netdev - Re: [PATCH 0/3] Patch series for a PTP Grandmaster use case using stmmac/gmac3 ptp clock

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Date:   Thu, 14 May 2020 17:37:06 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Olivier Dautricourt <olivier.dautricourt@...lia.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] Patch series for a PTP Grandmaster use case using
 stmmac/gmac3 ptp clock

On Thu, May 14, 2020 at 05:09:01PM +0200, Olivier Dautricourt wrote:
>   My issue is that the default behavior of the stmmac driver is to
>   set the mac into fine mode which implies to continuously do frequency
>   adjustment, So if i'm not mistaken using the nullf servo will not address
>   that issue even if we are not doing explicit clock_adjtime syscalls.

Why not use the external time stamps from your GPS to do one of the
following?

A) Feed them into a user space PI controller and do frequency
   correction, or

B) Feed the offsets into the hardware PLL (if that is present) using
   the new PHC ADJ_OFFSET support (found in net-next).

I don't see a need for changing any kernel interfaces at all.  If you
think there is such a need, then you will have to make some kind of
argument or justification for it.

BTW, support for B has been implemented in linuxptp, and the patches
will appear soon.  We can also add it into the ts2phc program if you
like.

Thanks,
Richard