linux-kernel - Re: [PATCH net-next] net: dsa: Deal with non-existing PHY/fixed-link

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Date:   Mon, 10 Jun 2019 23:11:33 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: dsa: Deal with non-existing PHY/fixed-link

> Hi Florian,
> 
> Can you give an example of when this is a valid use case, and why
> fixed-link is not appropriate?

A DSA link is used to connect two switches together. ZII devel b and c
are two boards which does this. Such links have the MACs connected
back to back, no PHYs involved. You can also connect a SoC interface
to the CPU port of a switch without having PHYs involved.

We have defined that CPU and DSA ports are always configured by the
driver to there maximum speed. Because of this, you often don't need a
fixed-link on CPU or DSA ports. So you will see most DT blobs don't
have any sort of PHY for the CPU or DSA ports.

You only need fixed-link when you need to slow a port down, e.g. a SoC
FE port connected to a switch 1G port.

    Andrew