[PATCH] tile: avoid overflow in ns2cycles

From: Henrik Austad
Date: Tue Mar 04 2014 - 03:23:44 EST


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 <johnstul@xxxxxxxxxx>
CC: Mike Galbraith <bitbucket@xxxxxxxxx>
CC: Salman Qazi <sqazi@xxxxxxxxxx>
Signed-off-by: Henrik Austad <henrik@xxxxxxxxx>
---
arch/tile/kernel/time.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
index 5d10642..9e74733 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)
--
1.7.10.4

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