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

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Fri, Sep 06, 2024 at 09:22:29AM -0700, Florian Fainelli wrote:
> 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...

Yes, there is no auto-negotiation on T1 PHY variants, so the DT property is
to fix broken or not existing for some reason HW straps.

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