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

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Date: Tue, 2 Jan 2024 12:02:09 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Heiner Kallweit <hkallweit1@...il.com>
Cc: ezra@...ergy-village.org, Andrew Lunn <andrew@...n.ch>,
	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:
> 
> Approach 1:
> Add a PHY driver flag to state: PHY is not c45-access-safe
> Then c45 scanning would be omitted if at least one c22 PHY
> with this flag was found.
> 
> Approach 2:
> Add a PHY driver flag to state: PHY is c45-access-safe
> Then c45 scanning would only be done if all found c22 devices

Anything based on PHY driver flags isn't going to work - the scan
happens _before_ we know what is on the bus and _before_ we have
any devices to even think about probing drivers (which could even
be in a module on a filesystem that has yet to be mounted.)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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