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: Mon, 19 Feb 2024 15:31:06 +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 Fri, 16 Feb 2024 08:47:14 +0100
Oleksij Rempel <o.rempel@...gutronix.de> wrote:
> >
> > So, either somebody needs to understand 1000BaseT and can say the
> > proposed binding works, or we explicitly document the binding is
> > limited to 10BaseT and 100BaseT.  
> 
> I asked the internet and found the answer: Some PSE/PD implementations
> are not compatible with 1000BaseT.
> 
> See Figure 33–4—10BASE-T/100BASE-TX Endpoint PSE location overview.
> Alternative B show a variant where power is injected directly to pairs
> without using magnetics as it is done for Alternative A (phantom
> delivery - over magnetics).
> 
> So, we have following variants of 2 pairs PoE:
> +---------+---------------+-------------------+---------------------+--------------------+
> | Variant | Alternative   | Polarity          | Power Feeding Type  |
> Compatibility with | |         | (a/b)         | (Direct/Reverse)  |
> (Direct/Phantom)    | 1000BaseT          |
> +=========+===============+===================+=====================+====================+
> | 1       | a             | Direct            | Phantom             | Yes
>            |
> +---------+---------------+-------------------+---------------------+--------------------+
> | 2       | a             | Reverse           | Phantom             | Yes
>            |
> +---------+---------------+-------------------+---------------------+--------------------+
> | 3       | b             | Direct            | Phantom             | Yes
>            |
> +---------+---------------+-------------------+---------------------+--------------------+
> | 4       | b             | Reverse           | Phantom             | Yes
>            |
> +---------+---------------+-------------------+---------------------+--------------------+
> | 5       | b             | Direct            | Direct              | No
>            |
> +---------+---------------+-------------------+---------------------+--------------------+
> | 6       | b             | Reverse           | Direct              | No
>            |
> +---------+---------------+-------------------+---------------------+--------------------+

Maybe we could remove the polarity column on this table as it does not bring
more information. It is also already explained on the PI pinout alternatives
table.

Also we should document that a 4pairs PSE supporting only 10/100BaseT (which
mean no magnetics on pinout AlternativeB) may not be compatible with a 4pairs
1GBaseT PD.

> For this case, it will be good if systems knows supported modes, so user
> can get this information  directly. For example with ethtool

Yes.

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