ktime: Optimize ktime_divns for constant divisors

At least on ARM, do_div() is optimized to turn constant divisors into
an inline multiplication by the reciprocal value at compile time.
However this optimization is missed entirely whenever ktime_divns() is
used and the slow out-of-line division code is used all the time.

Let ktime_divns() use do_div() inline whenever the divisor is constant
and small enough.  This will make things like ktime_to_us() and
ktime_to_ms() much faster.

Cc: Arnd Bergmann <arnd.bergmann@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Nicolas Pitre <nico@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 11 insertions, 1 deletions

diff --git a/include/linux/ktime.h b/include/linux/ktime.h
index c9d645ad98ff..411dd8bfe539 100644
--- a/include/linux/ktime.h
+++ b/include/linux/ktime.h
@@ -166,7 +166,17 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
 }
 
 #if BITS_PER_LONG < 64
-extern u64 ktime_divns(const ktime_t kt, s64 div);
+extern u64 __ktime_divns(const ktime_t kt, s64 div);
+static inline u64 ktime_divns(const ktime_t kt, s64 div)
+{
+	if (__builtin_constant_p(div) && !(div >> 32)) {
+		u64 ns = kt.tv64;
+		do_div(ns, div);
+		return ns;
+	} else {
+		return __ktime_divns(kt, div);
+	}
+}
 #else /* BITS_PER_LONG < 64 */
 # define ktime_divns(kt, div)		(u64)((kt).tv64 / (div))
 #endif