diff options
| -rw-r--r-- | lib/math/div64.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/lib/math/div64.c b/lib/math/div64.c index 55a81782e271..191761b1b623 100644 --- a/lib/math/div64.c +++ b/lib/math/div64.c @@ -22,6 +22,7 @@ #include <linux/export.h> #include <linux/math.h> #include <linux/math64.h> +#include <linux/minmax.h> #include <linux/log2.h> /* Not needed on 64bit architectures */ @@ -191,6 +192,20 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c) /* can a * b overflow ? */ if (ilog2(a) + ilog2(b) > 62) { /* + * Note that the algorithm after the if block below might lose + * some precision and the result is more exact for b > a. So + * exchange a and b if a is bigger than b. + * + * For example with a = 43980465100800, b = 100000000, c = 1000000000 + * the below calculation doesn't modify b at all because div == 0 + * and then shift becomes 45 + 26 - 62 = 9 and so the result + * becomes 4398035251080. However with a and b swapped the exact + * result is calculated (i.e. 4398046510080). + */ + if (a > b) + swap(a, b); + + /* * (b * a) / c is equal to * * (b / c) * a + |