netdev - RE: [PATCH v3 1/4] Add correlated clocksource deriving system time from an auxiliary clocksource

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Date:	Fri, 4 Sep 2015 21:12:47 +0000
From:	"Hall, Christopher S" <christopher.s.hall@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: RE: [PATCH v3 1/4] Add correlated clocksource deriving system time
 from an auxiliary clocksource

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Friday, September 04, 2015 7:28 AM
> To: Richard Cochran
> Cc: Hall, Christopher S; Thomas Gleixner; Kirsher, Jeffrey T;
> hpa@...or.com; mingo@...hat.com; john.stultz@...aro.org; x86@...nel.org;
> linux-kernel@...r.kernel.org; netdev@...r.kernel.org; intel-wired-
> lan@...ts.osuosl.org
> Subject: Re: [PATCH v3 1/4] Add correlated clocksource deriving system time
> from an auxiliary clocksource
> 
> > > (several milliseconds) and the result will be out of date by some
> fraction of that
> > > amount.
> >
> > Why does it take milliseconds to read one audio time stamp?
> 
> So what I suspect, but please correct me if I'm wrong Chris, is that a
> DSP will buffer and process audio signals, and only later wake up the
> main CPU.
> 
> So by the time the CPU is made aware of the data, it's 'old'.

That's about right.  The DSP runs on a 1 ms cadence.  Any access to registers controlled by the DSP will take 1-2 DSP ticks to access.
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