Re: [PATCH v1] dt-bindings: net: ethernet-phy: Add forced-master/slave properties for SPE PHYs

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

On 9/6/24 09:11, Andrew Lunn wrote:
>>> 10Base-T1 often does not have autoneg, so preferred-master &
>>> preferred-slave make non sense in this context, but i wounder if
>>> somebody will want these later. An Ethernet switch is generally
>>> preferred-master for example, but the client is preferred-slave.
>>>
>>> Maybe make the property a string with supported values 'forced-master'
>>> and 'forced-slave', leaving it open for the other two to be added
>>> later.
>>
>> My two cents, don't take it as a nack or any strong disagreement, my
>> experience with SPE is still limited. I agree that for SPE, it's
>> required that PHYs get their role assigned as early as possible,
>> otherwise the link can't establish. I don't see any other place but DT
>> to put that info, as this would be required for say, booting over the
>> network. This to me falls under 'HW representation', as we could do the
>> same with straps.
>>
>> However for preferred-master / preferred-slave, wouldn't we be crossing
>> the blurry line of "HW description => system configuration in the DT" ?
> 
> Yes, we are somewhere near the blurry line. This is why i gave the
> example of an Ethernet switch, vs a client. Again, it could be done
> with straps, so following your argument, it could be considered HW
> representation. But if it is set wrong, it probably does not matter,
> auto-neg should still work. Except for a very small number of PHYs
> whos random numbers are not random...

Having had to deal with an Ethernet PHY that requires operating in slave 
mode "preferably" in order to have a correct RXC duty cycle, if you 
force both sides of the link to "slave", auto-negotiation will fail, 
however thanks to auto-negotiation you can tell that there was a 
master/slave resolution failure. (This reminds me I need to send the 
patch for that PHY errata at some point).

In the case that Oleksij seems to be after, there is no auto-negotiation 
(is that correct?), so it seems to me that the Device Tree is coming to 
the rescue of an improperly strapped HW, and is used as a way to change 
the default HW configuration so as to have a fighting chance of having a 
functional link. That is not unprecedented, but it is definitively a bit 
blurry...
-- 
Florian