netdev - Re: [PATCH net-next 4/7] sfc: Add support for IEEE-1588 PTP

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Message-ID: <1342713051.2617.40.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Thu, 19 Jul 2012 16:50:51 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stuart Hodgson <smhodgson@...arflare.com>
CC:	Richard Cochran <richardcochran@...il.com>,
	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>,
	Andrew Jackson <ajackson@...arflare.com>
Subject: Re: [PATCH net-next 4/7] sfc: Add support for IEEE-1588 PTP

On Thu, 2012-07-19 at 16:29 +0100, Stuart Hodgson wrote:
> On 19/07/12 15:25, Richard Cochran wrote:
[...]
> > I am trying to purge the whole SYS thing (only blackfin is left)
> > because there is a much better way to go about this, namely
> > synchronizing the system time to the PHC time via an internal PPS
> > signal.
> 
> This may be possible in future. But leads us to another problem
> where the PPS event that is generated by the PHC subsystem to the 
> PPS subsystem is stamped with the current system_time. That may
> be fine for a PPS signal generated from an interrupt but not when
> the internal PPS event has implicit jitter from the handler/event_queue
> that we have in the driver.
[...]

We can certainly take a timestamp in the hard interrupt handler; in fact
that's what I originally expected we would do since we have a separate
MSI-X vector for PTP.  But even hard interrupt handling can be subject
to substantial jitter.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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