netdev - Re: [net-next 5/6] ixgbe: add hardware timestamping support

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Date:	Thu, 20 Oct 2011 16:56:37 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Jacob Keller <jacob.keller@...il.com>
Cc:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>, davem@...emloft.net,
	Jacob Keller <jacob.e.keller@...el.com>,
	netdev@...r.kernel.org, gospo@...hat.com, sassmann@...hat.com
Subject: Re: [net-next 5/6] ixgbe: add hardware timestamping support

On Wed, Oct 19, 2011 at 10:04:33AM -0700, Jacob Keller wrote:
> On Mon, Oct 17, 2011 at 9:44 AM, Richard Cochran <richardcochran@...il.com> wrote:
> > So, is this wrap around due to the fact that you are tied to the
> > system time via time_compare? Or, putting it another way, can't you
> > program the hardware time stamping unit so that the registers have
> > some reasonable resolution (like 64 bits worth of nanoseconds) and
> > just offer RAW timestamps?
> 
> The wrap around is due to hardware limitations. The ixgbe devices
> cannot support 64bits worth of nanoseconds and still have the ability
> to adjust the frequency in parts per billion. A larger increment
> increases the resolution available for frequency adjustments, but
> decreases the time it takes for the cycle counter to wrap around.

Oh, well. That stinks.

I think you do want to offer ppb adjustment.

> > I would really like to move away from the timecompare hacks and
> > towards a proper PHC->SYS PPS solution.
> >
> 
> I agree that this is the correct approach. The timecompare
> functionality does have issues.

And these cards are highlighting timecompare weaknesses I had not even
thought of.

I expect that if you offer the RAW time stamps, then it should be
possible to have the time stamp values always correct (or nearly so)
even with a changing link speed. If the link speed change gives an
interrupt, then the ISR can reprogram the frequency compensation
registers and let the counter continue.

> > Again, doing the update thing on every packet won't work for real
> > world PTP scenarios.
> >
> Which is why the PHC solution is better. Work on implementing this
> support is in progress. Out of curiosity, what is the sync rate for
> the scenario that breaks this? I would like to try that rate out on my
> setup.

For the audio/video profile, they have a max of 32 sync packets per
second. Not sure about delay request rate, maybe 16 per second.

Thanks,
Richard
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