DSA, through dsa_port_bridge_leave(), first notifies the port of the
fact that it left a bridge, then, if that bridge was VLAN-aware, it
notifies the port of the change in VLAN awareness state, towards
VLAN-unaware mode.

So ocelot_port_vlan_filtering() can be called when ocelot_port->bridge
is NULL, and this makes ocelot_add_vlan_unaware_pvid() create a struct
ocelot_bridge_vlan with a vid of 0 and an "untagged" setting of true on
that port.

In a way this structure correctly reflects the reality, but by design,
VID 0 (OCELOT_STANDALONE_PVID) was not meant to be kept in the bridge
VLAN list of the driver, but managed separately.

Having OCELOT_STANDALONE_PVID in ocelot->vlans makes us trip up on
several sanity checks that did not expect to have this VID there.
For example, after we leave a VLAN-aware bridge and we re-join it, we
can no longer program egress-tagged VLANs to hardware:

 # ip link add br0 type bridge vlan_filtering 1 && ip link set br0 up
 # ip link set swp0 master br0
 # ip link set swp0 nomaster
 # ip link set swp0 master br0
 # bridge vlan add dev swp0 vid 100
Error: mscc_ocelot_switch_lib: Port with more than one egress-untagged VLAN cannot have egress-tagged VLANs.

But this configuration is in fact supported by the hardware, since we
could use OCELOT_PORT_TAG_NATIVE. According to its comment:

/* all VLANs except the native VLAN and VID 0 are egress-tagged */

yet when assessing the eligibility for this mode, we do not check for
VID 0 in ocelot_port_uses_native_vlan(), instead we just ensure that
ocelot_port_num_untagged_vlans() == 1. This is simply because VID 0
doesn't have a bridge VLAN structure.

The way I identify the problem is that ocelot_port_vlan_filtering(false)
only means to call ocelot_add_vlan_unaware_pvid() when we dynamically
turn off VLAN awareness for a bridge we are under, and the PVID changes
from the bridge PVID to a reserved PVID based on the bridge number.

Since OCELOT_STANDALONE_PVID is statically added to the VLAN table
during ocelot_vlan_init() and never removed afterwards, calling
ocelot_add_vlan_unaware_pvid() for it is not intended and does not serve
any purpose.

Fix the issue by avoiding the call to ocelot_add_vlan_unaware_pvid(vid=0)
when we're resetting VLAN awareness after leaving the bridge, to become
a standalone port.

Fixes: 54c31984 ("net: mscc: ocelot: enforce FDB isolation when VLAN-unaware")
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>