linux-kernel - Re: drivers/net/dsa/dsa_loop

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Message-ID: <688e54ec-3b29-4e3b-a2c3-f2c83b9c97b7@lunn.ch>
Date: Mon, 13 May 2024 18:54:10 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Stephen Langstaff <stephenlangstaff1@...il.com>
Cc: Florian Fainelli <f.fainelli@...il.com>, linux-kernel@...r.kernel.org,
	"OlteanV@...il.com" <OlteanV@...il.com>
Subject: Re: drivers/net/dsa/dsa_loop_bdinfo.c build problems

> What I am trying to achieve is to use dsa_loop to connect a CPU
> running Linux to an Ethernet switch via a "conduit" Ethernet link
> (e.g. eth0) using the port-based Ethertype DSA (Marvell) frame tagging
> protocol, so that the Linux kernel will present the userspace with
> separate Ethernet links for each of the switch's user ports (e.g.
> lan1, lan2, lan3).
> 
> As I understand it this is usually done via a fixed phy-less conduit
> (e.g. xMII) and an MDIO link between the processor and the switch so
> that the processor can configure and control the physical user ports.
> 
> I want to do some software development ahead of receiving the
> hardware, so I want to "fake" the MDIO link, or in some other way
> configure the kernel, such that it uses port-based EDSA frame tagging
> over an ordinary (PHY-equipped) Ethernet port and it looks to me like
> dsa_loop should be capable of doing this.

That is an interesting use case. I don't know of anybody doing that
before.

As Florian pointed out, dsa_loop_bdinfo.o should be built into your
kernel. When you insmod dsa_loop.ko, it should match the data in
dsa_loop_bdinfo to the dsa_loop driver an instantiate an instance of
it. Do you see that happening?

You could hack dsa_loop_get_protocol() to return DSA_TAG_PROTO_EDSA,
in order to use the EDSA frame tagger. dsa_loop does not have an
implementation of .change_tag_protocol, so i don't think you can
change it at runtime. However an implementation of it should be
trivial.

   Andrew