selftests: forwarding: add a pvid_change test to bridge_vlan_unaware

Historically, DSA drivers have seen problems with the model in which
bridge VLANs work, particularly with them being offloaded to switchdev
asynchronously relative to when they become active (vlan_filtering=1).

This switchdev API peculiarity was papered over by commit 2ea7a679ca2a
("net: dsa: Don't add vlans when vlan filtering is disabled"), which
introduced other problems, fixed by commit 54a0ed0df496 ("net: dsa:
provide an option for drivers to always receive bridge VLANs") through
an opt-in ds->configure_vlan_while_not_filtering bool (which later
became an opt-out).

The point is that some DSA drivers still skip VLAN configuration while
VLAN-unaware, and there is a desire to get rid of that behavior.

It's hard to deduce from the wording "at least one corner case" what
Andrew saw, but my best guess is that there is a discrepancy of meaning
between bridge pvid and hardware port pvid which caused breakage.

On one side, the Linux bridge with vlan_filtering=0 is completely
VLAN-unaware, and will accept and process a packet the same way
irrespective of the VLAN groups on the ports or the bridge itself
(there may not even be a pvid, and this makes no difference).

On the other hand, DSA switches still do VLAN processing internally,
even with vlan_filtering disabled, but they are expected to classify all
packets to the port pvid. That pvid shouldn't be confused with the
bridge pvid, and there lies the problem.

When a switch port is under a VLAN-unaware bridge, the hardware pvid
must be explicitly managed by the driver to classify all received
packets to it, regardless of bridge VLAN groups. When under a VLAN-aware
bridge, the hardware pvid must be synchronized to the bridge port pvid.
To do this correctly, the pattern is unfortunately a bit complicated,
and involves hooking the pvid change logic into quite a few places
(the ones that change the input variables which determine the value to
use as hardware pvid for a port). See mv88e6xxx_port_commit_pvid(),
sja1105_commit_pvid(), ocelot_port_set_pvid() etc.

The point is that not all drivers used to do that, especially in older
kernels. If a driver is to blindly program a bridge pvid VLAN received
from switchdev while it's VLAN-unaware, this might in turn change the
hardware pvid used by a VLAN-unaware bridge port, which might result in
packet loss depending which other ports have that pvid too (in that same
note, it might also go unnoticed).

To capture that condition, it is sufficient to take a VLAN-unaware
bridge and change the [VLAN-aware] bridge pvid on a single port, to a
VID that isn't present on any other port. This shouldn't have absolutely
any effect on packet classification or forwarding. However, broken
drivers will take the bait, and change their PVID to 3, causing packet
loss.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20241210233541.1401837-1-vladimir.oltean@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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