netdev - Re: [PATCH RFC net-next] dt-bindings: ethernet-controller: document signal multiplexer

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Date:   Wed, 7 Jul 2021 01:22:24 +0200
From:   Marek Behún <kabel@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Peter Rosin <peda@...ntia.se>, netdev@...r.kernel.org,
        Russell King <rmk+kernel@...linux.org.uk>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH RFC net-next] dt-bindings: ethernet-controller: document
 signal multiplexer

On Fri, 2 Jul 2021 02:55:54 +0200
Andrew Lunn <andrew@...n.ch> wrote:

> On Thu, Jul 01, 2021 at 02:53:47AM +0200, Marek Behún wrote:
> > There are devices where the MAC signals from the ethernet controller are
> > not directly connected to an ethernet PHY or a SFP cage, but to a
> > multiplexer, so that the device can switch between the endpoints.
> > 
> > For example on Turris Omnia the WAN controller is connected to a SerDes
> > switch, which multiplexes the SerDes lanes between SFP cage and ethernet
> > PHY, depending on whether a SFP module is present (MOD_DEF0 GPIO from
> > the SFP cage).  
> 
> At the moment, i don't think phylink supports this. It does not have a
> way to dynamically switch PHY. If the SFP disappears, you probably
> want to configure the PHY, so that it is up, autoneg started,
> etc. When the SFP reappears, the PHY needs to be configured down, the
> SFP probably needs its TX GPIO line set active, etc. None of this
> currently exists.

Of course this is not supported by phylink: it can't be, since we don't
even have a binding description :) I am figuring out how to do correct
binding while working on implementing this into phylink.

> The Marvell switches have something similar but different. Which ever
> gets link first, SFP or PHY gets the data path. In this case, you
> probably want phylink to configure both the SFP and the PHY, and then
> wait and see what happens. The hardware will then set the mux when one
> of them gets link. phylink should then configure the other
> down. Again, non of this exists at the moment.
> 
> I would imaging a similar binding could be used for these two
> conditions. But until we get the needed code, it is hard for me to
> say. So i think i would prefer to wait until we do have code.
> 

I now have an idea that might be sane for bindings, so next time I will
send the code as well.

> I also wonder how wise it is to put this into the generic ethernet
> controller binding. Muxing based on MOD_DEF0 i expect to be very
> rare. Muxing based on first port having link seems more likely. But
> both i expect are pretty unusual. So i would be tempted to make it a
> standalone binding, which can be imported into an MAC binding which
> actually needs it. Or it actually becomes part of the phylink
> binding, since this all appears to be PHY related, not MAC.
> 
> 	  Andrew

We'll see. Stay tuned for my patch series. :)

Marek