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authorHenrik Austad <henrik@austad.us>2014-03-04 09:20:53 +0100
committerChris Metcalf <cmetcalf@tilera.com>2014-03-06 11:53:12 -0500
commit767f30210bee1bb74ab042e237c2a21e77874070 (patch)
tree83d5b3ea8f10688b80262172dda68e23b68816ff
parent0414855fdc4a40da05221fc6062cccbc0c30f169 (diff)
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 <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>
-rw-r--r--arch/tile/kernel/time.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
index 5d10642db63..462dcd0c170 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)