linux-kernel - Re: [PATCH v3 1/4] Add correlated clocksource deriving system time from an auxiliary clocksource

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Message-ID: <20150904142827.GE18489@twins.programming.kicks-ass.net>
Date:	Fri, 4 Sep 2015 16:28:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	"Hall, Christopher S" <christopher.s.hall@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"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

On Fri, Sep 04, 2015 at 10:11:22AM +0200, Richard Cochran wrote:
> On Thu, Sep 03, 2015 at 11:20:37PM +0000, Hall, Christopher S wrote:
> > In addition to the network interface, ART will be used in the audio interface as well.
> > We need to support the case where an audio co-processor will control the audio device.
> > In this case, the get_ts() function supplied by the audio driver will be very slow
> > (several milliseconds) and the result will be out of date by some fraction of that 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'.

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