linux-kernel - Re: [PATCH RFC net-next 4/5] net: phy: introduce is_c45_over

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Date:   Thu, 31 Mar 2022 14:06:16 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Michael Walle <michael@...le.cc>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Xu Liang <lxu@...linear.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 4/5] net: phy: introduce is_c45_over_c22 flag

On Thu, Mar 31, 2022 at 12:50:42PM +0100, Russell King (Oracle) wrote:
> On Thu, Mar 24, 2022 at 07:55:28PM +0100, Andrew Lunn wrote:
> > > The
> > > > only valid case i can think of is for a very oddball PHY which has C45
> > > > register space, but cannot actually do C45 transfers, and so C45 over
> > > > C22 is the only option.
> > > 
> > > And how would you know that the PHY has the needed registers in c22
> > > space? Or do we assume that every C45 PHY has these registers?
> > 
> > I think it is a reasonable assumption at the moment. We have around
> > 170 MDIO bus masters in Linux. All but one can do C22.
> 
> I don't think that is correct. I'm aware of the Marvell XMDIO driver
> that is C45 only, and also xgene's non-rgmii "xfi" variant which is
> also C45 only. Note that the xfi variant doesn't reject C22 and makes
> no distinction between a C22 and C45 access (so a C22 access to
> phy_id = 0 reg = 0 hits C45 phy_id = 0 mmd 0 reg 0.
> 
> MDIO drivers are IMHO an utter mess and are in dire need of fixing...
> and I'm coming to the conclusion that the bodge of passing both C22
> and C45 accesses through the same read/write functions is a huge
> mistake, one that is crying out for fixing to prevent more prolification
> of this kind of mess.
> 
> Yes, it's a lot of work, but I think it needs to be done. Retrofitting
> the MDIO drivers with checks etc sounds nice, but if we assume that
> patches will continue to be applied to net-next with little review,
> we have a losing battle - it would be better to have interfaces designed
> to make this kind of mistake impossible.

Hi Russell

So what i think you are saying is change the mii_bus structure:

diff --git a/include/linux/phy.h b/include/linux/phy.h
index 36ca2b5c2253..26322ee23867 100644
--- a/include/linux/phy.h
+++ b/include/linux/phy.h
@@ -353,10 +353,15 @@ struct mii_bus {
        const char *name;
        char id[MII_BUS_ID_SIZE];
        void *priv;
-       /** @read: Perform a read transfer on the bus */
-       int (*read)(struct mii_bus *bus, int addr, int regnum);
-       /** @write: Perform a write transfer on the bus */
-       int (*write)(struct mii_bus *bus, int addr, int regnum, u16 val);
+       /** @read: Perform a C22 read transfer on the bus */
+       int (*read_c22)(struct mii_bus *bus, int addr, int regnum);
+       /** @write: Perform a C22 write transfer on the bus */
+       int (*write_c22)(struct mii_bus *bus, int addr, int regnum, u16 val);
+       /** @read: Perform a C45 read transfer on the bus */
+       int (*read_c45)(struct mii_bus *bus, int addr, int devnum, int regnum);
+       /** @write: Perform a C45 write transfer on the bus */
+       int (*write_c45)(struct mii_bus *bus, int addr, int devnum,
+                        int regnum, u16 val);
        /** @reset: Perform a reset of the bus */
        int (*reset)(struct mii_bus *bus);

This way we get a cleaner interface, and the compiler helping us
finding drivers we miss during conversion?

	Andrew