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

[Semih Baskan]

Hi Vladimir,

On Thu, Aug 13, 2026 at 10:08 PM Vladimir Oltean <olteanv at gmail.com> wrote:
> Where is the code that configures this GMNGCFG register?

b53_brcm_hdr_setup(). Setting up port 8 ORs the 0b10 value into the
GMNGCFG management port field. Setting up port 5 ORs the whole field
mask, which sets 0b11, dual IMP mode. The chip has no IMP1 only
encoding (0b01 is reserved), so there was no single port value to use
there. Nothing clears the field. That gives 0xc2 whenever port 5 is a
CPU port, and 0x82 with port 8 alone.

> In the previous version, Rafal 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/

Yes. The IMPs never learn: b53_setup_port() disables learning
everywhere and only bridged user ports re-enable it, DIS_LEARNING
reads 0x01e1 on both conduit configurations, and the ARL holds the
host MAC only as the driver's static entry. So one MAC across gmac0
and gmac2 cannot flap anything, and the bridge MAC stops changing
every boot.

Best regards,
Semih