netdev - Re: [RFC PATCH 1/3] of: provide a binding for the 'fixed-link' property

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Date:	Mon, 12 Aug 2013 10:37:46 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	"David S. Miller" <davem@...emloft.net>,
	Florian Fainelli <florian@...nwrt.org>,
	Lior Amsalem <alior@...vell.com>, netdev@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 1/3] of: provide a binding for the 'fixed-link'
 property

On Mon, Aug 12, 2013 at 10:16:49AM +0200, Thomas Petazzoni wrote:
> Dear Sascha Hauer,
> 
> On Mon, 12 Aug 2013 08:38:06 +0200, Sascha Hauer wrote:
> 
> > > This patch adds:
> > > 
> > >  * A documentation for the Device Tree property "fixed-link".
> > > 
> > >  * A of_phy_register_fixed_link() OF helper, which provided an OF node
> > >    that contains a "fixed-link" property, registers the corresponding
> > >    fixed PHY.
> > > 
> > >  * Removes the warning on the of_phy_connect_fixed_link() that says
> > >    new drivers should not use it, since Grant Likely indicated that
> > >    this "fixed-link" property is indeed the way to go.
> > > 
> > 
> > Any progress with this series?
> 
> I am not sure there really was a consensus yet on what the DT binding
> looks like. As soon as there is a consensus, I'm definitely willing to
> make progress on this series.
> 
> > We have more and more boards here with exactly the same problem as
> > Thomas has. For reasons stated below I don't like this binding, but
> > still it would solve my problem.
> 
> Ok.
> 
> > > +Example:
> > > +
> > > +ethernet@0 {
> > > +	...
> > > +	fixed-link = <1 1 1000 0 0>;
> > > +	...
> > > +};
> > 
> > I must say I don't like this binding at all for two reasons.
> 
> As I explained, this binding was chosen for this RFC for two reasons:
> 
>  * It's the binding used on PowerPC platforms to represent fixed links.
>  * It allows to encode all the informations into a single property,
>    which avoids the need for a separate DT node for a "fake PHY", which
>    isn't a representation of the hardware.

The fake phy is avoided by making the other side of the link what it
really is: An ethernet switch. I'm currently not aware of a situation
where a fixed link is needed and the other side is not a switch. And I
can't think of a situation in which the other side of the other side of
the fixed link really is pure 'virtual', I mean there always must be
something connected, right?

> 
> > First the positional arguments make it impossible to add optional
> > arguments to the link.
> > 
> > Second the other side of the link is most likely a switch. Once this
> > switch has its own node in the devicetree it seems like having a phandle
> > to the switch here would be better.
> 
> So, in other words, what you're suggesting is something like:
> 
> 	ethernet@0 {
> 		reg = <...>;
> 		interrupt = <...>;
> 		phy = <&phy0>;
> 		phy0: phy@0 {
> 			fixed-link;
> 			speed = <1000>;
> 			full-duplex;
> 			...
> 		};
> 	};

Yes, this looks good. ePAPR suggests naming the phy property
"phy-handle" instead of just "phy", but that's just details. In case the
phy really is a switch the phandle could just point to a i2c device instead
of the ethernet node.

Sascha

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