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From: Andrew Lunn <andrew@lunn.ch>
Subject: Re: [PATCH v3 net-next 0/5] IGMP snooping for local traffic
Date: Wed, 8 Nov 2017 00:17:00 +0100
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Cc: Florian Fainelli <f.fainelli@gmail.com>,
        David Miller <davem@davemloft.net>,
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To: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
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On Tue, Nov 07, 2017 at 05:37:32PM -0500, Vivien Didelot wrote:
> Hi Andrew,
> 
> Andrew Lunn <andrew@lunn.ch> writes:
> 
> >> In a switch case, they all translate to programming a MDB entry for
> >> a given switch port, right?
> >
> > No, in fact it is the exact opposite.
> 
> Yes, they do. The proof is you call dsa_port_mdb_add.

Note that i always say switchdev.

switchdev has no concept of the CPU port. All switchdev has is the
concept of the external ports.

So when there is a join on the br0 interface, the bridge code will
iterative over each port in the bridge, and make a switchdev call to
each of the external ports in the bridge asking it to forward
multicast traffic for a group to the host.

Now, deep down in DSA, we can translate this to a dsa_port_mdb_add, on
the CPU port. And we do that for every call the bridge makes for each
of the external ports in the bridge.

However, a pure switchdev device won't do that. It does not have a CPU
port. It probably needs to add a match/action rule to its tables for
the actual external port saying to forward the frame out the slow
path.

> Still, what I see here _from a switch driver point of view_ is either
> program an MDB entry on a user port, or on its CPU port.

I agree with this, if you make one change:

_from a DSA switch driver point of view_

However, in the general case, this is not true. We need an API which
works for Mellonex and Netranome as well, systems without a CPU port.

      Andrew