netdev - Re: [net-next.git 2/9] stmmac: add IEEE 1588-2002 PTP support

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Date:	Sun, 10 Mar 2013 13:31:54 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Rayagond K <rayagond@...avyalabs.com>
Cc:	Giuseppe CAVALLARO <peppe.cavallaro@...com>,
	netdev@...r.kernel.org, bh74.an@...sung.com
Subject: Re: [net-next.git 2/9] stmmac: add IEEE 1588-2002 PTP support

On Fri, Mar 08, 2013 at 01:15:00PM +0530, Rayagond K wrote:
> On Fri, Mar 8, 2013 at 12:32 PM, Giuseppe CAVALLARO <peppe.cavallaro@...com>wrote:
> > On 3/8/2013 7:34 AM, Richard Cochran wrote:

> >> What are these coarse/fine method? Can't we always use the fine one?
> >>
> >
> > This is explained in the "4.1.2 System Time Register Module" of Synopsys
> > Databook. Summarizing, the MAC can have an optional module and use this
> > coarse correction method. In the fine correction method, a slave clock’s
> > frequency drift with respect to the master clock is corrected over a period
> > of time instead of in one clock, as in coarse correction.
> >
> > Pls, Rayagond feels free to provide more details...
> 
> 
> Yes Synopsys core support two method for correcting the system time
> ie COARSE method  and FINE method.
> 
> In the fine correction method, a slave clock’s frequency drift with respect
> to the master clock (as defined in IEEE 1588) is corrected over a period of
> time instead of in one clock, as in coarse correction. This helps maintain
> linear time and does not introduce drastic changes (or a large jitter) in
> the reference time between PTP Sync message intervals.

So it sounds like to me that you only should offer the COARSE
method. Smoothly changing the frequency of the clock is the job of the
clock servo which is running in user space.

Although some clocks (like this one and the phyter, for example) have
the ability to smoothly correct a given offset over a given time, we
do not have any user/kernel API for making use of this feature.

Thanks,
Richard
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