RE: [PATCH v1 net-next] net: phy: mdio_bus: make mdiobus_scan also cover PHY that only talks C45

From: Voon, Weifeng
Date: Tue Aug 27 2019 - 11:23:40 EST

> > > 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.