From b5776c4a6d0afc13697e8452b9ebe1cc4d961b74 Mon Sep 17 00:00:00 2001 From: john stultz Date: Thu, 16 Dec 2010 19:03:27 +0000 Subject: 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 Tested-by: Santosh Shilimkar Tested-by: Will Deacon Tested-by: Mikael Pettersson Tested-by: Eric Miao Tested-by: Olof Johansson Tested-by: Jamie Iles Signed-off-by: Russell King --- kernel/time/clocksource.c | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel') 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; -- cgit v1.2.3-70-g09d2