From 767f30210bee1bb74ab042e237c2a21e77874070 Mon Sep 17 00:00:00 2001 From: Henrik Austad Date: Tue, 4 Mar 2014 09:20:53 +0100 Subject: 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 --- arch/tile/kernel/time.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) (limited to 'arch/tile') 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) -- cgit v1.2.3