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

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Message-ID: <20120720061132.GA2330@netboy.at.omicron.at>
Date:	Fri, 20 Jul 2012 08:11:32 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Andrew Jackson <ajackson@...arflare.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com
Subject: Re: [PATCH net-next 4/7] sfc: Add support for IEEE-1588 PTP

On Thu, Jul 19, 2012 at 05:05:45PM +0100, Andrew Jackson wrote:
> On 19/07/2012 15:37, Ben Hutchings wrote:
> >On Thu, 2012-07-19 at 16:25 +0200, 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.
> >
> >Andrew, would that work for us?
> 
> I don't think so for the reason that Stu has pointed out (failed
> assumption).
> 
> The NIC's clock isn't directly accessible by the host from the PCIe
> bus and is "behind" the MC. Even when we process PPS events, we need
> a reliable way of determining the relationship between the two
> clocks (system <> NIC). We're trying to get that as accurately as we
> can but we know that some measurements will be incorrect/out of
> bounds because of loading on the system.

There is a better way to filter out bad measurements than to hard code
some black magic into the driver. Instead, just offer a PPS, and then
we can comfortably (and flexibly) filter using a user space program.

Thanks,
Richard
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