Commit-ID: 2353b47bffe4e6ab39042f470c55d41bb3ff3846
Gitweb: http://git.kernel.org/tip/2353b47bffe4e6ab39042f470c55d41bb3ff3846
Author: Bernd Faust <berndfaust@xxxxxxxxx>
AuthorDate: Wed, 5 Dec 2012 15:16:49 +0100
Committer: John Stultz <john.stultz@xxxxxxxxxx>
CommitDate: Tue, 15 Jan 2013 18:16:07 -0800
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@xxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
arch/x86/kernel/tsc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index cfa5d4f..8ed0857 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));
}
--
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