netdev - Re: [PATCH net-next 2/2] net: macb: Disable PCS auto-negotiation for SGMII fixed-link mode

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Date:   Sun, 14 Mar 2021 23:22:03 +0000
From:   Robert Hancock <robert.hancock@...ian.com>
To:     "andrew@...n.ch" <andrew@...n.ch>
CC:     "nicolas.ferre@...rochip.com" <nicolas.ferre@...rochip.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "claudiu.beznea@...rochip.com" <claudiu.beznea@...rochip.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>
Subject: Re: [PATCH net-next 2/2] net: macb: Disable PCS auto-negotiation for
 SGMII fixed-link mode

On Sat, 2021-03-13 at 02:45 +0100, Andrew Lunn wrote:
> On Thu, Mar 11, 2021 at 02:18:13PM -0600, Robert Hancock wrote:
> > When using a fixed-link configuration in SGMII mode, it's not really
> > sensible to have auto-negotiation enabled since the link settings are
> > fixed by definition. In other configurations, such as an SGMII
> > connection to a PHY, it should generally be enabled.
> 
> So how do you tell the PCS it should be doing 10Mbps over the SGMII
> link? I'm assuming it is the PCS which does the bit replication, not
> the MAC?

I'm not sure if this is the same for all devices using this Cadence IP, but the
register documentation I have for the Xilinx UltraScale+ MPSoC we are using
indicates this PCS is only capable of 1000 Mbps speeds:

https://www.xilinx.com/html_docs/registers/ug1087/gem___pcs_control.html

So it doesn't actually seem applicable in this case.

> 
> I'm surprised you are even using SGMII with a fixed link. 1000BaseX is
> the norm, and then you don't need to worry about the speed.
> 

That would be a bit simpler, yes - but it seems like this hardware is set up
more for SGMII mode - it's not entirely clear to me that 1000BaseX is supported
in the hardware, and it's not currently supported in the driver that I can see.

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