From: John Stultz <john.stultz@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
---
This is the final piece for my sched_clock() patches, which is required
by the patches to operate correctly with the known-constants-at-compile-
time platforms to avoid the kernel complaining that the constants are
wrong. The above changelog was written by me, patch was from John, and
I tweaked the spacing to match the kernel style.
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 c18d7ef..df140cd 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;
--
1.6.2.5
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