From: Bernd Faust Date: Wed, 5 Dec 2012 14:16:49 +0000 (+0100) Subject: Round the calculated scale factor in set_cyc2ns_scale() X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=2353b47bffe4e6ab39042f470c55d41bb3ff3846;p=linux-beck.git 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 Signed-off-by: John Stultz --- 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)); }