netdev - Re: DSA vs. SWTICHDEV ?

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Date:   Wed, 30 Nov 2016 17:44:14 +0000
From:   Joakim Tjernlund <Joakim.Tjernlund@...inera.com>
To:     "andrew@...n.ch" <andrew@...n.ch>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: DSA vs. SWTICHDEV ?

On Wed, 2016-11-30 at 17:55 +0100, Andrew Lunn wrote:
> > This is an embedded system with several boards in a subrack.
> > Each board has eth I/F connected to a switch to communicate with each other.
> > One of the board will also house the actual switch device and manage the switch.
> > Then the normal app just communicates over the physical eth I/F like any other board
> > in the system. There is a "manage switch app" which brings the switch up and partition
> > phys VLANs etc. (each phys I/F would be a a separate domain so no loop)
> 
> So you are planning on throwing away the "manage switch app", and just
> use standard linux networking commands? That is what switchdev is all
> about really, throwing away the vendor SDK for the switch, making a
> switch just a bunch on interfaces on the host which you manage as
> normal interfaces.

Something like that. I need to run routing protocols on the switch I/Fs and egress
pkgs on selected switch I/Fs bypassing ARP, just like DSA does with its vendor
tags.

> 
> > I guess I could skip the phys I/F and have the switch app create a virtual eth0 I/F over PCIe
> 
> No need to create this interface. It will exist if you go the
> switchdev route.
> 
> > > > And switchdev can do all this over PCIe instead? Can you have a
> > > > switch tree in switchdev too?
> > > 
> > > Mellonex says so, but i don't think they have actually implemented it.
> > 
> > Not impl. any of DSAs features? What can you do with a Mellonex switch then?
> 
> They don't have a tree of switches, as far as i know. Just a single
> switch. But DSA does support a tree of switches, that is what the D in
> DSA means, distributed. And there are a couple of boards which have 2
> to 4 switches in a tree.

We might have a tree as well so now I really wonder: Given we write a
proper switchdev driver, can it support switchtrees without touching
switchdev infra structure? If not I guess we will attach a physical
eth I/F to the switch and use both DSA and switchdev to support both trees
and HW offload. 

> 
> I think this is partially down to market segments. Mellonex market is
> top of rack switches. High port count, very high bandwidth. DSA is
> more wireless access points, set top boxes, generally up to 7 ports of
> 1Gbps and a few custom embedded products which need more ports, so
> build a tree of switches.

We have on an existing board with a BCM ROBO switch with lots of ports(>24),
managed over SPI. Looking at BCM DSA tag code it looks like it only supports
some 8 ports or so. I still have to find out if this is a limitation in BCM tagging
protocol or if just not impl. in DSA yet.