Re: [PATCH RFC 6/6] Modify tag_ksz.c to support other KSZ switch drivers

[Florian Fainelli <f.fainelli@gmail.com>]

On 09/18/2017 04:44 PM, Tristram.Ha@microchip.com wrote:
>>> In the old DSA implementation all the ports are partitioned into its own
>> device
>>> and the bridge joining them will do all the forwarding.  This is useful for
>> quick
>>> testing with some protocols like RSTP but it is probably useless for real
>>> operation.
>>
>> It is a good minimal driver, to get something into the kernel. You can
>> then add features to it.
>>
>>> The new switchdev model tries to use the switch hardware as much as
>>> possible.  This offload_fwd_mark bit means the frame is forwarded by the
>>> hardware switch, so the software bridge does not need to do it again.
>> Without
>>> this bit there will be duplicated multicast frames coming out the ports if
>> internal
>>> forwarding is enabled.
>>
>> Correct. Once you switch driver is clever enough, you can enable
>> offload_fwd_mark.
>>
>>> When RSTP is used the port can be put in blocked state and so the
>> forwarding
>>> will stop for that port.   Currently the switch driver will check that
>> membership
>>> to decide whether to set that bit.
>>
>> This i don't get. RSTP or STP just break loops. How does RSTP vs STP
>> mean you need to set offload_fwd_mark differently?
>>
> 
> The logic of the switch driver is if the membership of the port receiving
> the frame contains other ports--not counting cpu port--the bit
> offload_fwd_mark is set.  In RSTP closing the blocked port is generally good
> enough, but there are exceptions, so the port is removed from the
> membership of other forwarding ports.  A disabled port will have its
> membership completely reset so it cannot receive anything.  It does not
> matter much in RSTP as the software bridge should know whether to forward
> the frame or not.
> 
> We are back to square one.  Is there any plan to add this offload_fwd_mark
> support to DSA driver so that it can be reported properly?  It can be set all the
> time, except during port initialization or before bridge creation the forwarding
> state does not reflect reality.
> 
> If not the port membership can be fixed and there is no internal switch
> forwarding, leaving everything handled by the software bridge.

I am not really sure why this is such a concern for you so soon when
your driver is not even included yet. You should really aim for baby
steps here: get the basic driver(s) included, with a limited set of
features, and gradually add more features to the driver. When
fwd_offload_mark and RSTP become a real problem, we can most
definitively find a way to fix those in DSA and depending drivers.
-- 
Florian