Right now, address learning is disabled on DSA ports, which means that a
packet received over a DSA port from a cross-chip switch will be flooded
to unrelated ports.
It is desirable to eliminate that, but for that we need a breakdown of
the possibilities for the sja1105 driver. A DSA port can be:
- a downstream-facing cascade port. This is simple because it will
always receive packets from a downstream switch, and there should be
no other route to reach that downstream switch in the first place,
which means it should be safe to learn that MAC address towards that
switch.
- an upstream-facing cascade port. This receives packets either:
* autonomously forwarded by an upstream switch (and therefore these
packets belong to the data plane of a bridge, so address learning
should be ok), or
* injected from the CPU. This deserves further discussion, as normally,
an upstream-facing cascade port is no different than the CPU port
itself. But with "H" topologies (a DSA link towards a switch that
has its own CPU port), these are more "laterally-facing" cascade
ports than they are "upstream-facing". Here, there is a risk that
the port might learn the host addresses on the wrong port (on the
DSA port instead of on its own CPU port), but this is solved by
DSA's RX filtering infrastructure, which installs the host addresses
as static FDB entries on the CPU port of all switches in a "H" tree.
So even if there will be an attempt from the switch to migrate the
FDB entry from the CPU port to the laterally-facing cascade port, it
will fail to do that, because the FDB entry that already exists is
static and cannot migrate. So address learning should be safe for
this configuration too.
Ok, so what about other MAC addresses coming from the host, not
necessarily the bridge local FDB entries? What about MAC addresses
dynamically learned on foreign interfaces, isn't there a risk that
cascade ports will learn these entries dynamically when they are
supposed to be delivered towards the CPU port? Well, that is correct,
and this is why we also need to enable the assisted learning feature, to
snoop for these addresses and write them to hardware as static FDB
entries towards the CPU, to make the switch's learning process on the
cascade ports ineffective for them. With assisted learning enabled, the
hardware learning on the CPU port must be disabled.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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