netdev - Re: PTP_PEROUT_REQUEST and clock stepping

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Date:	Fri, 12 Sep 2014 16:40:25 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Daniel Glöckner <dg@...ix.com>
Cc:	Christian Riesch <christian.riesch@...cron.at>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: PTP_PEROUT_REQUEST and clock stepping

On Fri, Sep 12, 2014 at 01:40:32PM +0200, Daniel Glöckner wrote:
> You mean the hardware that you know of that currently exists.

Yes, that is the hardware we support. I am sorry that I don't know how
to support mythical hardware.
 
> I just think it is wrong to place the burden of stopping and starting
> the periodic output on the PTP demon.

I think it is the right place.

> The kernel knows when clock_adjtime
> and clock_settime are called and it also knows which periodic outputs
> are enabled and how they are configured. We just have to decide on the
> correct behavior (respect .start vs. ignore .start once running) to be
> implemented.

The only reasonable actions for kernel would be to either stop all
periodic outputs, or to just do nothing.
 
> Taking the Intel i210 as an example, the third option (doing nothing)
> would lead to the following behavior:
> 
>  - If the clock is stepped back, the periodic output stops until the
>    clock reaches the point when it was adjusted.
> 
>  - If the clock is stepped forward, the periodic output oscillates at
>    62,5 MHz to catch up.

That is a good example of "the results are undefined".

So, how would you reprogram the i210 to keep the period output
continuous?

Thanks,
Richard
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