netdev - Re: [PATCH net-next v2 3/3] net: phy: at803x: Support downstream SFP cage

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Date:   Wed, 12 Jan 2022 00:24:19 +0000
From:   Robert Hancock <robert.hancock@...ian.com>
To:     "andrew@...n.ch" <andrew@...n.ch>
CC:     "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
Subject: Re: [PATCH net-next v2 3/3] net: phy: at803x: Support downstream SFP
 cage

On Wed, 2022-01-12 at 01:14 +0100, Andrew Lunn wrote:
> On Tue, Jan 11, 2022 at 03:55:04PM -0600, Robert Hancock wrote:
> > Add support for downstream SFP cages for AR8031 and AR8033. This is
> > primarily intended for fiber modules or direct-attach cables, however
> > copper modules which work in 1000Base-X mode may also function. Such
> > modules are allowed with a warning.
> 
> The previous patch added:
> 
> AT803X_MODE_CFG_BASET_SGMII
> 
> So it seems it has some support for SGMII? Cannot it be used?

According to Qualcomm, the AR8031 PHY has one SERDES block which can either be
used in SGMII mode on the MAC side, or in 1000Base-X mode on the line side, but
not in SGMII mode on the line side, so only 1000Base-X mode can be used there.
So that means no SGMII support for SFP modules unfortunately.

In practice, it does seem to work with most of the copper modules we have
tried, though in some cases (like where the module defaults to SGMII mode, or
1000Base-X mode with auto-negotiation disabled) we need to disable auto-
negotiation on the interface to get the link to come up (the module still does
its own auto-negotiation on the copper side regardless). Of course without
SGMII, 100 or 10 Mbps speeds won't work.

> 
>    Andrew
-- 
Robert Hancock
Senior Hardware Designer, Calian Advanced Technologies
www.calian.com