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[PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count

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For large values of "mult" and long uptimes, the intermediate
result of "cycles * mult" can overflow 64 bits.  For example,
the tile platform calls clocksource_cyc2ns with a 1.2 GHz clock;
we have mult = 853, and after 208.5 days, we overflow 64 bits.

Since clocksource_cyc2ns() is intended to be used for relative
cycle counts, not absolute cycle counts, performance is more
importance than accepting a wider range of cycle values.
So, just use mult_frac() directly in tile's sched_clock().

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
---
Blackfin should make a similar change in their sched_clock().

 arch/tile/kernel/time.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/tile/kernel/time.c b/arch/tile/kernel/time.c
index 178989e6d3e3..ea960d660917 100644
--- a/arch/tile/kernel/time.c
+++ b/arch/tile/kernel/time.c
@@ -218,8 +218,8 @@ void do_timer_interrupt(struct pt_regs *regs, int fault_num)
  */
 unsigned long long sched_clock(void)
 {
-	return clocksource_cyc2ns(get_cycles(),
-				  sched_clock_mult, SCHED_CLOCK_SHIFT);
+	return mult_frac(get_cycles(),
+			 sched_clock_mult, 1ULL << SCHED_CLOCK_SHIFT);
 }
 
 int setup_profiling_timer(unsigned int multiplier)
-- 
2.7.2

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[PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count Chris Metcalf <cmetcalf@mellanox.com> - 2016-11-16 20:40 +0100
  Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count John Stultz <john.stultz@linaro.org> - 2016-11-16 21:00 +0100
    Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count John Stultz <john.stultz@linaro.org> - 2016-11-16 21:30 +0100
      Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count John Stultz <john.stultz@linaro.org> - 2016-11-16 21:40 +0100
    Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count Peter Zijlstra <peterz@infradead.org> - 2016-11-17 11:00 +0100
      Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count Peter Zijlstra <peterz@infradead.org> - 2016-11-18 11:40 +0100
        Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute  cycle count Peter Zijlstra <peterz@infradead.org> - 2016-11-18 16:00 +0100

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