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

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Message-ID: <BYAPR11MB355872F35F8758BA7F761B6DEC862@BYAPR11MB3558.namprd11.prod.outlook.com>
Date: Tue, 13 Aug 2024 20:59:36 +0000
From: <Tristram.Ha@...rochip.com>
To: <andrew@...n.ch>
CC: <Woojung.Huh@...rochip.com>, <UNGLinuxDriver@...rochip.com>,
	<devicetree@...r.kernel.org>, <f.fainelli@...il.com>, <olteanv@...il.com>,
	<robh@...nel.org>, <krzk+dt@...nel.org>, <conor+dt@...nel.org>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <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.
> >
> > The SGMII module can only report basic link status of the SFP, so it is
> > simulated as a regular internal PHY.
> >
> > Since the regular PHY in the switch uses interrupt instead of polling the
> > driver has to handle the SGMII interrupt indicating link on/off.
> >
> > One issue for the 1000BaseT SFP is there is no link down interrupt, so
> > the driver has to use polling to detect link down when the link is up.
> >
> > Recent change in the DSA operation can setup the port before the PHY
> > interrupt handling function is registered.  As the SGMII interrupt can
> > be triggered after port setup there is extra code in the interrupt
> > processing to handle this situation.  Otherwise a kernel fault can be
> > triggered.
> >
> > Note the SGMII interrupt cannot be masked in hardware.  Also the module
> > is not reset when the switch is reset.  It is important to reset the
> > module properly to make sure the interrupt is not triggered prematurely.
> 
> Why not model this as a PCS? Russell has been converting all PCS like
> things in DSA into try PCS drivers. So i suspect Russell will not like
> this code, and would prefer a PCS driver.

I am not familiar with PCS.  It seems too complicated after looking at
the phylink_pcs structure and associated phylink_pcs_ops functions.

The SGMII module can only report link and does not even restart
auto-negotiation.  It is a set once and forget operation.