linux-kernel - Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

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Message-Id: <201008191428.04546.arnd@arndb.de>
Date:	Thu, 19 Aug 2010 14:28:04 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	john stultz <johnstul@...ibm.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org,
	Krzysztof Halasa <khc@...waw.pl>,
	Rodolfo Giometti <giometti@...ux.it>
Subject: Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

On Thursday 19 August 2010, Richard Cochran wrote:
> On Wed, Aug 18, 2010 at 05:12:56PM -0700, john stultz wrote:
> > On Wed, 2010-08-18 at 09:19 +0200, Richard Cochran wrote:
> > > The timer/alarm stuff is "ancillary" and is not at all necessary. It
> > > is just a "nice to have." I will happily remove it, if it is too
> > > troubling for people.
> > 
> > If there's a compelling argument for it, I'm interested to hear. But
> > again, it seems like just
> > yet-another-way-to-get-alarm/timer-functionality, so before we add an
> > extra API (or widen an existing API) I'd like to understand the need.
> 
> We don't really need it, IMHO.
> 
> But if we offer clockid_t CLOCK_PTP, then we get timer_settime()
> without any extra effort.

Makes sense.

> > So in thinking about this, try to focus on what the new clock_id
> > provides that the other existing clockids do not? Are they at comparable
> > levels of abstraction? 15 years from now, are folks likely to still be
> > using it? Will it be maintainable? etc...
> 
> Arnd convinced me that clockid_t=CLOCK_PTP is a good fit.

My point was that a syscall is better than an ioctl based interface here,
which I definitely still think. Given that John knows much more about
clocks than I do, we still need to get agreement on the question that
he raised, which is whether we actually need to expose this clock to the
user or not.

If we can find a way to sync system time accurate enough with PTP and
PPS, user applications may not need to see two separate clocks at all.

> My plan would be to introduce just one additional syscall:
> 
> SYSCALL_DEFINE3(clock_adjtime, const clockid_t, clkid,
> 		int, ppb, struct timespec __user *, ts)
> 
> ppb - desired frequency adjustment in parts per billion
> ts  - desired time step (or jump) in <sec,nsec> to correct
>       a measured offset
> 
> Arguably, this syscall might be useful for other clocks, too.

This is a mix of adjtime and adjtimex with the addition of
the clkid parameter, right?

Have you considered passing a struct timex instead of ppb and ts?

Is using ppb instead of the timex ppm required to get the accuracy
you want?

	Arnd
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