netdev - Re: PTP_PEROUT_REQUEST and clock stepping

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Message-ID: <CABkLObpyxgxsPZDEyf=Wr+Cfy5Kav9KmxkKOtWXP_4S5Jh8+Fg@mail.gmail.com>
Date:	Fri, 12 Sep 2014 08:17:45 +0200
From:	Christian Riesch <christian.riesch@...cron.at>
To:	Daniel Glöckner <dg@...ix.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: PTP_PEROUT_REQUEST and clock stepping

Hi Daniel,

On Thu, Sep 11, 2014 at 11:52 AM, Daniel Glöckner <dg@...ix.com> wrote:
> On Wed, Sep 10, 2014 at 07:26:20PM +0200, Richard Cochran wrote:
>> On Wed, Sep 10, 2014 at 06:16:37PM +0200, Daniel Glöckner wrote:
>> > I was wondering how periodic output is supposed to behave if the clock
>> > is stepped from a time after ptp_perout_request.start to a time before
>> > ptp_perout_request.start.
>>
>> I would say the result is undefined.
>>
>> User space should first stop the periodic output, then reprogram the
>> clock, then restart the periodic output. Anything else makes no sense
>> at all.
>
> But then the PTP demon has to be responsible for configuring the periodic
> outputs as well.

I guess yes, but is that a problem?

> IMHO there is no use for a .start value > .period. It is good for defining

There is, for the small fraction of people you note below. But you are
talking about PTP_PEROUT_REQUEST here, right?

> the phase of the periodic output but is just annoying if you want to have
> the periodic output running all the time.

How about PTP_ENABLE_PPS? Is this the ptp clock feature you are looking for?

> Only a small fraction of people
> using periodic outputs will want to schedule the start of the signal at
> a specific time in the future and only a fraction of those will use a
> .period value that is so small that they can't use a timer to configure
> the periodic output within .period/2 before the desired start time.

What timer?

Regards,
Christian
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