linux-kernel - Re: [PATCH net-next v3 14/17] dt-bindings: net: pse-pd: Add bindings for PD692x0 PSE controller

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Date: Thu, 15 Feb 2024 11:41:23 +0100
From: Köry Maincent <kory.maincent@...tlin.com>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: Andrew Lunn <andrew@...n.ch>, Rob Herring <robh@...nel.org>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Jonathan
 Corbet <corbet@....net>, Luis Chamberlain <mcgrof@...nel.org>, Russ Weight
 <russ.weight@...ux.dev>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Krzysztof Kozlowski
 <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>,
 Mark Brown <broonie@...nel.org>, Frank Rowand <frowand.list@...il.com>,
 Heiner Kallweit <hkallweit1@...il.com>, Russell King
 <linux@...linux.org.uk>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, devicetree@...r.kernel.org, Dent Project
 <dentproject@...uxfoundation.org>
Subject: Re: [PATCH net-next v3 14/17] dt-bindings: net: pse-pd: Add
 bindings for PD692x0 PSE controller

On Thu, 15 Feb 2024 09:17:48 +0100
Oleksij Rempel <o.rempel@...gutronix.de> wrote:

> On Wed, Feb 14, 2024 at 06:41:54PM +0100, Andrew Lunn wrote:
> > > Alternative A and B Overview
> > > ----------------------------
> > > 
> > > - **Alternative A:** Utilizes the data-carrying pairs for power
> > > transmission in 10/100BaseT networks. The power delivery's polarity in
> > > this alternative can vary based on the MDI (Medium Dependent Interface)
> > > or MDI-X (Medium Dependent Interface Crossover) configuration.
> > > 
> > > - **Alternative B:** Delivers power over the spare pairs not used for
> > > data in 10/100BaseT networks. Unlike Alternative A, Alternative B's
> > > method separates power from data lines within the cable. Though it is
> > > less influenced by data transmission direction, Alternative B includes
> > > two configurations with different polarities, known as variant X and
> > > variant S, to accommodate different network requirements and device
> > > specifications.  
> > 
> > Thanks for this documentation.
> > 
> > It might be worth pointing out that RJ-45 supports up to 4
> > pairs. However, 10/100BaseT only makes use of two pairs for data
> > transfer from the four. 1000BaseT and above make use of all four pairs
> > for data transfer. If you don't know this, it is not so obvious what
> > 'data-carrying pairs' and 'spare pairs' mean.  
> 
> @Kory, can you please update it.
> 
> > And what happens for 1000BaseT when all four pairs are in use?  
> 
> Hm.. good question. I didn't found the answer in the spec. By combining all
> puzzle parts I assume, different Alternative configurations are designed
> to handle conflict between "PSE Physical Layer classification" and PHY
> autoneg.

Oleksij how did you get the definition of Alternative A uses the "data-carrying"
pairs for power transmission and Alternative B Delivers power over the "spare
pairs"?

On my understanding of the 2022 standard the definition is: 
- Alternative A is for pinout conductors 1, 2, 3 and 6
- Alternative B is for pinout conductors 4, 5, 7, 8.

Then indeed if we are in 10/100BaseT Alternative A are "data-carrying
pairs" and Alternative B are "spare pairs" but that's not the case on
1000BaseT.

You can see it in the figures in the paragraph 145.2.3.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com