netdev - Re: [PATCH net-next 5/6] net: phy: dp83869: Support SGMII SFP modules

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Message-ID: <ZoQaNnbYJ1c5SH0c@shell.armlinux.org.uk>
Date: Tue, 2 Jul 2024 16:18:14 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Romain Gantois <romain.gantois@...tlin.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 5/6] net: phy: dp83869: Support SGMII SFP modules

On Tue, Jul 02, 2024 at 03:21:09PM +0200, Andrew Lunn wrote:
> The SFP PHY is however a PHY which phylib is managing. And you have
> phylink on top of that, which knows about both PHYs. Architecturally,
> i really think phylink should be dealing with all this
> configuration.
> 
> The MAC driver has told phylink its pause capabilities.
> phylink_bringup_phy() will tell phylib these capabilities by calling
> phy_support_asym_pause(). Why does this not work for the SFP PHY?
> 
> phylink knows when the SFP PHY is plugged in, and knows if the link is
> admin up. It should be starting the state machine, not the PHY.

phylink only knows about SFPs that are directly connected to the
MAC/PCS. It has no knowledge of SFPs that are behind a PHY (like
on the Macchiatobin with 88x3310 PHYs.)

Due to the structure of the networking layer, I don't see how we
could sanely make stacked PHYs work - we expect the ethtool APIs
to target the media PHY, but in the case of a platform such as
Macchiatobin, we potentially have _two_ media facing PHYs on one
network interface. There's the 88x3310 which has its own RJ45
socket, and then if one plugs in a copper SFP, you get another
media-facing PHY with its own RJ45 socket. Which PHY should
ethtool ksettings_set interact with?

The ethtool API wasn't designed for this kind of thing!

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