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

[Florian Fainelli <florian.fainelli@broadcom.com>]

On 11/11/26 02:11, Jonas Gorski wrote:
> Hi,
> 
> On Wed, Nov 11, 2026 at 7:31 AM Joseph Telaak <joseph@telaak.dev> 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@telaak.dev>
>> ---
>> 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.

> 
>> +               dev->bpdu_mirror = true;
>> +       }
>> +
>>          /* B53_BRCM_HDR not present on devices with legacy tags */
>>          if (dev->tag_protocol == DSA_TAG_PROTO_BRCM_LEGACY ||
>>              dev->tag_protocol == DSA_TAG_PROTO_BRCM_LEGACY_FCS)
>> @@ -2604,6 +2622,12 @@ int b53_mirror_add(struct dsa_switch *ds, int port,
>>          struct b53_device *dev = ds->priv;
>>          u16 reg, loc;
>>
>> +       if (dev->bpdu_mirror) {
>> +               NL_SET_ERR_MSG_MOD(extack,
>> +                                  "The mirror carries BPDUs to CPU port 5");
>> +               return -EBUSY;
> 
> I think this should be rather -EOPNOTSUPP, as with this setup we will
> never be able to support it; -EBUSY feels more like a temporary state.

Agreed.
-- 
Florian