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Subject: Re: [PATCH net V2 1/1] net: phy: fix invalid phy id when probe using
 C22
To:     Heiner Kallweit <hkallweit1@gmail.com>,
        Wong Vee Khee <vee.khee.wong@intel.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Russell King <linux@armlinux.org.uk>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Voon Weifeng <weifeng.voon@intel.com>,
        Ong Boon Leong <boon.leong.ong@intel.com>
References: <20210318090937.26465-1-vee.khee.wong@intel.com>
 <b63c5068-1203-fcb6-560d-1d2419bb39b0@gmail.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <c921bf7f-e4d1-eefa-c5ae-024d5e8a4845@gmail.com>
Date:   Thu, 18 Mar 2021 09:02:22 -0700
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On 3/18/2021 6:25 AM, Heiner Kallweit wrote:
> On 18.03.2021 10:09, Wong Vee Khee wrote:
>> When using Clause-22 to probe for PHY devices such as the Marvell
>> 88E2110, PHY ID with value 0 is read from the MII PHYID registers
>> which caused the PHY framework failed to attach the Marvell PHY
>> driver.
>>
>> Fixed this by adding a check of PHY ID equals to all zeroes.
>>
> 
> I was wondering whether we have, and may break, use cases where a PHY,
> for whatever reason, reports PHY ID 0, but works with the genphy
> driver. And indeed in swphy_read_reg() we return PHY ID 0, therefore
> the patch may break the fixed phy.
> Having said that I think your patch is ok, but we need a change of
> the PHY ID reported by swphy_read_reg() first.
> At a first glance changing the PHY ID to 0x00000001 in swphy_read_reg()
> should be sufficient. This value shouldn't collide with any real world
> PHY ID.

It most likely would not, but it could be considered an ABI breakage,
unless we filter out what we report to user-space via SIOGCMIIREG and
/sys/class/mdio_bus/*/*/phy_id

Ideally we would have assigned an unique PHY OUI to the fixed PHY but
that would have required registering Linux as a vendor, and the process
is not entirely clear to me about how to go about doing that.
--
Florian