linux-kernel - Re: [PATCH v1 net-next] net: phy: mdio_bus: make mdiobus

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Date:   Tue, 27 Aug 2019 08:37:26 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     "Voon, Weifeng" <weifeng.voon@...el.com>,
        Andrew Lunn <andrew@...n.ch>
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



On 8/27/2019 8:23 AM, 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.

It is not so much the messing up that concerns me other than the
increased scan time. Assuming you are making this change to support your
stmmac PCI patch series with SGMII/RGMII, etc. cannot you introduce a
bitmask of C45 PHY addresses that should be scanned and the logic could
look like (pseudo code):

- 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

or something along those lines?
--
Florian