On September 6, 2017 8:54:33 AM PDT, Roopa Prabhu [off-list ref] wrote:

On Tue, Sep 5, 2017 at 5:47 PM, Andrew Lunn [off-list ref] wrote:

quoted

quoted

The third and last issue will be explained in a followup email.

Hi DSA hackers

So there is the third issue. It affects just DSA, but it possible
affects all DSA drivers.

This patchset broken broadcast with the Marvell drivers. It could
break broadcast on others drivers as well.

What i found is that the Marvell chips don't flood broadcast frames
between bridged ports. What appears to happen is there is a fdb miss,
so it gets forwarded to the CPU port for the host to deal with. The
software bridge when floods it out all ports of the bridge.

But the set offload_fwd_mark patch changes this. The software bridge
now assumes the hardware has already flooded broadcast out all ports
of the switch as needed. So it does not do any flooding itself. As a
result, on Marvell devices, broadcast packets don't get flooded at
all.

The issue can be fixed. I just need to add an mdb entry for the
broadcast address to each port of the bridge in the switch, and the
CPU port.  But i don't know at what level to do this.

Should this be done at the DSA level, or at the driver level?  Do any
chips do broadcast flooding in hardware already? Hence they currently
see broadcast duplication? If i add a broadcast mdb at the DSA level,
and the chip is already hard wired to flooding broadcast, is it going
to double flood?

On the switch asics we work with, the driver has information if the
packet was
forwarded in hardware. This is per packet reason code telling why the
CPU is seeing the packet.
The driver can use this information to reset skb->offload_fwd_mark to
allow software forward.

I am not positive this is universally available across different switch vendors. In Broadcom tag (net/dsa/tag_brcm.c) the reason code definitely tells you that but it also largely depends on whether you have configured SW managed forwarding or not and that translates in having either the HW do all the address learning itself or having SW do it which is less desirable since you end-up with a possibility huge FDB of 4k entries to manage in SW.


-- 
Florian