netdev - Re: [PATCH net] net: mdio: Prevent Clause 45 scan on SMSC PHYs

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Date: Tue, 2 Jan 2024 14:42:17 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Heiner Kallweit <hkallweit1@...il.com>
Cc: ezra@...ergy-village.org, Russell King <linux@...linux.org.uk>,
	Tristram Ha <Tristram.Ha@...rochip.com>,
	Michael Walle <michael@...le.cc>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: mdio: Prevent Clause 45 scan on SMSC PHYs

On Mon, Jan 01, 2024 at 11:44:38PM +0100, Heiner Kallweit wrote:
> On 01.01.2024 22:31, Ezra Buehler wrote:
> > Since commit 1a136ca2e089 ("net: mdio: scan bus based on bus
> > capabilities for C22 and C45") our AT91SAM9G25-based GARDENA smart
> > Gateway will no longer boot.
> > 
> > Prior to the mentioned change, probe_capabilities would be set to
> > MDIOBUS_NO_CAP (0) and therefore, no Clause 45 scan was performed.
> > Running a Clause 45 scan on an SMSC/Microchip LAN8720A PHY will (at
> > least with our setup) considerably slow down kernel startup and
> > ultimately result in a board reset.
> > 
> > AFAICT all SMSC/Microchip PHYs are Clause 22 devices. Some have a
> > "Clause 45 protection" feature (e.g. LAN8830) and others like the
> > LAN8804 will explicitly state the following in the datasheet:
> > 
> >     This device may respond to Clause 45 accesses and so must not be
> >     mixed with Clause 45 devices on the same MDIO bus.
> > 
> 
> I'm not convinced that some heuristic based on vendors is a
> sustainable approach. Also I'd like to avoid (as far as possible)
> that core code includes vendor driver headers. Maybe we could use
> a new PHY driver flag. Approaches I could think of:

We already have a core hack for these broken PHYs:

/*
 * There are some C22 PHYs which do bad things when where is a C45
 * transaction on the bus, like accepting a read themselves, and
 * stomping over the true devices reply, to performing a write to
 * themselves which was intended for another device. Now that C22
 * devices have been found, see if any of them are bad for C45, and if we
 * should skip the C45 scan.
 */
static bool mdiobus_prevent_c45_scan(struct mii_bus *bus)
{
        int i;

        for (i = 0; i < PHY_MAX_ADDR; i++) {
                struct phy_device *phydev;
                u32 oui;

                phydev = mdiobus_get_phy(bus, i);
                if (!phydev)
                        continue;
                oui = phydev->phy_id >> 10;

                if (oui == MICREL_OUI)
                        return true;
        }
        return false;
}

So it seems we need to extend this with another OUI.

	Andrew