linux-kernel - Re: [PATCH net-next 1/3] dt-bindings: net: ethernet-phy: add property mac-series-termination-ohms

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Message-ID: <de68ea7e-1ca5-4983-9824-3fb432b00e82@lunn.ch>
Date: Tue, 11 Mar 2025 19:06:09 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Rob Herring <robh@...nel.org>
Cc: Dimitri Fedrau <dimitri.fedrau@...bherr.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dimitri Fedrau <dima.fedrau@...il.com>
Subject: Re: [PATCH net-next 1/3] dt-bindings: net: ethernet-phy: add
 property mac-series-termination-ohms

On Tue, Mar 11, 2025 at 12:33:44PM -0500, Rob Herring wrote:
> On Fri, Mar 07, 2025 at 11:30:01AM +0100, Dimitri Fedrau wrote:
> > Add property mac-series-termination-ohms in the device tree bindings for
> > selecting the resistance value of the builtin series termination resistors
> > of the PHY. Changing the resistance to an appropriate value can reduce
> > signal reflections and therefore improve signal quality.
> > 
> > Signed-off-by: Dimitri Fedrau <dimitri.fedrau@...bherr.com>
> > ---
> >  Documentation/devicetree/bindings/net/ethernet-phy.yaml | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/net/ethernet-phy.yaml b/Documentation/devicetree/bindings/net/ethernet-phy.yaml
> > index 824bbe4333b7ed95cc39737d3c334a20aa890f01..4a710315a83ccf15bfc210ae432ae988cf31e04c 100644
> > --- a/Documentation/devicetree/bindings/net/ethernet-phy.yaml
> > +++ b/Documentation/devicetree/bindings/net/ethernet-phy.yaml
> > @@ -238,6 +238,11 @@ properties:
> >        peak-to-peak specified in ANSI X3.263. When omitted, the PHYs default
> >        will be left as is.
> >  
> > +  mac-series-termination-ohms:
> 
> A property of the MAC (or associated with it) should be in the MAC's 
> node.

But it is the PHY which uses the property, and the PHY which is
implementing the resistor.

Also, a PHY has two sides, one towards the MAC and a second media side
to the network peer via the Ethernet cable. Both sides need
termination resistors. So we need something in the name to make it
clear which side of the PHY we are talking about. So we might end up
with something like mac-termination-ohms and media-termination-ohms,
in the PHY node.

> Also, sounds like either either end could have a property.

True, the MAC could also need a similar property, since the outputs
from the MAC to the PHY needs termination resistors.  For the MAC,
termination-ohms is probably sufficient, or phy-termination-ohms to
indicate it is towards the PHY?

	Andrew