[PATCH V3 1/4 net] net: dsa: b53: deal with Northstar devices using CPU port 5 or 7

[Semih Baskan]

Hi Rafał,

Tested on an Asus RT-N18U (BCM47081 + BCM53011 rev 5), with a local
DTS change enabling ports 7 and 8 like your 4/4 does for the R8000
(the in-tree bcm-ns.dtsi state). With patches 1 and 2 applied:

- The op works as designed: DSA picks port 8, the conduit is gmac2,
  the 1/4 warning stays silent, and management over the bridged LAN
  ports runs through the port 8 path with no issues.
- Trap delivery is restored: 20 of 20 BPDUs injected into a front port
  reach the CPU. On the port 5 topology the same probe measured 0 across
  every front port.

With all three CPU ports enabled, GMNGCFG ends up in dual IMP mode
(0xc2 read back). In that mode a tagged frame whose VID is missing
from the VLAN table is delivered to no CPU port at all: an 8021q
upper on a standalone port receives 0 of 7 tagged probes, whether
the port 5 conduit is up or down. With port 8 as the only enabled
CPU port in the DT, where GMNGCFG reads 0b10, the same frames were
delivered. So the preferred CPU port alone does not revive
standalone 8021q uppers on DTs that enable all three ports.

The same effect touches 4/4: the R8000 today has ports 5 and 7
disabled, the single IMP configuration, and enabling them lands the
switch in dual IMP mode through the same driver path. If its switch
behaves like this BCM53011, standalone 8021q uppers on the R8000
stop receiving with 4/4 applied.

On the RT-N18U the boot firmware provisions a MAC only for gmac0,
so gmac2 comes up with a random MAC and the bridge inherits it,
changing per boot.

Best regards,
Semih