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| author |   john stultz <johnstul@us.ibm.com> | 2010-12-16 19:03:27 +0000 |
|---|---|---|
| committer |   Russell King <rmk+kernel@arm.linux.org.uk> | 2010-12-22 22:44:43 +0000 |
| commit | b5776c4a6d0afc13697e8452b9ebe1cc4d961b74 (patch) | |
| tree | 0b60b679d636a9759849fea1cd36a33438fff5ef /kernel | |
| parent | ARM: ensure all sched_clock() implementations are notrace marked (diff) | |
| download | linux-dev-b5776c4a6d0afc13697e8452b9ebe1cc4d961b74.tar.xz linux-dev-b5776c4a6d0afc13697e8452b9ebe1cc4d961b74.zip | |
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')
| -rw-r--r-- | kernel/time/clocksource.c | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index c18d7efa1b4b..df140cd3ea47 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; |