linux-kernel - Re: [PATCH 1/5] Add functions producing system time given a backing counter value

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Message-ID: <20150729123555.GP19282@twins.programming.kicks-ass.net>
Date:	Wed, 29 Jul 2015 14:35:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Christopher Hall <christopher.s.hall@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	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
Subject: Re: [PATCH 1/5] Add functions producing system time given a backing
 counter value

On Wed, Jul 29, 2015 at 01:48:41PM +0200, Richard Cochran wrote:
> On Wed, Jul 29, 2015 at 12:49:44PM +0200, Peter Zijlstra wrote:
> > This is still fuzzy, right? The hardware ART timestamp could be from
> > _before_ the NTP adjust; or the NTP adjust could happen while we do this
> > conversion and we'll take the retry loop.
> 
> In the original series, yes.
>  
> > In both cases, the resulting value is computed using a different slope
> > than was in effect at the time of the stamp. With the end result being
> > slightly off from what it should be.
> 
> In Thomas' patch the get_ts() is meant to read fresh pairs of time
> stamps from within the loop.

Ah, indeed, I missed that. Yes if that's possible we get it right.
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