From f2bbaf99a1d7f51e6c98997ecb07a6287a428c97 Mon Sep 17 00:00:00 2001
From: Nicolas Pitre <nicolas.pitre@linaro.org>
Date: Wed, 3 Dec 2014 14:43:06 -0500
Subject: [PATCH] ktime: Optimize ktime_divns for constant divisors

[ Upstream commit 8b618628b2bf83512fc8df5e8672619d65adfdfb ]

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.

Change-Id: I6a672b10d6d6b373b327f9548135ededfd1e1835
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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>
---
 include/linux/ktime.h | 12 +++++++++++-
 kernel/hrtimer.c      |  4 ++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/ktime.h b/include/linux/ktime.h
index b07a9a976aa..ad2c587013e 100644
--- a/include/linux/ktime.h
+++ b/include/linux/ktime.h
@@ -302,7 +302,17 @@ static inline int ktime_compare(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
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index d18f6216190..6ad5e5940e3 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -323,7 +323,7 @@ EXPORT_SYMBOL_GPL(ktime_sub_ns);
 /*
  * Divide a ktime value by a nanosecond value
  */
-u64 ktime_divns(const ktime_t kt, s64 div)
+u64 __ktime_divns(const ktime_t kt, s64 div)
 {
 	u64 dclc;
 	int sft = 0;
@@ -339,7 +339,7 @@ u64 ktime_divns(const ktime_t kt, s64 div)
 
 	return dclc;
 }
-EXPORT_SYMBOL_GPL(ktime_divns);
+EXPORT_SYMBOL_GPL(__ktime_divns);
 #endif /* BITS_PER_LONG >= 64 */
 
 /*