On Wed, 29 Jul 2015, Hall, Christopher S wrote: > > > -----Original Message----- > > From: John Stultz [mailto:john.stu...@linaro.org] > > Sent: Monday, July 27, 2015 8:44 PM > > To: Hall, Christopher S > > Cc: Thomas Gleixner; Richard Cochran; Ingo Molnar; Kirsher, Jeffrey T; > > Ronciak, John; H. Peter Anvin; x...@kernel.org; lkml; > > netdev@vger.kernel.org > > Subject: Re: [PATCH 1/5] Add functions producing system time given a > > backing counter value > > > > On Mon, Jul 27, 2015 at 5:46 PM, Christopher Hall > > <christopher.s.h...@intel.com> wrote: > > > * counter_to_rawmono64 > > > * counter_to_mono64 > > > * counter_to_realtime64 > > > > > > Enables drivers to translate a captured system clock counter to system > > > time. This is useful for network and audio devices that capture > > timestamps > > > in terms of both the system clock and device clock. > > > > Huh. So for counter_to_realtime64 & mono64, this seems to ignore the > > fact that the multiplier is constantly adjusted and corrected. So that > > calling the function twice with the same counter value may result in > > different returned values. > > > > I've not yet groked the whole patchset, but it seems like there needs > > to be some mechanism that ensures the counter value is captured and > > used in the same (or at least close) interval that the timekeeper data > > is valid for. > > The ART (and derived TSC) values are always in the past. There's no > chance that we could exceed the interval. I don't think any similar > usage would be a problem either. > > Are you suggesting that, for completeness, this be enforced by the > conversion function? > > I do a check here to make sure that the current counter value isn't before > the beginning of the current interval: > > timekeeping_get_delta() > ... > if (cycle_now < tkr->cycle_last && > tkr->cycle_last - cycle_now < ROLLOVER_THRESHOLD) > return -EAGAIN; > > If tkr->cycle_last - cycle_now is large, the assumption is that > rollover occurred. Otherwise, the caller should re-read the counter > so that it falls within the current interval. In my "normal use" > testing, re-read never occurred.
Sure that never happens, because your rollover value is 2 << 39 for whatever reasons. So on a 1GHz machine that is (2 << 39) / 1e9 ~= 1099.51 seconds. Oh well. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
