netdev - Re: [PATCH 5/5] arm64: dts: renesas: r8a779f0: spider: Enable Ethernet Switch

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Message-ID: <YyHFnZdGTJL8uLxn@lunn.ch>
Date:   Wed, 14 Sep 2022 14:14:21 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "geert+renesas@...der.be" <geert+renesas@...der.be>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 5/5] arm64: dts: renesas: r8a779f0: spider: Enable
 Ethernet Switch

> > > +		port@2 {
> > > +			reg = <2>;
> > > +			phy-handle = <&etha2>;
> > > +			phy-mode = "sgmii";
> > > +			#address-cells = <1>;
> > > +			#size-cells = <0>;
> > > +			etha2: ethernet-phy@2 {
> > > +				reg = <3>;
> > > +				compatible = "ethernet-phy-ieee802.3-c45";
> > > +			};
> > > +		};
> > 
> > I find it interesting you have PHYs are address 1, 2, 3, even though
> > they are on individual busses. Why pay for the extra pullup/down
> > resistors when they could all have the same address?
> 
> I don't know why. But, the board really configured such PHY addresses...

That is not wrong. It could be the hardware engineer is used to shared
MDIO busses, and just copy/pasted an existing design, but then
separated the busses?

You might see actual customer boards putting all the PHYs on one MDIO
bus, to save pins. Linux has no problem with that, the phy-handle can
point anywhere.

One last thought. Is there anything in the data sheet about the switch
hardware directly talking the PHY? Some of the Marvell switches can do
that, but we disable that feature. The hardware has no idea what the
PHY driver is doing, such as selecting different pages.

	  Andrew