linux-kernel - Re: [PATCH devicetree 3/4] arm64: dts: fsl: ls1028a: add node for Felix switch

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Date:   Wed, 19 Feb 2020 01:15:20 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Shawn Guo <shawnguo@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Claudiu Manoil <claudiu.manoil@....com>
Subject: Re: [PATCH devicetree 3/4] arm64: dts: fsl: ls1028a: add node for
 Felix switch

Hi Andrew,

On Mon, 17 Feb 2020 at 19:24, Vladimir Oltean <olteanv@...il.com> wrote:
>
> On Mon, 17 Feb 2020 at 17:33, Vladimir Oltean <olteanv@...il.com> wrote:
> >
> > Hi Andrew,
> >
> > On Mon, 17 Feb 2020 at 17:29, Andrew Lunn <andrew@...n.ch> wrote:
> > >
> > > Hi Vladimir
> > >
> > > > +                                     /* Internal port with DSA tagging */
> > > > +                                     mscc_felix_port4: port@4 {
> > > > +                                             reg = <4>;
> > > > +                                             phy-mode = "gmii";
> > >
> > > Is it really using gmii? Often in SoC connections use something else,
> > > and phy-mode = "internal" is more appropriate.
> > >
> >
> > What would be that "something else"? Given that the host port and the
> > switch are completely different hardware IP blocks, I would assume
> > that a parallel GMII is what's connecting them, no optimizations done.
> > Certainly no serializer. But I don't know for sure.
> > Does it matter, in the end?
> >
>
> To clarify, the reason I'm asking whether it matters is because I'd
> have to modify PHY_INTERFACE_MODE_GMII in
> drivers/net/dsa/ocelot/felix_vsc9959.c too, for the internal ports.
> Then I'm not sure anymore what tree this device tree patch should go
> in through.
>
> > > > +                                             ethernet = <&enetc_port2>;
> > > > +
> > > > +                                             fixed-link {
> > > > +                                                     speed = <2500>;
> > > > +                                                     full-duplex;
> > > > +                                             };
> > >
> > > gmii and 2500 also don't really go together.
> >
> > Not even if you raise the clock frequency?
> >
> > >
> > >      Andrew
> >
> > Thanks,
> > -Vladimir

Correct me if I'm wrong, but I think that PHY_INTERFACE_MODE_INTERNAL
is added by Florian in 2017 as a generalization of the BCM7445 DSA
switch bindings with internal PHY ports, and later became "popular"
with other DSA drivers (ar9331, lantiq gswip). Of those, ar9331 is
actually using phy-mode = "gmii" for the CPU port, and phy-mode =
"internal" for the embedded copper PHYs.
I hate to be making this sort of non-binary decision. Is it a GMII
interface _or_ an internal interface? Prior to 2017, this would have
probably been a non-question. The patch series which adds it does not
clarify "you should use this mode in situation A, and this mode in
situation B" either.

Regards,
-Vladimir