linux-kernel - [PATCH v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative dividends

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Message-Id: <1346425339-17138-1-git-send-email-linux@roeck-us.net>
Date:	Fri, 31 Aug 2012 08:02:19 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jean Delvare <khali@...ux-fr.org>, lm-sensors@...sensors.org,
	Guenter Roeck <linux@...ck-us.net>
Subject: [PATCH v4] linux/kernel.h: Fix DIV_ROUND_CLOSEST to support negative dividends

DIV_ROUND_CLOSEST returns a bad result for negative dividends:
	DIV_ROUND_CLOSEST(-2, 2) = 0

Most of the time this does not matter. However, in the hardware monitoring
subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
negative (such as temperatures).

Signed-off-by: Guenter Roeck <linux@...ck-us.net>
---
v4: Don't address negative divisors to improve chances for compile-time
    optimization. Document DIV_ROUND_CLOSEST macro.

v3: Instead of adding a new macro, fix DIV_ROUND_CLOSEST.
    This version works for negative dividend and divisor.

v2: v1 did not work if typeof(divisor) was an unsigned variable type
    (which can obviously not be negative).
    Rework to revert to DIV_ROUND_CLOSEST if the dividend is unsigned,
    or if it is signed but non-negative.

 include/linux/kernel.h |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 6043821..594b419 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -82,10 +82,18 @@
 	__x - (__x % (y));				\
 }							\
 )
+
+/*
+ * Divide positive or negative dividend by positive divisor and round
+ * to closest integer. Result is undefined for negative divisors.
+ */
 #define DIV_ROUND_CLOSEST(x, divisor)(			\
 {							\
-	typeof(divisor) __divisor = divisor;		\
-	(((x) + ((__divisor) / 2)) / (__divisor));	\
+	typeof(x) __x = x;				\
+	typeof(divisor) __d = divisor;			\
+	(((typeof(x))-1) >= 0 || (__x) >= 0) ?		\
+		(((__x) + ((__d) / 2)) / (__d)) :	\
+		(((__x) - ((__d) / 2)) / (__d));	\
 }							\
 )
 
-- 
1.7.9.7

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