Messages in this thread Patch in this message | | | From | Bernd Faust <> | Subject | [PATCH] Round the calculated scale factor in set_cyc2ns_scale() | Date | Wed, 5 Dec 2012 15:16:49 +0100 |
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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> --- 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)); } -- 1.7.9.5
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