From: Henrik Austad Date: Tue, 4 Mar 2014 08:20:53 +0000 (+0100) Subject: tile: avoid overflow in ns2cycles X-Git-Url: http://drtracing.org/?a=commitdiff_plain;h=767f30210bee1bb74ab042e237c2a21e77874070;p=deliverable%2Flinux.git tile: avoid overflow in ns2cycles In commit 4cecf6d401a ("sched, x86: Avoid unnecessary overflow in sched_clock") and in recent patch "clocksource: avoid unnecessary overflow in cyclecounter_cyc2ns()" https://lkml.org/lkml/2014/3/4/17, the mult-shift approach is replaced by 2 steps to avoid storing a large, intermediate value that could overflow. arch/tile/kernel/time.c has a similar pattern in cycles2ns, and this copies the same pattern in this function CC: John Stultz CC: Mike Galbraith CC: Salman Qazi Signed-off-by: Henrik Austad Signed-off-by: Chris Metcalf --- diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c index 5d10642db63e..462dcd0c1700 100644 --- a/arch/tile/kernel/time.c +++ b/arch/tile/kernel/time.c @@ -236,7 +236,15 @@ cycles_t ns2cycles(unsigned long nsecs) * clock frequency. */ struct clock_event_device *dev = &__raw_get_cpu_var(tile_timer); - return ((u64)nsecs * dev->mult) >> dev->shift; + + /* + * as in clocksource.h and x86's timer.h, we split the calculation + * into 2 parts to avoid unecessary overflow of the intermediate + * value. This will not lead to any loss of precision. + */ + u64 quot = (u64)nsecs >> dev->shift; + u64 rem = (u64)nsecs & ((1ULL << dev->shift) - 1); + return quot * dev->mult + ((rem * dev->mult) >> dev->shift); } void update_vsyscall_tz(void)