linux-kernel - Re: [PATCH 2/2] net: phy: dp83869: Fix RGMII-SGMII and 1000BASE-X

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Date: Wed, 15 May 2024 10:15:33 +0200 (CEST)
From: Thomas Geßler <gessler_t@...eckmann-gmbh.de>
To: Andrew Lunn <andrew@...n.ch>
cc: Thomas Gessler <thomas.gessler@...eckmann-gmbh.de>, 
    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>, netdev@...r.kernel.org, 
    linux-kernel@...r.kernel.org, MD Danish Anwar <danishanwar@...com>, 
    Ravi Gunasekaran <r-gunasekaran@...com>
Subject: Re: [PATCH 2/2] net: phy: dp83869: Fix RGMII-SGMII and 1000BASE-X

On Tue, 14 May 2024, Andrew Lunn wrote:
> On Tue, May 14, 2024 at 02:27:28PM +0200, Thomas Gessler wrote:
> > The RGMII-to-SGMII mode is special, because the chip does not really act
> > as a PHY in this mode but rather as a bridge.
> 
> It is known as a media converter. You see them used between an RGMII
> port and an SFP cage. Is that your use case?

Basically. I would call this an RGMII-SGMII bridge. A "media converter" I
would call a device that changes the physical medium, like 1000BASE-T
copper/RJ45 to 1000BASE-X fiber/SFP.

We have this chip on a daughter card with exposed SGMII lines that can be
plugged into customer-specific motherboards. The motherboard will have
either an SGMII-copper PHY (this can also be a DP83869) with 10/100/1000
auto-neg enabled but without MDIO exposed to the CPU on the daughter card;
or an SFP cage. The SFP module, in turn, can be for 1000BASE-X fiber,
1000BASE-X-to-1000-BASE-T copper, or SGMII copper supporting 10/100/1000
auto-neg.

So I would like to support all those configurations, which can be done
with this chip.

> > SGMII PHY and gets the negotiated speed and duplex from it through SGMII
> > auto-negotiation. To use the DP83869 as a virtual PHY, we assume that
> > the connected SGMII PHY supports 10/100/1000M half/full duplex and
> > therefore support and always advertise those settings.
> 
> Not all copper SFP modules support auto-neg. This is all really messy
> because there is no standardisation. Also 1000BaseT_Half is often not
> supported.

I agree. Is there a better way to implement this use case? The problem
remains that in this mode the chip is not really a PHY, but rather a
bridge to an external PHY. See also Russell's e-mail.

I actually started out by NOT supporting or advertising any of the
10/100/1000BASE-T speeds when in RGMII-to-SGMII mode. This also works for
the SGMII auto-negotiation, since all it does is get the negotiated
speed/duplex from the connected PHY. However, this leads to a problem when
trying to disable auto-neg and force speed with ethtool.
phy_sanitize_settings() will then limit the speed to 10M because 100M and
1000M do not match any supported speeds.

Thomas