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

[Vladimir Oltean]

On Thu, Aug 13, 2026 at 01:53:56PM +0300, Semih Baskan wrote:
> 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.

Where is the code that configures this GMNGCFG register?

> 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.

In the previous version, Rafał included a patch so that gmac2 uses the
same MAC address as gmac0. Is this better?
https://lore.kernel.org/netdev/20260811193658.14304-2-zajec5@gmail.com/