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

[Ong, Boon Leong]

>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.

Andrew & Florian, thanks for your review on this patch and insights on it.
We will look into the implementation as suggested as follow. 

- for each bit clear in mii_bus::phy_mask, scan it as C22
- for each bit clear in mii_bus::phy_c45_mask, scan it as C45

We will work on this and resubmit soonest.