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: <20250307192305.GA8328@debian>
Date: Fri, 7 Mar 2025 20:23:05 +0100
From: Dimitri Fedrau <dima.fedrau@...il.com>
To: Andrew Lunn <andrew@...n.ch>
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

Am Fri, Mar 07, 2025 at 04:34:59PM +0100 schrieb Andrew Lunn:
> > 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.
>
Ok.

> 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?
>
I think they are also needed for the PHY inputs, but termination
resistors should be placed near the driver. In case the MAC doesn't have
them integrated they should be placed near the MAC outputs. From PHY
perspective we just care about the PHY outputs.

https://resources.pcb.cadence.com/blog/termination-resistors-in-pcb-design

I don't know if there is a general rule that states wether to use series
termination or not. I think it depends on the PCB design and if there are
issues with signal quality. Maybe this one helps, there is a small
chapter regarding series termination:

https://resources.altium.com/p/gigabit-ethernet-101-basics-implementation

Found it reading this:

https://community.nxp.com/t5/i-MX-Processors/Why-doesn-t-NXP-use-termination-resistors-for-MII-data-lines/td-p/1360873

> 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.
>
Ok.

Best regards,
Dimitri Fedrau