netdev - RE: [PATCH v1 net-next] net: phy: mdio_bus: make mdiobus

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Date:   Tue, 27 Aug 2019 15:23:34 +0000
From:   "Voon, Weifeng" <weifeng.voon@...el.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
CC:     "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jose Abreu <joabreu@...opsys.com>,
        "Heiner Kallweit" <hkallweit1@...il.com>,
        "Ong, Boon Leong" <boon.leong.ong@...el.com>
Subject: RE: [PATCH v1 net-next] net: phy: mdio_bus: make mdiobus_scan also
 cover PHY that only talks C45

> > > Make mdiobus_scan() to try harder to look for any PHY that only
> talks C45.
> > If you are not using Device Tree or ACPI, and you are letting the MDIO
> > bus be scanned, it sounds like there should be a way for you to
> > provide a hint as to which addresses should be scanned (that's
> > mii_bus::phy_mask) and possibly enhance that with a mask of possible
> > C45 devices?
> 
> Yes, i don't like this unconditional c45 scanning. A lot of MDIO bus
> drivers don't look for the MII_ADDR_C45. They are going to do a C22
> transfer, and maybe not mask out the MII_ADDR_C45 from reg, causing an
> invalid register write. Bad things can then happen.
> 
> With DT and ACPI, we have an explicit indication that C45 should be used,
> so we know on this platform C45 is safe to use. We need something
> similar when not using DT or ACPI.
> 
> 	  Andrew

Florian and Andrew,
The mdio c22 is using the start-of-frame ST=01 while mdio c45 is using ST=00
as identifier. So mdio c22 device will not response to mdio c45 protocol.
As in IEEE 802.1ae-2002 Annex 45A.3 mention that:
" Even though the Clause 45 MDIO frames using the ST=00 frame code
will also be driven on to the Clause 22 MII Management interface,
the Clause 22 PHYs will ignore the frames. "

Hence, I am not seeing any concern that the c45 scanning will mess up with 
c22 devices.

Weifeng