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 <[email protected]>
Acked-by: john stultz <[email protected]>
Cc: John Kacur <[email protected]>
Cc: Clark Williams <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

0 files changed, 0 insertions, 0 deletions