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Date:   Wed, 8 Jul 2020 04:00:53 -0700
From:   Richard Cochran <richardcochran@gmail.com>
To:     Vladimir Oltean <olteanv@gmail.com>
Cc:     Sergey Organov <sorganov@gmail.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Fugang Duan <fugang.duan@nxp.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, andrew@lunn.ch
Subject: Re: [PATCH  1/5] net: fec: properly support external PTP PHY for
 hardware time stamping
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On Tue, Jul 07, 2020 at 10:04:37AM +0300, Vladimir Oltean wrote:
> > > We do it like this:
> > > - DSA: If there is a timestamping switch stacked on top of a
> > >   timestamping Ethernet MAC, the switch hijacks the .ndo_do_ioctl of the
> > >   host port, and you are supposed to use the PTP clock of the switch,
> > >   through the .ndo_do_ioctl of its own (virtual) net devices. This
> > >   approach works without changing any code in each individual Ethernet
> > >   MAC driver.
> > > - PHY: The Ethernet MAC driver needs to be kind enough to check whether
> > >   the PHY supports hw timestamping, and pass this ioctl to that PHY
> > >   while making sure it doesn't do anything stupid in the meanwhile, like
> > >   also acting upon that timestamping request itself.
> > >
> > > Both are finicky in their own ways. There is no real way for the user to
> > > select which PHC they want to use. The assumption is that you'd always
> > > want to use the outermost one, and that things in the kernel side always
> > > collaborate towards that end.

Vladimir, your explanations in this thread are valuable.  Please
consider converting them into a patch to expand

   Documentation/networking/timestamping.rst


Thanks,
Richard