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Subject[patch 10/15] add xtime_shift and ntp_error_shift to struct timekeeper
From: Martin Schwidefsky <schwidefsky@de.ibm.com>

The xtime_nsec value in the timekeeper structure is shifted by a few
bits to improve precision. This happens to be the same value as the
clock->shift. To improve readability add xtime_shift to the timekeeper
and use it instead of the clock->shift. Likewise add ntp_error_shift
and replace all (NTP_SCALE_SHIFT - clock->shift) expressions.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
kernel/time/timekeeping.c | 36 +++++++++++++++++++++---------------
1 file changed, 21 insertions(+), 15 deletions(-)
Index: linux-2.6/kernel/time/timekeeping.c
===================================================================
--- linux-2.6.orig/kernel/time/timekeeping.c
+++ linux-2.6/kernel/time/timekeeping.c
@@ -23,6 +23,8 @@
struct timekeeper {
/* Current clocksource used for timekeeping. */
struct clocksource *clock;
+ /* The shift value of the current clocksource. */
+ int shift;

/* Number of clock cycles in one NTP interval. */
cycle_t cycle_interval;
@@ -36,6 +38,9 @@ struct timekeeper {
/* Difference between accumulated time and NTP time in ntp
* shifted nano seconds. */
s64 ntp_error;
+ /* Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds. */
+ int ntp_error_shift;
};

struct timekeeper timekeeper;
@@ -75,8 +80,10 @@ static void timekeeper_setup_internals(s
((u64) interval * clock->mult_orig) >> clock->shift;

timekeeper.xtime_nsec = 0;
+ timekeeper.shift = clock->shift;

timekeeper.ntp_error = 0;
+ timekeeper.ntp_error_shift = NTP_SCALE_SHIFT - clock->shift;
}

/*
@@ -641,8 +648,7 @@ static __always_inline int timekeeping_b
* Now calculate the error in (1 << look_ahead) ticks, but first
* remove the single look ahead already included in the error.
*/
- tick_error = tick_length >>
- (NTP_SCALE_SHIFT - timekeeper.clock->shift + 1);
+ tick_error = tick_length >> (timekeeper.ntp_error_shift + 1);
tick_error -= timekeeper.xtime_interval >> 1;
error = ((error - tick_error) >> look_ahead) + tick_error;

@@ -673,8 +679,7 @@ static void timekeeping_adjust(s64 offse
s64 error, interval = timekeeper.cycle_interval;
int adj;

- error = timekeeper.ntp_error >>
- (NTP_SCALE_SHIFT - timekeeper.clock->shift - 1);
+ error = timekeeper.ntp_error >> (timekeeper.ntp_error_shift - 1);
if (error > interval) {
error >>= 2;
if (likely(error <= interval))
@@ -696,7 +701,7 @@ static void timekeeping_adjust(s64 offse
timekeeper.xtime_interval += interval;
timekeeper.xtime_nsec -= offset;
timekeeper.ntp_error -= (interval - offset) <<
- (NTP_SCALE_SHIFT - timekeeper.clock->shift);
+ timekeeper.ntp_error_shift;
}

/**
@@ -708,7 +713,7 @@ void update_wall_time(void)
{
struct clocksource *clock;
cycle_t offset;
- s64 nsecs;
+ u64 nsecs;

/* Make sure we're fully resumed: */
if (unlikely(timekeeping_suspended))
@@ -720,7 +725,7 @@ void update_wall_time(void)
#else
offset = timekeeper.cycle_interval;
#endif
- timekeeper.xtime_nsec = (s64)xtime.tv_nsec << clock->shift;
+ timekeeper.xtime_nsec = (s64)xtime.tv_nsec << timekeeper.shift;

/* normally this loop will run just once, however in the
* case of lost or late ticks, it will accumulate correctly.
@@ -731,8 +736,9 @@ void update_wall_time(void)
clock->cycle_last += timekeeper.cycle_interval;

timekeeper.xtime_nsec += timekeeper.xtime_interval;
- if (timekeeper.xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
- timekeeper.xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
+ nsecs = (u64) NSEC_PER_SEC << timekeeper.shift;
+ if (timekeeper.xtime_nsec >= nsecs) {
+ timekeeper.xtime_nsec -= nsecs;
xtime.tv_sec++;
second_overflow();
}
@@ -746,7 +752,7 @@ void update_wall_time(void)
/* accumulate error between NTP and clock interval */
timekeeper.ntp_error += tick_length;
timekeeper.ntp_error -= timekeeper.xtime_interval <<
- (NTP_SCALE_SHIFT - clock->shift);
+ timekeeper.ntp_error_shift;
}

/* correct the clock when NTP error is too big */
@@ -771,16 +777,16 @@ void update_wall_time(void)
if (unlikely((s64)timekeeper.xtime_nsec < 0)) {
s64 neg = -(s64)timekeeper.xtime_nsec;
timekeeper.xtime_nsec = 0;
- timekeeper.ntp_error += neg << (NTP_SCALE_SHIFT - clock->shift);
+ timekeeper.ntp_error += neg << timekeeper.ntp_error_shift;
}

/* store full nanoseconds into xtime after rounding it up and
* add the remainder to the error difference.
*/
- xtime.tv_nsec = ((s64)timekeeper.xtime_nsec >> clock->shift) + 1;
- timekeeper.xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
- timekeeper.ntp_error += timekeeper.xtime_nsec <<
- (NTP_SCALE_SHIFT - clock->shift);
+ xtime.tv_nsec = ((s64) timekeeper.xtime_nsec >> timekeeper.shift) + 1;
+ timekeeper.xtime_nsec -= (s64) xtime.tv_nsec << timekeeper.shift;
+ timekeeper.ntp_error += timekeeper.xtime_nsec <<
+ timekeeper.ntp_error_shift;

nsecs = clocksource_cyc2ns(offset, clock->mult, clock->shift);
update_xtime_cache(nsecs);
--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.



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