linux-kernel - Re: [PATCH net-next 3/3] net: phy: dp83869: Support 1000Base-X SFP modules

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Message-ID: <10709391.nUPlyArG6x@fw-rgant>
Date: Wed, 14 May 2025 14:32:00 +0200
From: Romain Gantois <romain.gantois@...tlin.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: 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>,
 Maxime Chevallier <maxime.chevallier@...tlin.com>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject:
 Re: [PATCH net-next 3/3] net: phy: dp83869: Support 1000Base-X SFP modules

On Wednesday, 14 May 2025 14:22:48 CEST Andrew Lunn wrote:
> > +static int dp83869_port_configure_serdes(struct phy_port *port, bool
> > enable, +					 phy_interface_t interface)
> > +{
> > +	struct phy_device *phydev = port_phydev(port);
> > +	struct dp83869_private *dp83869;
> > +	int ret;
> > +
> > +	if (!enable)
> > +		return 0;
> > +
> > +	dp83869 = phydev->priv;
> > +
> > +	switch (interface) {
> > +	case PHY_INTERFACE_MODE_1000BASEX:
> > +		dp83869->mode = DP83869_RGMII_1000_BASE;
> > +		break;
> > +	default:
> > +		phydev_err(phydev, "Incompatible SFP module inserted\n");
> > +		return -EINVAL;
> > +	}
> 
> There is also DP83869_RGMII_SGMII_BRIDGE. Can this be used with the
> SERDES? Copper SFPs often want SGMII.

It can definitely be used to support non-DAC copper modules. In fact, I've 
implemented support for these modules locally, but I'm planning to upstream 
this part of the SFP support later, as there is some additional trickiness to 
solve beforehand.

To quickly summarize the issue, non-DAC copper module support requires reading 
autonegotiation results from the downstream PHY and relaying them back to the 
MAC. This requires some kind of stacked PHY support in the kernel, which has 
been in discussion for a while now:

https://lore.kernel.org/all/20241119115136.74297db7@fedora.home/

So as things currently stand, SGMII SFP support in this driver is blocked 
until some kind of generic stacked PHY support is implemented in the kernel.

Thanks,

-- 
Romain Gantois, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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