netdev - Re: [PATCH v2] net: dsa: ksz8795: adjust CPU link to host interface

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Date:   Tue, 8 Dec 2020 15:15:27 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Jean Pihet <jean.pihet@...oldbits.com>
Cc:     netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Ryan Barnett <ryan.barnett@...kwellcollins.com>,
        Conrad Ratschan <conrad.ratschan@...kwellcollins.com>,
        Hugo Cornelis <hugo.cornelis@...ensium.com>,
        Arnout Vandecappelle <arnout.vandecappelle@...ensium.com>
Subject: Re: [PATCH v2] net: dsa: ksz8795: adjust CPU link to host interface

> > Hi Jean
> >
> > I never said i was too specific to your board. There are other boards
> > using different switches like this. This is where the commit message
> > is so important. Without understanding Why? it is hard to point you in
> > the right direction.
> >
> > So you setup is:
> >
> > SoC - MAC - PHY - PHY - MAC - Switch.
> >
> > The SoC MAC driver is looking after the first PHY?
> 
> No, the connection is at the MAC level, via RGMII but it is missing the MDC/
> MDIO signals. That means we have to fix the auto-neg parameters from the DT.

So the PHY is there, but you cannot talk to it? It has strapping
resisters to make it auto-neg to the other PHY?

Some switches default their CPU port to the maximum speed the port can
do. But not all do. It is worth checking that.

> On the 4.14 LTS kernel we are working with, the setup of the parameters is done
> via adjust_link. Since the phylink conversion adjust_link is not required
> anymore, is this correct?

4.14 is dead in terms of development. Anything you contribute needs to
be for net-next, and then you need to figure out how to backport
it. Using v5.4 will help with that, since it is much closer, and v5.10
will be LTS. Given the change to phylink, you probably want as new a
kernel as possible. If you put a fixed-link property in the "CPU"
node, phylink should do the right thing for you.

      Andrew