linux-kernel - [PATCH v2] tile: avoid using clocksource

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Date:   Wed, 16 Nov 2016 14:35:33 -0500
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Salman Qazi <sqazi@...gle.com>, Paul Turner <pjt@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Steven Miao <realmz6@...il.com>, linux-kernel@...r.kernel.org
Cc:     Chris Metcalf <cmetcalf@...lanox.com>
Subject: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count

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@...lanox.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