linux-kernel - Re: [PATCH net v2] net: dsa: b53: bcm531x5: fix cpu rgmii mode interpretation

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Message-ID: <aQ4RR4OQI9f2bBOG@shell.armlinux.org.uk>
Date: Fri, 7 Nov 2025 15:33:27 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Andrew Lunn <andrew@...n.ch>, Jonas Gorski <jonas.gorski@...il.com>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Álvaro Fernández Rojas <noltari@...il.com>
Subject: Re: [PATCH net v2] net: dsa: b53: bcm531x5: fix cpu rgmii mode
 interpretation

On Fri, Nov 07, 2025 at 04:45:15PM +0200, Vladimir Oltean wrote:
> On Fri, Nov 07, 2025 at 03:07:48PM +0100, Andrew Lunn wrote:
> > > There is allwinner/sun7i-a20-lamobo-r1.dts, which uses "rgmii-txid",
> > > which is untouched by this patch. The ethernet interface uses "rgmii".
> > 
> > Which is odd, but lets leave it alone.
> > 
> > > And there is arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-elbert.dts,
> > > where a comment says that it has a BCM53134, but there is no such
> > > node. The ethernet node uses "rgmii".
> > 
> > aspeed pretty much always get phy-mode wrong. So i would not worry too
> > much about this.
> > 
> > > So one doesn't define one, one uses rgmii-id on the switch / phy side
> > > and rgmii on the ethernet mac side, and one only defines the ethernet
> > > mac side as rgmii.
> > 
> > That is reasonable. It is a lot less clear what is correct for a
> > MAC-MAC connection. For a MAC-PHY connection we do have documentation,
> > the preference is that the PHY adds the delays, not the MAC. If the
> > switch is playing PHY, then having it add delays is sensible.
> > 
> > > > I would maybe add a dev_warn() here, saying the DT blob is out of date
> > > > and needs fixing. And fix all the in kernel .dts files.
> > > 
> > > Sure I can add a warning.
> > 
> > Great, thanks.
> > 
> > 	Andrew
> 
> +Russell

As this is discussing the applicability of RGMII delays for DSA
switches, I've long held out that the situation is a mess, and
diverges from what we decide to do for MACs - so I'd prefer not
to get involved in this, except to say...

> Since there is no 'correct' way to apply RGMII delays on a MAC according
> to phy-mode, my advice, if possible, would be to leave sleeping dogs lie
> and fix broken setups by adding the explicit device tree properties in
> the MAC, and adding driver support for parsing these.

Indeed - let's not break existing working setups. If there is a
problem with them, then that's the time to start thinking about
changing them.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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