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>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index cfa5d4f..8ed0857 100644 (file)
--- 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));
        }