netdev - Re: [RFC PATCH 00/13] hardware time stamping + igb example implementation

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Message-Id: <1226507118.31699.91.camel@ecld0pohly>
Date:	Wed, 12 Nov 2008 17:25:18 +0100
From:	Patrick Ohly <patrick.ohly@...el.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Octavian Purdila <opurdila@...acom.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Ingo Oeser <netdev@...eo.de>,
	"Ronciak, John" <john.ronciak@...el.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Oliver Hartkopp <oliver@...tkopp.net>
Subject: Re: [RFC PATCH 00/13] hardware time stamping + igb example
	implementation

On Wed, 2008-11-12 at 16:06 +0000, Andi Kleen wrote:
> As a general comment on the patch series I'm still a little sceptical
> the time stamp offset method is a good idea. Since it tries to approximate
> several unsynchronized clocks the result will always be of a little poor
> quality, which will likely lead to problems sooner or later (or rather
> require ugly workarounds in the user).
> 
> I think it would be better to just bite the bullet and add new fields
> for this to the skbs. Hardware timestamps are useful enough to justify
> this.

I'm all for it, as long as it doesn't keep this feature out of the
mainline.

At least one additional ktime_t field would be needed for the raw
hardware time stamp. Transformation to system time (as needed by PTP)
would have to be delayed until the packet is delivered via a socket. The
code would be easier (and a bit more accurate) if also another ktime_t
was added to store the transformed value directly after generating it.

An extra field would also solve one of the open problems (tstamp set to
time stamp when dev_start_xmit_hard is called for IP_MULTICAST_LOOP).

-- 
Best Regards, Patrick Ohly

The content of this message is my personal opinion only and although
I am an employee of Intel, the statements I make here in no way
represent Intel's position on the issue, nor am I authorized to speak
on behalf of Intel on this matter.

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