DSA tries to prevent having a VLAN added by a bridge and by an 802.1Q
upper at the same time. It does that by checking the VID in
.ndo_vlan_rx_add_vid(), since that's something that the 8021q module
calls, via vlan_vid_add(). When a VLAN matches in both subsystems, this
check returns -EBUSY.

However the vlan_vid_add() function isn't specific to the 8021q module
in any way at all. It is simply the kernel's way to tell an interface to
add a VLAN to its RX filter and not drop that VLAN. So there's no reason
to return -EBUSY when somebody tries to call vlan_vid_add() for a VLAN
that was installed by the bridge. The proper behavior is to accept that
configuration.

So what's wrong is how DSA checks that it has an 8021q upper. It should
look at the actual uppers for that, not just assume that the 8021q
module was somewhere in the call stack of .ndo_vlan_rx_add_vid().
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>