Similar to commit 6087175b ("net: dsa: mt7530: use independent VLAN
learning on VLAN-unaware bridges"), software forwarding between an
unoffloaded LAG port (a bonding interface with an unsupported policy)
and a mv88e6xxx user port directly under a bridge is broken.

We adopt the same strategy, which is to make the standalone ports not
find any ATU entry learned on a bridge port.

Theory: the mv88e6xxx ATU is looked up by FID and MAC address. There are
as many FIDs as VIDs (4096). The FID is derived from the VID when
possible (the VTU maps a VID to a FID), with a fallback to the port
based default FID value when not (802.1Q Mode is disabled on the port,
or the classified VID isn't present in the VTU).

The mv88e6xxx driver makes the following use of FIDs and VIDs:

- the port's DefaultVID (to which untagged & pvid-tagged packets get
  classified) is 0 and is absent from the VTU, so this kind of packets is
  processed in FID 0, the default FID assigned by mv88e6xxx_setup_port.

- every time a bridge VLAN is created, mv88e6xxx_port_vlan_join() ->
  mv88e6xxx_atu_new() associates a FID with that VID which increases
  linearly starting from 1. Like this:

  bridge vlan add dev lan0 vid 100 # FID 1
  bridge vlan add dev lan1 vid 100 # still FID 1
  bridge vlan add dev lan2 vid 1024 # FID 2

The FID allocation made by the driver is sub-optimal for the following
reasons:

(a) A standalone port has a DefaultPVID of 0 and a default FID of 0 too.
    A VLAN-unaware bridged port has a DefaultPVID of 0 and a default FID
    of 0 too. The difference is that the bridged ports may learn ATU
    entries, while the standalone port has the requirement that it must
    not, and must not find them either. Standalone ports must not use
    the same FID as ports belonging to a bridge. All standalone ports
    can use the same FID, since the ATU will never have an entry in
    that FID.

(b) Multiple VLAN-unaware bridges will all use a DefaultPVID of 0 and a
    default FID of 0 on all their ports. The FDBs will not be isolated
    between these bridges. Every VLAN-unaware bridge must use the same
    FID on all its ports, different from the FID of other bridge ports.

(c) Each bridge VLAN uses a unique FID which is useful for Independent
    VLAN Learning, but the same VLAN ID on multiple VLAN-aware bridges
    will result in the same FID being used by mv88e6xxx_atu_new().
    The correct behavior is for VLAN 1 in br0 to have a different FID
    compared to VLAN 1 in br1.

This patch cannot fix all the above. Traditionally the DSA framework did
not care about this, and the reality is that DSA core involvement is
needed for the aforementioned issues to be solved. The only thing we can
solve here is an issue which does not require API changes, and that is
issue (a), aka use a different FID for standalone ports vs ports under
VLAN-unaware bridges.

The first step is deciding what VID and FID to use for standalone ports,
and what VID and FID for bridged ports. The 0/0 pair for standalone
ports is what they used up till now, let's keep using that. For bridged
ports, there are 2 cases:

- VLAN-aware ports will never end up using the port default FID, because
  packets will always be classified to a VID in the VTU or dropped
  otherwise. The FID is the one associated with the VID in the VTU.

- On VLAN-unaware ports, we _could_ leave their DefaultVID (pvid) at
  zero (just as in the case of standalone ports), and just change the
  port's default FID from 0 to a different number (say 1).

However, Tobias points out that there is one more requirement to cater to:
cross-chip bridging. The Marvell DSA header does not carry the FID in
it, only the VID. So once a packet crosses a DSA link, if it has a VID
of zero it will get classified to the default FID of that cascade port.
Relying on a port default FID for upstream cascade ports results in
contradictions: a default FID of 0 breaks ATU isolation of bridged ports
on the downstream switch, a default FID of 1 breaks standalone ports on
the downstream switch.

So not only must standalone ports have different FIDs compared to
bridged ports, they must also have different DefaultVID values.
IEEE 802.1Q defines two reserved VID values: 0 and 4095. So we simply
choose 4095 as the DefaultVID of ports belonging to VLAN-unaware
bridges, and VID 4095 maps to FID 1.

For the xmit operation to look up the same ATU database, we need to put
VID 4095 in DSA tags sent to ports belonging to VLAN-unaware bridges
too. All shared ports are configured to map this VID to the bridging
FID, because they are members of that VLAN in the VTU. Shared ports
don't need to have 802.1QMode enabled in any way, they always parse the
VID from the DSA header, they don't need to look at the 802.1Q header.

We install VID 4095 to the VTU in mv88e6xxx_setup_port(), with the
mention that mv88e6xxx_vtu_setup() which was located right below that
call was flushing the VTU so those entries wouldn't be preserved.
So we need to relocate the VTU flushing prior to the port initialization
during ->setup(). Also note that this is why it is safe to assume that
VID 4095 will get associated with FID 1: the user ports haven't been
created, so there is no avenue for the user to create a bridge VLAN
which could otherwise race with the creation of another FID which would
otherwise use up the non-reserved FID value of 1.

[ Currently mv88e6xxx_port_vlan_join() doesn't have the option of
  specifying a preferred FID, it always calls mv88e6xxx_atu_new(). ]

mv88e6xxx_port_db_load_purge() is the function to access the ATU for
FDB/MDB entries, and it used to determine the FID to use for
VLAN-unaware FDB entries (VID=0) using mv88e6xxx_port_get_fid().
But the driver only called mv88e6xxx_port_set_fid() once, during probe,
so no surprises, the port FID was always 0, the call to get_fid() was
redundant. As much as I would have wanted to not touch that code, the
logic is broken when we add a new FID which is not the port-based
default. Now the port-based default FID only corresponds to standalone
ports, and FDB/MDB entries belong to the bridging service. So while in
the future, when the DSA API will support FDB isolation, we will have to
figure out the FID based on the bridge number, for now there's a single
bridging FID, so hardcode that.

Lastly, the tagger needs to check, when it is transmitting a VLAN
untagged skb, whether it is sending it towards a bridged or a standalone
port. When we see it is bridged we assume the bridge is VLAN-unaware.
Not because it cannot be VLAN-aware but:

- if we are transmitting from a VLAN-aware bridge we are likely doing so
  using TX forwarding offload. That code path guarantees that skbs have
  a vlan hwaccel tag in them, so we would not enter the "else" branch
  of the "if (skb->protocol == htons(ETH_P_8021Q))" condition.

- if we are transmitting on behalf of a VLAN-aware bridge but with no TX
  forwarding offload (no PVT support, out of space in the PVT, whatever),
  we would indeed be transmitting with VLAN 4095 instead of the bridge
  device's pvid. However we would be injecting a "From CPU" frame, and
  the switch won't learn from that - it only learns from "Forward" frames.
  So it is inconsequential for address learning. And VLAN 4095 is
  absolutely enough for the frame to exit the switch, since we never
  remove that VLAN from any port.

Fixes: 57e661aa

 ("net: dsa: mv88e6xxx: Link aggregation support")
Reported-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>