docs: net: dsa: sja1105: document limitations of tc-flower rule VLAN awareness

docs: net: dsa: sja1105: document limitations of tc-flower rule VLAN awareness

After change "net: dsa: tag_8021q: replace the SVL bridging with
VLAN-unaware IVL bridging", tag_8021q enforces two different pvids on a
port, depending on whether it is standalone or in a VLAN-unaware bridge.

Up until now, there was a single pvid, represented by
dsa_tag_8021q_rx_vid(), and that was used as the VLAN for VLAN-unaware
virtual link rules, regardless of whether the port was bridged or
standalone.

To keep VLAN-unaware virtual links working, we need to follow whether
the port is in a bridge or not, and update the VLAN ID from those rules.

In fact we can't fully do that. Depending on whether the switch is
VLAN-aware or not, we can accept Virtual Link rules with just the MAC
DA, or with a MAC DA and a VID. So we already deny changes to the VLAN
awareness of the switch. But the VLAN awareness may also change as a
result of joining or leaving a bridge.

One might say we could just allow the following: a port may leave a
VLAN-unaware bridge while it has VLAN-unaware VL (tc-flower) rules, and
the driver will update those with the new tag_8021q pvid for standalone
mode, but the driver won't accept joining a bridge at all while VL rules
were installed in standalone mode. This is sort of a compromise made
because leaving a bridge is an operation that cannot be vetoed.
But this sort of setup change is not fully supported, either: as
mentioned, VLAN filtering changes can also be triggered by leaving a
bridge, therefore, the existing veto we have in place for turning VLAN
filtering off with VLAN-aware VL rules active still isn't fully
effective.

I really don't know how to deal with this in a way that produces
predictable behavior for user space. Since at the moment, keeping this
feature fully functional on constellation changes (not changing the
tag_8021q port pvid when joining a bridge) is blocking progress for the
DSA FDB isolation, I'd rather document it as a (potentially temporary)
limitation and go on without it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>