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Subject[22/85] time: Compensate for rounding on odd-frequency clocksources
2.6.33-longterm review patch.  If anyone has any objections, please let us know.

------------------
From: Kasper Pedersen <kkp2010@kasperkp.dk>

commit a386b5af8edda1c742ce9f77891e112eefffc005 upstream.

When the clocksource is not a multiple of HZ, the clock will be off. For
acpi_pm, HZ=1000 the error is 127.111 ppm:

The rounding of cycle_interval ends up generating a false error term in
ntp_error accumulation since xtime_interval is not exactly 1/HZ. So, we
subtract out the error caused by the rounding.

This has been visible since 2.6.32-rc2
commit a092ff0f90cae22b2ac8028ecd2c6f6c1a9e4601
time: Implement logarithmic time accumulation
That commit raised NTP_INTERVAL_FREQ and exposed the rounding error.

testing tool: http://n1.taur.dk/permanent/testpmt.c
Also tested with ntpd and a frequency counter.

Signed-off-by: Kasper Pedersen <kkp2010@kasperkp.dk>
Acked-by: john stultz <johnstul@us.ibm.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/time/timekeeping.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -32,6 +32,8 @@ struct timekeeper {
cycle_t cycle_interval;
/* Number of clock shifted nano seconds in one NTP interval. */
u64 xtime_interval;
+ /* shifted nano seconds left over when rounding cycle_interval */
+ s64 xtime_remainder;
/* Raw nano seconds accumulated per NTP interval. */
u32 raw_interval;

@@ -62,7 +64,7 @@ struct timekeeper timekeeper;
static void timekeeper_setup_internals(struct clocksource *clock)
{
cycle_t interval;
- u64 tmp;
+ u64 tmp, ntpinterval;

timekeeper.clock = clock;
clock->cycle_last = clock->read(clock);
@@ -70,6 +72,7 @@ static void timekeeper_setup_internals(s
/* Do the ns -> cycle conversion first, using original mult */
tmp = NTP_INTERVAL_LENGTH;
tmp <<= clock->shift;
+ ntpinterval = tmp;
tmp += clock->mult/2;
do_div(tmp, clock->mult);
if (tmp == 0)
@@ -80,6 +83,7 @@ static void timekeeper_setup_internals(s

/* Go back from cycles -> shifted ns */
timekeeper.xtime_interval = (u64) interval * clock->mult;
+ timekeeper.xtime_remainder = ntpinterval - timekeeper.xtime_interval;
timekeeper.raw_interval =
((u64) interval * clock->mult) >> clock->shift;

@@ -771,7 +775,8 @@ static cycle_t logarithmic_accumulation(

/* Accumulate error between NTP and clock interval */
timekeeper.ntp_error += tick_length << shift;
- timekeeper.ntp_error -= timekeeper.xtime_interval <<
+ timekeeper.ntp_error -=
+ (timekeeper.xtime_interval + timekeeper.xtime_remainder) <<
(timekeeper.ntp_error_shift + shift);

return offset;



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