netdev - RE: [PATCH net-next v2] net: dsa: microchip: Add SGMII port support to KSZ9477 switch

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Message-ID:
 <DM3PR11MB873695894DC7B99A15357CCBECE52@DM3PR11MB8736.namprd11.prod.outlook.com>
Date: Sat, 18 Jan 2025 00:59:25 +0000
From: <Tristram.Ha@...rochip.com>
To: <andrew@...n.ch>
CC: <maxime.chevallier@...tlin.com>, <olteanv@...il.com>,
	<Woojung.Huh@...rochip.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <UNGLinuxDriver@...rochip.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next v2] net: dsa: microchip: Add SGMII port support
 to KSZ9477 switch

> > The KSZ9477 SGMII module does use DesignWare IP, but its implementation
> > is probably too old as some registers do not match.
> 
> Is there a revision value somewhere in the registers? Maybe the lower
> nibble of ID registers 2 and 3?
> 

I am not aware of getting the version of the DesignWare IP to
differentiate different implementations.  I will find out from hardware
designers.

> > When using XPCS
> > driver link detection works but the SGMII port does not pass traffic for
> > some SFPs.  It is probably doable to update the XPCS driver to work in
> > KSZ9477, but there is no way to submit that patch as that may affect
> > other hardware implementation.
> 
> We have PHY drivers which change their behaviour based on the
> revision. So it is possible. And XPCS is used quite a bit, so i don't
> think it will be an issue finding somebody to do some regression
> testing.
> 
> Using a PCS driver is the correct way to go here. So either you need
> to copy/paste/edit the XPCS driver to create a version specific for
> you hardware, or you need to extend the XPCS driver so it supports
> your hardware.

Some of the register definitions are not present in the XPCS driver so I
need to add them.

Some register bits programmed by the XPCS driver do not have effect.

Actually KSZ9477 has a bug in SGMII implementation and needs a software
workaround.  I am not sure if the generic XCPS driver can cover that.