netdev - Re: [PATCH net-next] net: dsa: mark parsed interface mode for legacy switch drivers

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Date: Thu, 17 Aug 2023 21:27:29 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Andrew Lunn <andrew@...n.ch>,
	"Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Linus Walleij <linus.walleij@...aro.org>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: mark parsed interface mode for legacy
 switch drivers

On Mon, Aug 14, 2023 at 07:05:19PM +0200, Andrew Lunn wrote:
> arch/arm/boot/dts/nxp/vf/vf610-zii-dev-rev-b.dts:
>                                        switch0port5: port@5 {
>                                                 reg = <5>;
>                                                 label = "dsa";
>                                                 phy-mode = "rgmii-txid";
>                                                 link = <&switch1port6
>                                                         &switch2port9>;
>                                                 fixed-link {
>                                                         speed = <1000>;
>                                                         full-duplex;
>                                                 };
>                                         };
> 
> and the other end of this link:
> 
>                                         switch1port6: port@6 {
>                                                 reg = <6>;
>                                                 label = "dsa";
>                                                 phy-mode = "rgmii-txid";
>                                                 link = <&switch0port5>;
>                                                 fixed-link {
>                                                         speed = <1000>;
>                                                         full-duplex;
>                                                 };
>                                         };
> 
> imx7d-zii-rpu2.dts:
>                                 port@5 {
>                                         reg = <5>;
>                                         label = "cpu";
>                                         ethernet = <&fec1>;
>                                         phy-mode = "rgmii-id";
> 
>                                         fixed-link {
>                                                 speed = <1000>;
>                                                 full-duplex;
>                                         };
>                                 };
> 
> With the 'cpu' case, it is clearly acting like a PHY to the SoCs MAC,
> so it does not seem too unreasonable for it to act upon it. For a DSA
> link there is not a clear MAC-PHY relationship, but we do somehow need
> to specify delays.

Andrew, I'd argue that the MAC-PHY relationship between the DSA master
and the CPU port is equally clear as between 2 arbitrary cascade ports.
Which is: not clear at all. The RGMII standard does not talk about the
existence of a MAC role and a PHY role, to my knowledge. These are 2
MACs back to back, in both cases.

With rx-internal-delay-ps and tx-internal-delay-ps in each MAC node, you
get the freedom of specifying RGMII delays in whichever way is needed,
without baking in any assumption that the port plays the role of a PHY
or not.