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: <20250307142252.GA2326148@legfed1>
Date: Fri, 7 Mar 2025 15:22:52 +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 02:39:32PM +0100 schrieb Andrew Lunn:
> On Fri, Mar 07, 2025 at 11:30:00AM +0100, Dimitri Fedrau via B4 Relay wrote:
> > The dp83822 provides the possibility to set the resistance value of the
> > the MAC series termination. Modifying the resistance to an appropriate
> > value can reduce signal reflections and therefore improve signal quality.
> 
> I have some dumb questions....
> 
> By referring to MAC from the perspective of the PHY, do you mean the
> termination of the bus between the MAC and the PHY? The SGMII SERDES,
> or RGMII?
>
- Yes, the perspective is from the PHY.
- Yes, but only the outputs towards the MAC. Resistors can be saved on
  the PCB when they are integrated into the PHY.
- The PHY is able to operate with RGMII, RMII and MII.

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

> I'm assuming the terminology is direct from the datasheet of the PHY?
> But since this is a bit of a niche area, no other PHY driver currently
> supports anythings like this, the terminology is not well known. So it
> would be good to expand the description, to make it really clear what
> you are talking about, so if anybody else wants to add the same
> feature, they make use of the property, not add a new property.

The datasheet calls it MAC impedance control, that is what someone might
want to achieve when selecting the proper resistance value. I named it
"mac-series-termination-ohms" instead to make sure what is done to
achieve impedance matching.

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

Best regards,
Dimitri Fedrau