Commit 767f3021 authored Mar 04, 2014 by Henrik Austad Committed by Chris Metcalf Mar 06, 2014

tile: avoid overflow in ns2cycles

In commit 4cecf6d4 ("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 <johnstul@us.ibm.com>
CC: Mike Galbraith <bitbucket@online.de>
CC: Salman Qazi <sqazi@google.com>
Signed-off-by: Henrik Austad <henrik@austad.us>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

parent 0414855f

arch/tile/kernel/time.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -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)

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