linux/kernel.h: fix DIV_ROUND_CLOSEST to support negative divisors

While working on a thermal driver I encounter a scenario where the
divisor could be negative, instead of adding local code to handle this I
though I first try to add support for this in DIV_ROUND_CLOSEST.

Add support to DIV_ROUND_CLOSEST for negative divisors if both dividend
and divisor variable types are signed.  This should not alter current
behavior for users of the macro as previously negative divisors where
not supported.

Before:

DIV_ROUND_CLOSEST(  59,  4) =  15
DIV_ROUND_CLOSEST(  59, -4) = -14
DIV_ROUND_CLOSEST( -59,  4) = -15
DIV_ROUND_CLOSEST( -59, -4) =  14

After:

DIV_ROUND_CLOSEST(  59,  4) =  15
DIV_ROUND_CLOSEST(  59, -4) = -15
DIV_ROUND_CLOSEST( -59,  4) = -15
DIV_ROUND_CLOSEST( -59, -4) =  15

[akpm@linux-foundation.org: fix comment, per Guenter]
Link: http://lkml.kernel.org/r/20161222102217.29011-1-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index cb09238f6d32be355a9b7e2347e48746de7eae77..4c26dc3a8295a9fb2910dcb18946ee86f54ceac8 100644 (file)
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -100,16 +100,18 @@
 )
 
 /*
- * Divide positive or negative dividend by positive divisor and round
- * to closest integer. Result is undefined for negative divisors and
- * for negative dividends if the divisor variable type is unsigned.
+ * Divide positive or negative dividend by positive or negative divisor
+ * and round to closest integer. Result is undefined for negative
+ * divisors if he dividend variable type is unsigned and for negative
+ * dividends if the divisor variable type is unsigned.
  */
 #define DIV_ROUND_CLOSEST(x, divisor)(                 \
 {                                                      \
        typeof(x) __x = x;                              \
        typeof(divisor) __d = divisor;                  \
        (((typeof(x))-1) > 0 ||                         \
-        ((typeof(divisor))-1) > 0 || (__x) > 0) ?      \
+        ((typeof(divisor))-1) > 0 ||                   \
+        (((__x) > 0) == ((__d) > 0))) ?                \
                (((__x) + ((__d) / 2)) / (__d)) :       \
                (((__x) - ((__d) / 2)) / (__d));        \
 }                                                      \