linux-kernel - Re: [PATCH net-next 0/3] net: phy: dp83822: Add support for changing the MAC series termination

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Message-ID: <d57aff5b-7d1d-43bf-95a1-ee90689f5ac0@lunn.ch>
Date: Fri, 7 Mar 2025 16:34:59 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Dimitri Fedrau <dima.fedrau@...il.com>
Cc: 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>,
	Rob Herring <robh@...nel.org>,
	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
Subject: Re: [PATCH net-next 0/3] net: phy: dp83822: Add support for changing
 the MAC series termination

> Should I add the proper description in the bindings ? Description of the
> properties are somehow short. However will expand the description.

Yes, please expand the description. For well known concepts, we can
keep the binding description short. But i would not consider this a
well known concept, so we need to spell out in detail what it is.

My knowledge of transmission lines and termination is not so good....

So this configures the resistor on the PHY outputs. Do PHY inputs also
need termination resistors? Could there be PHYs which also allow such
resistors to be configured? Are there use cases where you need
asymmetric termination resistors?

My questions are trying to lead to an answer to your question:

> Should I rename then "mac-series-termination-ohms" to
> "output-mac-series-termination-ohms" or similar ?

We should think about this from the general case, not one specific
PHY, and ideally from thinking about the physics of termination.

https://electronics.stackexchange.com/questions/524620/impedance-termination-of-marvell-phy

This seems to suggest RGMII only has termination resistors at the
outputs. So "mac-series-termination-ohms" would be O.K.

	Andrew