time: Compensate for rounding on odd-frequency clocksources

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>

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 49010d8..5bb86da 100644 (file)
--- 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(struct clocksource *clock)
        /* 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(struct clocksource *clock)
 
        /* 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;
 
@@ -719,7 +723,8 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift)
 
        /* 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;