netdev - Re: mv88e6xxx: 2500base-x inband AN is broken on Amethyst? what to do?

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Date:   Wed, 13 Jan 2021 22:37:29 +0100
From:   Marek Behún <kabel@...nel.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     andrew@...n.ch, netdev@...r.kernel.org, olteanv@...il.com,
        pavana.sharma@...i.com
Subject: Re: mv88e6xxx: 2500base-x inband AN is broken on Amethyst? what to
 do?

On Wed, 13 Jan 2021 21:21:25 +0000
Russell King - ARM Linux admin <linux@...linux.org.uk> wrote:

> > Russell, could we, for now, just edit the code so that when
> >   mv88e6390_serdes_pcs_config
> > is being configured with inband mode in 2500base-x, the inband mode
> > won't be enabled and the function will print a warning?
> > This could come with a Fixes tag so that it is backported to stable.  
> 
> I don't see any other easy option, so yes, please do that.

Will do.

> > Afterwards we can work on refactoring the phylink code so that either
> > the driver can inform phylink whether 2500base-x inband AN is supported,
> > or maybe we can determine from some documentation or whatnot whether
> > inband AN is supported on 2500base-x at all.  
> 
> I suspect there is no definitive documentation on exactly what
> 2500base-x actually is. I suspect it may just be easier to turn off AN
> for 2500base-x everywhere, so at least all Linux systems are compatible
> irrespective of the hardware.
> 
> Yes, it means losing pause negotiation, and people will have to
> manually set pause on each end.

This would need a little refactoring of the phylink_sfp_config
function, but I don't think it will be that hard.

> One thing that I don't know is whether the GPON SFP ONT modules that
> use 2500base-x will still function with AN disabled - although I have
> the modules, it appeared that they both needed a connection to the ONU
> to switch from 1000base-x to 2500base-x on the host side - and as I
> don't have an ONU I can test with, I have no way to check their
> behaviour.

We have an ISP here in Prague who is willing to lend us some GPON
ONU devices to get GPON SFP modules work on Turris. I will ask him if he
has one capable of 2.5g and try to borrow it.

Marek