netdev - Re: PTP_PEROUT_REQUEST and clock stepping

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Message-ID: <20140911095245.GA5823@emlix.com>
Date:	Thu, 11 Sep 2014 11:52:45 +0200
From:	Daniel Glöckner <dg@...ix.com>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: PTP_PEROUT_REQUEST and clock stepping

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.

IMHO there is no use for a .start value > .period. It is good for defining
the phase of the periodic output but is just annoying if you want to have
the periodic output running all the time. 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.

Best regards,

  Daniel

-- 
Dipl.-Math. Daniel Glöckner, emlix GmbH, http://www.emlix.com
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