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authorjohn stultz <johnstul@us.ibm.com>2010-12-16 19:03:27 +0000
committerRussell King <rmk+kernel@arm.linux.org.uk>2010-12-22 22:44:43 +0000
commitb5776c4a6d0afc13697e8452b9ebe1cc4d961b74 (patch)
tree0b60b679d636a9759849fea1cd36a33438fff5ef /kernel/time
parent5e06b6492e53ab2a4e467763a9ee9f70b032c301 (diff)
Fix rounding in clocks_calc_mult_shift()
Russell King reports: | On the ARM dev boards, we have a 32-bit counter running at 24MHz. Calling | clocks_calc_mult_shift(&mult, &shift, 24MHz, NSEC_PER_SEC, 60) gives | us a multiplier of 2796202666 and a shift of 26. | | Over a large counter delta, this produces an error - lets take a count | from 362976315 to 4280663372: | | (4280663372-362976315) * 2796202666 / 2^26 - (4280663372-362976315) * (1000/24) | => -38.91872422891230269990 | | Can we do better? | | (4280663372-362976315) * 2796202667 / 2^26 - (4280663372-362976315) * (1000/24) | 19.45936211449532822051 | | which is about twice as good as the 2796202666 multiplier. | | Looking at the equivalent divisions obtained, 2796202666 / 2^26 gives | 41.66666665673255920410ns per tick, whereas 2796202667 / 2^26 gives | 41.66666667163372039794ns. The actual value wanted is 1000/24 = | 41.66666666666666666666ns. Fix this by ensuring we round to nearest when calculating the multiplier. Signed-off-by: John Stultz <john.stultz@linaro.org> Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com> Tested-by: Will Deacon <will.deacon@arm.com> Tested-by: Mikael Pettersson <mikpe@it.uu.se> Tested-by: Eric Miao <eric.y.miao@gmail.com> Tested-by: Olof Johansson <olof@lixom.net> Tested-by: Jamie Iles <jamie@jamieiles.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/clocksource.c1
1 files changed, 1 insertions, 0 deletions
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index c18d7efa1b4..df140cd3ea4 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -152,6 +152,7 @@ clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 minsec)
*/
for (sft = 32; sft > 0; sft--) {
tmp = (u64) to << sft;
+ tmp += from / 2;
do_div(tmp, from);
if ((tmp >> sftacc) == 0)
break;