netdev - Re: [PATCH net-next v2 1/8] net-timestamp: explicit SO

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Date:	Mon, 7 Jul 2014 22:11:56 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Chad Reese <kreese@...ium.com>
Cc:	Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Chad Reese <kreese@...iumnetworks.com>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH net-next v2 1/8] net-timestamp: explicit SO_TIMESTAMPING
 ancillary data struct

On Mon, Jul 07, 2014 at 12:44:44PM -0700, Chad Reese wrote:
> 
> A hardware timer used for ethernet timestamps is completely
> independent from the kernel's software view of time. Since the
> hardware timestamps are only exposed in the driver, how can they be
> correlated with system time? If the driver doesn't do it, then
> nobody else knows how.

Um, implement a PTP Hardware Clock device?

Don't reimplement clock servos in your driver. Instead, leave that to
the PTP stack (like using linuxptp's phc2sys).
 
> For Octeon, you can optionally use the hardware timestamp as the
> system clock reference. Most people don't, but it is the only way to
> get the system time to be accurate. 1588 can synchronize two Octeon
> boards to less than 1ns for the hardware timer. The Linux software
> timers is always farther off.

1588 cannot synchronize boards unless you expose the clock to the
userland PTP stack. Why don't you do that?

Thanks,
Richard
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