linux-kernel - Re: [PATCH v2] drm/msm: Use 64-bit timekeeping

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Date:	Thu, 21 Apr 2016 14:21:28 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Tina Ruchandani <ruchandani.tina@...il.com>
Cc:	y2038 <y2038@...ts.linaro.org>, dri-devel@...ts.freedesktop.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Wentao Xu <wentaox@...eaurora.org>,
	Thierry Reding <treding@...dia.com>,
	Hai Li <hali@...eaurora.org>
Subject: Re: [PATCH v2] drm/msm: Use 64-bit timekeeping

On Thursday 21 April 2016 04:39:04 Tina Ruchandani wrote:
> >> which only does one 64-bit division, and it's one that we can probably
> >> optimize out in the future (we can check in ktime_ms_delta whether the
> >> difference is more than 2^32 nanoseconds as the fast path).
> 
> It looks like ktime_divns already has that optimization for 32-bit divisor,
> so your solution should avoid the 64-bit division.

I meant an optimization for a 32-bit dividend, not divisor,
e.g. doing:

diff --git a/include/linux/ktime.h b/include/linux/ktime.h
index 2b6a204bd8d4..4fbf735ec0af 100644
--- a/include/linux/ktime.h
+++ b/include/linux/ktime.h
@@ -169,13 +169,17 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
 extern s64 __ktime_divns(const ktime_t kt, s64 div);
 static inline s64 ktime_divns(const ktime_t kt, s64 div)
 {
+	s64 ns = kt.tv64;
+
 	/*
 	 * Negative divisors could cause an inf loop,
 	 * so bug out here.
 	 */
 	BUG_ON(div < 0);
-	if (__builtin_constant_p(div) && !(div >> 32)) {
-		s64 ns = kt.tv64;
+
+	if ((ns >> 32) == 0) {
+		return (s32)ns / div;
+	else if (__builtin_constant_p(div) && !(div >> 32)) {
 		u64 tmp = ns < 0 ? -ns : ns;
 
 		do_div(tmp, div);

I also just looked at the implementation of do_div() in
include/asm-generic/div64.h, and it already does that for
non-constant divisors, but I don't understand __div64_const32()
enough to know if the compiler end up doing the same
optimization for the constant divisor we have here.

	Arnd