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Re: [RFC v5 3/6] Add history to cross timestamp interface supporting slower devices

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On Thu, 7 Jan 2016, Christopher Hall wrote:
> On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz <john.stultz@linaro.org>
> wrote:
> > I've not done a super close reading here. But is it very likely the
> > the history_ref->cs_seq is the same as the captured seq? I thought
> > this history_ref was to allow old cross stamps to be used to improve
> > the back-calculation of the time at the given cycle value. So throwing
> > them out if they are older then the last tick seems strange.
> 
> Maybe this needs more explanation. The clocksource sequence (cs_seq) is
> incremented for each change in clocksource. I use this to detect a rare corner
> case where the clocksource is changed from (on x86 anyway) TSC and then back.
> If the history crosses one of these changes then interpolation shouldn't be
> attempted (return error). It's not really enough when using the history to
> just check that the current clocksource is equal to the one used at the start
> of the history. The clocksource must not have changed at all. To answer your
> question, it's not at all likely that this would occur.

You can flush the history when a clocksource change happens. No need to add
extra data to the core structures.

Thanks,

	tglx

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[RFC v5 0/6] Patchset enabling hardware based cross-timestamps for next gen Intel platforms "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
  [RFC v5 4/6] Remove duplicate code from ktime_get_raw_and_real code "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
    Re: [RFC v5 4/6] Remove duplicate code from ktime_get_raw_and_real code John Stultz <john.stultz@linaro.org> - 2016-01-06 20:50 +0100
  [RFC v5 3/6] Add history to cross timestamp interface supporting slower devices "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
    Re: [RFC v5 3/6] Add history to cross timestamp interface supporting  slower devices John Stultz <john.stultz@linaro.org> - 2016-01-06 20:40 +0100
      Re: [RFC v5 3/6] Add history to cross timestamp interface supporting  slower devices "Christopher Hall" <christopher.s.hall@intel.com> - 2016-01-08 02:10 +0100
        Re: [RFC v5 3/6] Add history to cross timestamp interface supporting  slower devices John Stultz <john.stultz@linaro.org> - 2016-01-08 02:20 +0100
        Re: [RFC v5 3/6] Add history to cross timestamp interface supporting  slower devices Thomas Gleixner <tglx@linutronix.de> - 2016-01-08 15:10 +0100
          Re: [RFC v5 3/6] Add history to cross timestamp interface supporting  slower devices "Christopher Hall" <christopher.s.hall@intel.com> - 2016-01-08 23:30 +0100
  [RFC v5 6/6] Adds hardware supported cross timestamp "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
  [RFC v5 2/6] Always Running Timer (ART) correlated clocksource "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
  [RFC v5 1/6] Timekeeping cross timestamp interface for device drivers "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
    Re: [RFC v5 1/6] Timekeeping cross timestamp interface for device drivers John Stultz <john.stultz@linaro.org> - 2016-01-06 20:00 +0100
      Re: [RFC v5 1/6] Timekeeping cross timestamp interface for device  drivers "Christopher Hall" <christopher.s.hall@intel.com> - 2016-01-08 01:50 +0100
        Re: [RFC v5 1/6] Timekeeping cross timestamp interface for device drivers John Stultz <john.stultz@linaro.org> - 2016-01-08 02:10 +0100
          Re: [RFC v5 1/6] Timekeeping cross timestamp interface for device  drivers Richard Cochran <richardcochran@gmail.com> - 2016-01-08 10:20 +0100
  [RFC v5 5/6] Add PTP_SYS_OFFSET_PRECISE for driver crosstimestamping "Christopher S. Hall" <christopher.s.hall@intel.com> - 2016-01-04 21:00 +0100
    Re: [RFC v5 5/6] Add PTP_SYS_OFFSET_PRECISE for driver  crosstimestamping Richard Cochran <richardcochran@gmail.com> - 2016-01-05 16:30 +0100
      Re: [RFC v5 5/6] Add PTP_SYS_OFFSET_PRECISE for driver  crosstimestamping "Christopher Hall" <christopher.s.hall@intel.com> - 2016-01-07 02:50 +0100

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