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:

quoted

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