Date:   Wed, 30 Nov 2016 16:35:18 +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 16:25 +0100, Andrew Lunn wrote:
> On Wed, Nov 30, 2016 at 02:30:43PM +0000, Joakim Tjernlund wrote:
> > On Wed, 2016-11-30 at 14:52 +0100, Andrew Lunn wrote:
> > > On Wed, Nov 30, 2016 at 08:50:34AM +0000, Joakim Tjernlund wrote:
> > > > I am trying to wrap my head around these two "devices" and have a hard time telling them apart.
> > > > We are looking att adding a faily large switch(over PCIe) to our board and from what I can tell
> > > > switchdev is the new way to do it but DSA is still there. Is it possible to just list
> > > > how they differ?
> > > 
> > > Hi Joakim
> > 
> > Hi Andrew, thanks for answering
> > 
> > > 
> > > If the interface you use to send frames from the host to the switch is
> > > PCIe, you probably want to use switchdev directly.
> > 
> > OK, we will have a few ethernet I/F's connected too but I these should be used
> > as normal interfaces just interfacing a switch.
> 
> That does not make much sense.
> 
> Maybe time to backtrack a bit. The Linux concept for switch/router
> chips is that they are just hardware accelerators for what Linux can
> already do in software. Each port of the switch is just a normal Linux
> interface.  ip link show will list each port. ip addr add can be used
> to add an IP address to the interface.  You want to switch frames
> between two ports: Create a linux bridge and put the interfaces into
> it. Via switchdev you get a call into the hardware to accelerate
> this. If the hardware cannot accelerate it, it is done in software as
> normal.  Want to combine two ports into a trunk: Add a team interface
> and make the port interfaces slaves of the team interface. Via
> switchdev, you ask the hardware to accelerate this. If it cannot, it
> is done in software.
> 
> So back your connecting a few host interfaces to the switch. This is
> logically putting a cable between two interfaces on the same host. You
> are making a loopback. Why do that? Sure it is possible, but it is an
> odd architecture.

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)

I guess I could skip the phys I/F and have the switch app create a virtual eth0 I/F over PCIe
instead to save eth MACS but the above is safer should there be some problems/limitations in
swicthdev plus switchdev does not exist in u-boot so it would be a lot of effort to
get a working eth I/F inside u-boot.
I can still can still create a bridge I/F etc. should I need to.
Does the above make sense to you ?
 
> 
> > 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?

 Jocke

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