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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

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.)

-- 
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