Re: [PATCH net 1/2] net: dsa: let drivers offload 8021q uppers on standalone ports

[Semih Baskan]

Hi Vladimir,

> What about VLAN-unaware bridging? Can packets with VID=100 be forwarded
> autonomously between two ports of a vlan_filtering=0 bridge if that VLAN
> is not programmed to hardware?

No. On every b53 switch measured so far it cannot, and it fails in the
same place on all of them. The frame passes ingress admission (with
filtering off the driver programs VC4_NO_ING_VID_CHK and clears
VC5_DROP_VTABLE_MISS, one shared code path for every chip), then the
forwarding stage resolves the egress set from the VLAN table entry for
that VID. A VID that is not programmed resolves to an empty member set,
so there is nothing to forward to on any port.

> Does the behaviour depend on silicon capabilities, or is it the same for
> all switches handled by the b53 driver?

The port-to-port part has been the same on everything measured. What
depends on the silicon is only whether the CPU gets a copy of the miss
frame. Three chips have data across these two threads:

  - BCM53011 (bcm5301x, my RT-N18U): measured for the CPU direction.
    Every ingress-check setting (NO_ING_VID_CHK, VIO_FWD, VIO_TO_IMP,
    plus clearing the VC0 miss-drop bit) delivers 0 of 7; the frame dies
    at member-set resolution, not at admission. Whether a miss frame
    reaches the CPU at all depends on the IMP routing from the other
    subthread: with port 8 as IMP it does (indiscriminately), with the
    in-tree port 5 topology it never does for LAN-class ports.

  - BCM63268 and BCM53115: Jonas measured exactly this case there.
    Standalone RX works even on a table miss, so the CPU copy exists,
    but forwarding between ports does not. That is the case you are
    asking about, on two other generations of this silicon.

My own measurements were CPU-directed, so for bcm5301x the two-port
case is inferred from the empty member set rather than measured
directly. For your question the measured answer is Jonas's, and the
mechanism says it generalizes.

Best regards,
Semih