netdev - Re: [PATCH RFC net-next v2 11/12] net: dsa: Separate C22 and C45 MDIO bus transaction methods

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Date:   Tue, 3 Jan 2023 17:56:33 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Michael Walle <michael@...le.cc>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Sergey Shtylyov <s.shtylyov@....ru>,
        Wei Fang <wei.fang@....com>,
        Shenwei Wang <shenwei.wang@....com>,
        Clark Wang <xiaoning.wang@....com>,
        NXP Linux Team <linux-imx@....com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Luiz Angelo Daros de Luca <luizluca@...il.com>
Subject: Re: [PATCH RFC net-next v2 11/12] net: dsa: Separate C22 and C45
 MDIO bus transaction methods

On Tue, Jan 03, 2023 at 04:48:59PM +0100, Andrew Lunn wrote:
> > Since clause 45 PHYs are identified by the "ethernet-phy-ieee802.3-c45"
> > compatible string (otherwise they are C22), then a PHY which is not
> > described in the device tree can only be C22. So this is why
> > ds->slave_mii_bus only deals with clause 22 methods, and the true reason
> > behind the comment above.
> > 
> > But actually this premise is no longer true since Luiz' commit
> > fe7324b93222 ("net: dsa: OF-ware slave_mii_bus"), which introduced the
> > strange concept of an "OF-aware helper for internal PHYs which are not
> > described in the device tree". After his patch, it is possible to have
> > something like this:
> > 
> > 	ethernet-switch {
> > 		ethernet-ports {
> > 			port@1 {
> > 				reg = <1>;
> > 			};
> > 		};
> > 
> > 		mdio {
> > 			ethernet-phy@1 {
> > 				compatible = "ethernet-phy-ieee802.3-c45"
> > 				reg = <1>;
> > 			};
> > 		};
> > 	};
> > 
> > As you can see, this is a clause 45 internal PHY which lacks a
> > phy-handle, so its bus must be put in ds->slave_mii_bus in order for
> > dsa_slave_phy_connect() to see it without that phy-handle (based on the
> > port number matching with the PHY number). After Luiz' patch, this kind
> > of device tree is possible, and it invalidates the assumption about
> > ds->slave_mii_bus only driving C22 PHYs.
> 
> My memory is hazy, but i think at the time i wrote these patches,
> there was no DSA driver which made use of ds->slave_mii_bus with
> C45. So i took the short cut of only supporting C22.

Actually I believe that in v1 you did extend ds->ops with C45 methods,
but it's me who told you to remove them:
https://patchwork.kernel.org/project/netdevbpf/patch/20220508153049.427227-10-andrew@lunn.ch/#24852813

> 
> Those DSA drivers which do support C45 all register their bus directly
> with the MDIO core.

And rightfully so. IMHO, letting DSA allocate ds->slave_mii_bus out of
driver writer sheer convenience (a secondary purpose) should be deprecated,
unless the reason for using ds->slave_mii_bus is the lack of a phy-handle
(the primary purpose). It becomes that more confusing to have to extend
dsa_switch_ops with 2 more methods which serve the secondary purpose but
not the primary one.

> So Luiz patches may allow a C45 bus, but are there any drivers today
> actually using it?

I sent a private email to Luiz a few minutes ago asking him to confirm.