netdev - RE: [PATCH 1/5] Add functions producing system time given a backing counter value

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Date:	Wed, 29 Jul 2015 01:41:34 +0000
From:	"Hall, Christopher S" <christopher.s.hall@...el.com>
To:	'John Stultz' <john.stultz@...aro.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH 1/5] Add functions producing system time given a backing
 counter value


> -----Original Message-----
> From: John Stultz [mailto:john.stultz@...aro.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@...nel.org; lkml;
> netdev@...r.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@...el.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