netdev - Re: [PATCH net-next 2/2] net: dsa: lan9303: Add basic offloading of unicast traffic

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Message-ID: <20170922200810.GJ3470@lunn.ch>
Date:   Fri, 22 Sep 2017 22:08:10 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Egil Hjelmeland <privat@...l-hjelmeland.no>
Cc:     vivien.didelot@...oirfairelinux.com, f.fainelli@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] net: dsa: lan9303: Add basic offloading of
 unicast traffic

> >I'm wondering how this is supposed to work. Please add a good comment
> >here, since the hardware is forcing you to do something odd.
> >
> >Maybe it would be a good idea to save the STP state in chip.  And then
> >when chip->is_bridged is set true, change the state in the hardware to
> >the saved value?
> >
> >What happens when port 0 is added to the bridge, there is then a
> >minute pause and then port 1 is added? I would expect that as soon as
> >port 0 is added, the STP state machine for port 0 will start and move
> >into listening and then forwarding. Due to hardware limitations it
> >looks like you cannot do this. So what state is the hardware
> >effectively in? Blocking? Forwarding?
> >
> >Then port 1 is added. You can then can respect the states. port 1 will
> >do blocking->listening->forwarding, but what about port 0? The calls
> >won't get repeated? How does it transition to forwarding?
> >
> >   Andrew
> >
> 
> I see your point with the "minute pause" argument. Although a bit
> contrived use case, it is easy to fix by caching the STP state, as
> you suggest. So I can do that.

I don't think it is contrived. I've done bridge configuration by hand
for testing purposes. I've also set the forwarding delay to very small
values, so there is a clear race condition here.

> How does other DSA HW chips handle port separation? Knowing that
> could perhaps help me know what to look for.

They have better hardware :-)

Generally each port is totally independent. You can change the STP
state per port without restrictions.

      Andrew