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

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Message-ID: <24097072-1d54-4a24-aaf3-c6b28f31a6cb@gmail.com>
Date: Fri, 6 Sep 2024 09:22:29 -0700
From: Florian Fainelli <f.fainelli@...il.com>
To: Andrew Lunn <andrew@...n.ch>,
 Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: Oleksij Rempel <o.rempel@...gutronix.de>,
 Heiner Kallweit <hkallweit1@...il.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, kernel@...gutronix.de,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 Russell King <linux@...linux.org.uk>, devicetree@...r.kernel.org
Subject: Re: [PATCH v1] dt-bindings: net: ethernet-phy: Add
 forced-master/slave properties for SPE PHYs

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