netdev - Re: [PATCH net-next v6 06/14] net: phy: Introduce generic SFP handling for PHY drivers

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Message-ID: <aDliS9uMFaLf2lCV@shell.armlinux.org.uk>
Date: Fri, 30 May 2025 08:46:19 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Romain Gantois <romain.gantois@...tlin.com>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, thomas.petazzoni@...tlin.com,
	Andrew Lunn <andrew@...n.ch>, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	linux-arm-kernel@...ts.infradead.org,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Herve Codina <herve.codina@...tlin.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Marek Behún <kabel@...nel.org>,
	Oleksij Rempel <o.rempel@...gutronix.de>,
	Nicolò Veronese <nicveronese@...il.com>,
	Simon Horman <horms@...nel.org>, mwojtas@...omium.org,
	Antoine Tenart <atenart@...nel.org>, devicetree@...r.kernel.org,
	Conor Dooley <conor+dt@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Rob Herring <robh@...nel.org>, Daniel Golle <daniel@...rotopia.org>,
	Dimitri Fedrau <dimitri.fedrau@...bherr.com>
Subject: Re: [PATCH net-next v6 06/14] net: phy: Introduce generic SFP
 handling for PHY drivers

On Fri, May 30, 2025 at 09:28:11AM +0200, Romain Gantois wrote:
> On Thursday, 29 May 2025 15:23:22 CEST Russell King (Oracle) wrote:
> > On Wed, May 28, 2025 at 09:35:35AM +0200, Romain Gantois wrote:
> > > > In that regard, you can consider 1000BaseX as a MII mode (we do have
> > > > PHY_INTERFACE_MODE_1000BASEX).
> > > 
> > > Ugh, the "1000BaseX" terminology never ceases to confuse me, but yes
> > > you're
> > > right.
> > 
> > 1000BASE-X is exactly what is described in IEEE 802.3. It's a PHY
> > interface mode because PHYs that use SerDes can connect to the host
> > using SGMII or 1000BASE-X over the serial link.
> > 
> > 1000BASE-X's purpose in IEEE 802.3 is as a protocol for use over
> > fibre links, as the basis for 1000BASE-SX, 1000BASE-LX, 1000BASE-EX
> > etc where the S, L, E etc are all to do with the properties of the
> > medium that the electrical 1000BASE-X is sent over. It even includes
> > 1000BASE-CX which is over copper cable.
> 
> Ah makes sense, thanks for the explanation. I guess my mistake was assuming 
> that MAC/PHY interface modes were necessarily strictly at the reconciliation 
> sublayer level, and didn't include PCS/PMA functions.

When a serdes protocol such as SGMII, 1000BASE-X, or 10GBASE-R is being
used with a PHY, the IEEE 802.3 setup isn't followed exactly - in
effect there are more layers.

On the SoC:

	MAC
	Reconciliation (RS)
	PCS
	SerDes (part of the PMA layer)

On the PHY side of the SerDes host-to-phy link:

	SerDes
	PCS (which may or may not be exposed in the PHY register set,
	     and is normally managed by the PHY itself)
	(maybe other layers, could include MACs	back-to-back)
	PCS
	PMA
	PMD

Hope that helps explain what's going on a little more.

Another way to look at it is that with SGMII, 1000BASE-X etc between
the PHY and host, the PHY is a media converter.

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