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

From: Andrew Lunn
Date: Tue Aug 27 2019 - 12:13:26 EST

On Tue, Aug 27, 2019 at 03:23:34PM +0000, Voon, Weifeng wrote:
> > > > 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.

Hi Voon

Take for example mdio-hisi-femac.c

static int hisi_femac_mdio_read(struct mii_bus *bus, int mii_id, int regnum)
struct hisi_femac_mdio_data *data = bus->priv;
int ret;

ret = hisi_femac_mdio_wait_ready(data);
if (ret)
return ret;

writel((mii_id << BIT_PHY_ADDR_OFFSET) | regnum,
data->membase + MDIO_RWCTRL);

There is no check here for MII_ADDR_C45. So it will perform a C22
transfer. And regnum will still have MII_ADDR_C45 in it, so the
writel() is going to set bit 30, since #define MII_ADDR_C45
(1<<30). What happens on this hardware under these conditions?

You cannot unconditionally ask an MDIO driver to do a C45
transfer. Some drivers are going to do bad things.