netdev - Re: [PATCH 1/2] posix clocks: introduce a syscall for clock tuning.

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Date:	Thu, 9 Sep 2010 23:31:52 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	john stultz <johnstul@...ibm.com>
cc:	Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] posix clocks: introduce a syscall for clock
 tuning.

On Thu, 9 Sep 2010, john stultz wrote:
> On Thu, 2010-09-09 at 12:49 +0200, Thomas Gleixner wrote:
> > But what I see is an approach which tries to implement disconnected
> > special purpose clocks which have the ability to be adjusted
> > independently. What's the purpose of this ? Why can't we just use the
> > existing clocks and make PTP work on them ?
> 
> So this too was my initial gut response. It seems ridiculous to expose
> two clock_ids (CLOCK_REALTIME and CLOCK_PTP)that conceptually represent
> the same thing (ie: number of seconds,nanoseconds since 1970).
> 
> It doesn't help that one of the use cases that Richard suggests is "for
> example in an embedded control application. The userland software can
> simply ignore the wrong system time." 
> 
> As someone who's spent a *lot* of time trying to fix the "wrong system
> time" these use cases reek of work-around solutions instead of properly
> fixing whatever keeps the system time from being accurately sycned.
> 
> However, as I've worked on understanding the issue, I realize that there
> is some validity to needing to expose more then one hardware clock the
> conceptually is the same as CLOCK_REALTIME. And that most of my gut
> reaction to this was me being a bit oversensitive. :)

Yup. It still scares me that we might end up with a dozen different
notions of ONE second elapsed on the same machine :)

> However, since there may be multiple PTP clocks or audio clocks or
> whatever, allocating static clockids for each type isn't quite useful,

Yeah, I corrected myself on that one, but I really want to see some
confinement into well defined clock classes rather than the "hooray
here is my clock of the day" approach.

Thanks,

	tglx
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