linux-kernel - Re: [PATCH net-next 4/4] net: dsa: microchip: add SGMII port support to KSZ9477 switch

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Message-ID: <ede735e5-cbf1-48ea-a93e-1b4f21a48a4c@lunn.ch>
Date: Sun, 11 Aug 2024 18:32:31 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Tristram.Ha@...rochip.com
Cc: Woojung Huh <woojung.huh@...rochip.com>, UNGLinuxDriver@...rochip.com,
	devicetree@...r.kernel.org, Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Marek Vasut <marex@...x.de>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 4/4] net: dsa: microchip: add SGMII port support
 to KSZ9477 switch

On Fri, Aug 09, 2024 at 04:38:40PM -0700, Tristram.Ha@...rochip.com wrote:
> From: Tristram Ha <tristram.ha@...rochip.com>
> 
> The SGMII module of KSZ9477 switch can be setup in 3 ways: 0 for direct
> connect, 1 for 1000BaseT SFP, and 2 for 10/100/1000 SFP.
> 
> SFP is typically used so the default is 1.  The driver can detect
> 10/100/1000 SFP and change the mode to 2.  For direct connect this mode
> has to be explicitly set to 0 as driver cannot detect that
> configuration.

Is 1 actually 1000BaseX? An SFP module using fibre would typically
want 1000BaseX, and only support one speed. An SFP module using copper
typically has a PHY in it, it performs auto-neg on the media side, and
then uses SGMII inband signalling to tell the MAC what data rate,
symbol duplication to do. And maybe mode 0 has in-band signalling
turned off, in which case 1000BaseX and SGMII become identical,
because it is the signalling which is different.

	Andrew