netdev - Re: [RFC PATCH 0/4] net: dsa: link aggregation support

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Message-ID: <20201027190034.utk3kkywc54zuxfn@skbuf>
Date:   Tue, 27 Oct 2020 21:00:34 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     Andrew Lunn <andrew@...n.ch>, Marek Behun <marek.behun@....cz>,
        vivien.didelot@...il.com, f.fainelli@...il.com,
        netdev@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] net: dsa: link aggregation support

On Tue, Oct 27, 2020 at 07:25:16PM +0100, Tobias Waldekranz wrote:
> > 1) trunk user ports, with team/bonding controlling it
> > 2) trunk DSA ports, i.e. the ports between switches in a D in DSA setup
> > 3) trunk CPU ports.
[...]
> I think that (2) and (3) are essentially the same problem, i.e. creating
> LAGs out of DSA links, be they switch-to-switch or switch-to-cpu
> connections. I think you are correct that the CPU port can not be a
> LAG/trunk, but I believe that limitation only applies to TO_CPU packets.

Which would still be ok? They are called "slow protocol PDUs" for a reason.

> In order for this to work on transmit, we need to add forward offloading
> to the bridge so that we can, for example, send one FORWARD from the CPU
> to send an ARP broadcast to swp1..4 instead of four FROM_CPUs.

That surely sounds like an interesting (and tough to implement)
optimization to increase the throughput, but why would it be _needed_
for things to work? What's wrong with 4 FROM_CPU packets?