Re: [PATCH net-next] net: dsa: b53: mirror BPDUs to CPU port 5 when port 8 is not a CPU port

[Jonas Gorski]

On Wed, Nov 11, 2026 at 6:22 PM Florian Fainelli
<florian.fainelli@xxxxxxxxxxxx> wrote:
>
> On 11/11/26 02:11, Jonas Gorski wrote:
> > Hi,
> >
> > On Wed, Nov 11, 2026 at 7:31 AM Joseph Telaak <joseph@xxxxxxxxxx> wrote:
> >>
> >> A BCM531x5 can be connected to the host through its IMP port (8) or
> >> through port 5. With port 5 as the CPU port, b53_brcm_hdr_setup() puts
> >> the switch in dual-IMP mode, in which frames trapped for the CPU, BPDUs
> >> among them, go to port 8 when they come from a LAN port. b53 sets up no
> >> WAN ports, so that is every user port. On boards that use only port 5
> >> and leave port 8 unconnected, BPDUs are lost: STP never sees its peers,
> >> and a loop between two user ports takes the host off the network.
> >>
> >> Mirror frames sent to the bridge group address from the user ports to
> >> port 5 instead. The multiport address registers cannot do this, since
> >> the RoboSwitch data sheets resolve reserved multicast before them, and
> >> marking the user ports as WAN ports stopped forwarding between them
> >> when tried. Other frames trapped for the CPU still go to port 8, as the
> >> mirror matches a single destination address. The switch has one mirror,
> >> so tc mirroring is refused on these boards.
> >>
> >> This was observed on a BCM53125 board in real use, when both user ports
> >> were cabled into the same LAN. Tested on that board with a host sending
> >> BPDUs into a user port: before, none reached the CPU and the bridge
> >> stayed its own root; after, all of them did, the bridge took the sender
> >> as root, and forwarding between the user ports kept working. No
> >> selftests were run.
> >
> > So on your board only port 5 is wired up, and port 8 is unpopulated
> > and no way to make it work?
> >
> > Be aware that more than BPDUs are broken by this, as anything trapped
> > to CPU is not working. This also affects traffic on ports outside of
> > the bridge, other trapped packets in the 01:80:c2:00:00:0* range, or
> > IGMP/MLD snooped packets (or any other L2 protocol snooping).
> >
> >>
> >> Signed-off-by: Joseph Telaak <joseph@xxxxxxxxxx>
> >> ---
> >> Sent to net-next rather than net because it takes the mirror away from
> >> tc on these boards. I can resend to net with Fixes: 8fab459e69ab ("net:
> >> dsa: b53: Enable Broadcom tags for 531x5/539x families") if you prefer.
> >>
> >> Only checked on a BCM53125. BCM5301x boards with port 5 as the only CPU
> >> port may have the same problem, but I could not confirm it.
> >
> > They do, though there it's "easy" to fix by switching to port 8 in the
> > device tree, since all three ports are AFAIK always hardwired in the
> > SoC.
>
> Yes, I have a series of patches doing that, because using port 5 is just
> not working for everything.
>
> >
> >> ---
> >>   drivers/net/dsa/b53/b53_common.c | 24 ++++++++++++++++++++++++
> >>   drivers/net/dsa/b53/b53_priv.h   |  1 +
> >>   2 files changed, 25 insertions(+)
> >>
> >> diff --git a/drivers/net/dsa/b53/b53_common.c b/drivers/net/dsa/b53/b53_common.c
> >> index a2a5754c413c..6b33775c7598 100644
> >> --- a/drivers/net/dsa/b53/b53_common.c
> >> +++ b/drivers/net/dsa/b53/b53_common.c
> >> @@ -797,6 +797,24 @@ void b53_brcm_hdr_setup(struct dsa_switch *ds, int port)
> >>                  hdr_ctl |= GC_FRM_MGMT_PORT_M;
> >>          b53_write8(dev, B53_MGMT_PAGE, B53_GLOBAL_CONFIG, hdr_ctl);
> >>
> >> +       /* With port 5 as the CPU port, a BCM531x5 runs in dual-IMP mode and
> >> +        * traps BPDUs from LAN ports to port 8. When port 8 is not a CPU
> >> +        * port, mirror them to port 5 instead. This takes the switch's only
> >> +        * mirror, so tc mirroring is refused.
> >> +        */
> >> +       if (tag_en && port == 5 && is531x5(dev) &&
> >> +           !dsa_is_cpu_port(ds, B53_CPU_PORT)) {
> >
> > So this should be extended to all dual IMP capable chips, because they
> > are all equally affected.
> >
> >> +               b53_write48(dev, B53_MGMT_PAGE, B53_IG_MIR_MAC,
> >> +                           ether_addr_to_u64(eth_stp_addr));
> >> +               b53_write16(dev, B53_MGMT_PAGE, B53_IG_MIR_CTL,
> >> +                           (MIRROR_DA << MIRROR_FILTER_SHIFT) |
> >> +                           (dsa_user_ports(ds) & MIRROR_MASK));
> >> +               b53_write16(dev, B53_MGMT_PAGE, B53_EG_MIR_CTL, 0);
> >> +               b53_write16(dev, B53_MGMT_PAGE, B53_MIR_CAP_CTL,
> >> +                           MIRROR_EN | port);
> >
> > Have you tried a catch-all port 8 egress to port 5 mirror? If that
> > happens to work, it should catch any trapped traffic. May not work
> > while the port is disabled in software though.
>
> We could also try to use the CFP to perform a full redirection to port 8.

CFP isn't supported in all bcm531x5 chip variants. E.g BCM53115M and
BCM53125 do, wile BCM53115S and BCM53125S don't. And likely the
presumably lower cost S variants will be found most (at least the
board I use for testing has one).

So we could at most use it as an alternative path where supported to
free up mirroring. Checking if we have CFP can be done via the Bonding
Pad Status Register.

Best regards,
Jonas