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| author | Bernd Faust <berndfaust@gmail.com> | 2012-12-05 15:16:49 +0100 |
|---|---|---|
| committer | John Stultz <john.stultz@linaro.org> | 2013-01-16 03:16:07 +0100 |
| commit | 2353b47bffe4e6ab39042f470c55d41bb3ff3846 (patch) | |
| tree | 75d4c5bd873f97c029530349f875153f0d68119a /arch/x86/kernel/tsc.c | |
| parent | NTP: Add a CONFIG_RTC_SYSTOHC configuration (diff) | |
| download | linux-2353b47bffe4e6ab39042f470c55d41bb3ff3846.tar.xz linux-2353b47bffe4e6ab39042f470c55d41bb3ff3846.zip | |
Round the calculated scale factor in set_cyc2ns_scale()
During some experiments with an external clock (in a FPGA), we saw that
the TSC clock drifted approx. 2.5ms per second.
This drift was caused by the current way of calculating the scale.
In our case cpu_khz had a value of 3292725. This resulted in a scale
value of 310. But when doing the calculation by hand it shows that the
actual value is 310.9886188491, so a value of 311 would be more precise.
With this change the value is rounded.
Signed-off-by: Bernd Faust <berndfaust@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Diffstat (limited to 'arch/x86/kernel/tsc.c')
| -rw-r--r-- | arch/x86/kernel/tsc.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index cfa5d4f7ca56..8ed085733773 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -617,7 +617,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu) ns_now = __cycles_2_ns(tsc_now); if (cpu_khz) { - *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz; + *scale = ((NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR) + + cpu_khz / 2) / cpu_khz; *offset = ns_now - mult_frac(tsc_now, *scale, (1UL << CYC2NS_SCALE_FACTOR)); } |