netdev - Re: [PATCH V2 net-next 2/5] net: Introduce a new MII time stamping interface.

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Date:   Sun, 7 Oct 2018 23:14:05 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     netdev@...r.kernel.org, devicetree@...r.kernel.org,
        David Miller <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH V2 net-next 2/5] net: Introduce a new MII time stamping
 interface.

On Sun, Oct 07, 2018 at 01:59:06PM -0700, Richard Cochran wrote:
> On Sun, Oct 07, 2018 at 09:54:00PM +0200, Andrew Lunn wrote:
> > Sure, but things have moved on since then.
> 
> If you have a specific suggestion on how to better implement this,
> please tell us what it is.
>   
> > I can think of three obvious use cases where this does not work:
> > 
> > 1) phylink, not phdev. We have been pushing some MAC drivers towards
> > phylink, especially those which support >1Gbp.
> 
> If a phylink device appears that wants time stamping, can't we add the
> call to register_mii_timestamper()?

Hi Richard

The problem is you depend on skbuf->dev->phydev. phydev will be NULL.
net_device does not currently have a phylink member. Even if it did,
you end up add more and more tests looking every place a
mii_timestamper could be placed.
  
> > 2) When an SFP is connected to the MAC, not a copper PHY. The class of
> > device you are adding a driver for will work just as well for an SFP
> > as for a copper PHY. The SERDES interface remains the same,
> > independent of if a copper PHY is used, or a SFP. But an SFP does not
> > have an instance of a phydv.
> 
> Well, as I said before in v1, CONFIG_NETWORK_PHY_TIMESTAMPING depends
> on phylib, plain and simple, and expanding beyond phylib is not within
> the scope of the this series.

True. But we also should be forward looking, to make sure we are not
heading into a dead end.

I'm currently thinking register_mii_timestamper() should take a netdev
argument, and the net_device structure should gain a struct
mii_timestamper.

But we have to look at the lifetime problems. A phydev does not know
what netdev it is associated to until phy_connect() is called. It is
at that point you can call register_mii_timestamper().

   Andrew