Re: [PATCH net V2 1/1] net: phy: fix invalid phy id when probe using C22

[Florian Fainelli <f.fainelli@gmail.com>]

On 3/18/2021 11:14 AM, Greg KH wrote:
> On Thu, Mar 18, 2021 at 09:02:22AM -0700, Florian Fainelli wrote:
>>
>>
>> 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.
> 
> If you need me to do that under the umbrella of the Linux Foundation,
> I'll be glad to do so if you point me at the proper group to do that
> with.
> 
> We did this for a few years with the USB-IF and have a vendor id
> assigned to us for Linux through them, until they kicked us out because.
> But as the number is in a global namespace, it can't be reused so we
> keep it :)

We would still be creating what is technically an user-space interface
breakage since prior to a given kernel version we would return 0 for
that PHY OUI, and after another point it would be something different. I
don't know what software out there may be expecting to find 0 and not
determine that the PHY was fixed already because it is under
/sys/class/mdio_bus/fixed-0
-- 
Florian