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author | John Stultz <john.stultz@linaro.org> | 2011-10-28 01:12:42 (GMT) |
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committer | Ingo Molnar <mingo@elte.hu> | 2011-10-28 06:57:38 (GMT) |
commit | c2bc11113c50449f23c40b724fe410fc2380a8e9 (patch) | |
tree | f5a6a5f9c199d6b3ed7a973dfb7556ad2af2bf5a /kernel/time/timekeeping.c | |
parent | e35f95b36e43f67a6f806172555a152c11ea0a78 (diff) | |
download | linux-fsl-qoriq-c2bc11113c50449f23c40b724fe410fc2380a8e9.tar.xz |
time: Improve documentation of timekeeeping_adjust()
After getting a number of questions in private emails about the
math around admittedly very complex timekeeping_adjust() and
timekeeping_big_adjust(), I figure the code needs some better
comments.
Hopefully the explanations are clear enough and don't muddy the
water any worse.
Still needs documentation for ntp_error, but I couldn't recall
exactly the full explanation behind the code that's there
(although I do recall once working it out when Roman first
proposed it). Given a bit more time I can probably work it out,
but I don't want to hold back this documentation until then.
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Chen Jie <chenj@lemote.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1319764362-32367-1-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel/time/timekeeping.c')
-rw-r--r-- | kernel/time/timekeeping.c | 81 |
1 files changed, 80 insertions, 1 deletions
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 2b021b0e..025e136 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -802,14 +802,44 @@ static void timekeeping_adjust(s64 offset) s64 error, interval = timekeeper.cycle_interval; int adj; + /* + * The point of this is to check if the error is greater then half + * an interval. + * + * First we shift it down from NTP_SHIFT to clocksource->shifted nsecs. + * + * Note we subtract one in the shift, so that error is really error*2. + * This "saves" dividing(shifting) intererval twice, but keeps the + * (error > interval) comparision as still measuring if error is + * larger then half an interval. + * + * Note: It does not "save" on aggrivation when reading the code. + */ error = timekeeper.ntp_error >> (timekeeper.ntp_error_shift - 1); if (error > interval) { + /* + * We now divide error by 4(via shift), which checks if + * the error is greater then twice the interval. + * If it is greater, we need a bigadjust, if its smaller, + * we can adjust by 1. + */ error >>= 2; + /* + * XXX - In update_wall_time, we round up to the next + * nanosecond, and store the amount rounded up into + * the error. This causes the likely below to be unlikely. + * + * The properfix is to avoid rounding up by using + * the high precision timekeeper.xtime_nsec instead of + * xtime.tv_nsec everywhere. Fixing this will take some + * time. + */ if (likely(error <= interval)) adj = 1; else adj = timekeeping_bigadjust(error, &interval, &offset); } else if (error < -interval) { + /* See comment above, this is just switched for the negative */ error >>= 2; if (likely(error >= -interval)) { adj = -1; @@ -817,9 +847,58 @@ static void timekeeping_adjust(s64 offset) offset = -offset; } else adj = timekeeping_bigadjust(error, &interval, &offset); - } else + } else /* No adjustment needed */ return; + /* + * So the following can be confusing. + * + * To keep things simple, lets assume adj == 1 for now. + * + * When adj != 1, remember that the interval and offset values + * have been appropriately scaled so the math is the same. + * + * The basic idea here is that we're increasing the multiplier + * by one, this causes the xtime_interval to be incremented by + * one cycle_interval. This is because: + * xtime_interval = cycle_interval * mult + * So if mult is being incremented by one: + * xtime_interval = cycle_interval * (mult + 1) + * Its the same as: + * xtime_interval = (cycle_interval * mult) + cycle_interval + * Which can be shortened to: + * xtime_interval += cycle_interval + * + * So offset stores the non-accumulated cycles. Thus the current + * time (in shifted nanoseconds) is: + * now = (offset * adj) + xtime_nsec + * Now, even though we're adjusting the clock frequency, we have + * to keep time consistent. In other words, we can't jump back + * in time, and we also want to avoid jumping forward in time. + * + * So given the same offset value, we need the time to be the same + * both before and after the freq adjustment. + * now = (offset * adj_1) + xtime_nsec_1 + * now = (offset * adj_2) + xtime_nsec_2 + * So: + * (offset * adj_1) + xtime_nsec_1 = + * (offset * adj_2) + xtime_nsec_2 + * And we know: + * adj_2 = adj_1 + 1 + * So: + * (offset * adj_1) + xtime_nsec_1 = + * (offset * (adj_1+1)) + xtime_nsec_2 + * (offset * adj_1) + xtime_nsec_1 = + * (offset * adj_1) + offset + xtime_nsec_2 + * Canceling the sides: + * xtime_nsec_1 = offset + xtime_nsec_2 + * Which gives us: + * xtime_nsec_2 = xtime_nsec_1 - offset + * Which simplfies to: + * xtime_nsec -= offset + * + * XXX - TODO: Doc ntp_error calculation. + */ timekeeper.mult += adj; timekeeper.xtime_interval += interval; timekeeper.xtime_nsec -= offset; |