Messages in this thread | | | From | "Hall, Christopher S" <> | Subject | RE: [PATCH 1/5] Add functions producing system time given a backing counter value | Date | Wed, 29 Jul 2015 01:41:34 +0000 |
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> -----Original Message----- > From: John Stultz [mailto:john.stultz@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; x86@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.hall@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.
Thanks for your input.
Chris
> > thanks > -john
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