;
+                /* interpolator bits */
+                time_interpolator_update(clock->interval_snsecs
+                                                >> clock->shift);
+                /* increment the NTP state machine */
+                update_ntp_one_tick();
+
+                /* accumulate error between NTP and clock interval */
+                error += (ntp_snsecs - (s64)clock->interval_snsecs);
+
+                /* correct the clock when NTP error is too big */
+                remainder_snsecs += make_ntp_adj(clock, offset, &error);
+
+                if (remainder_snsecs >= snsecs_per_sec) {
+                        remainder_snsecs -= snsecs_per_sec;
                         xtime.tv_sec++;
                         second_overflow();
                 }
         }
+        /* store full nanoseconds into xtime */
+        xtime.tv_nsec = remainder_snsecs >> clock->shift;
+        remainder_snsecs -= (s64)xtime.tv_nsec << clock->shift;
 }
 
 /*