netdev - Re: [RFC net-next v2 1/4] net: dsa: mt7530: enable assisted learning on CPU port

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Date:   Mon, 2 Aug 2021 16:07:23 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Eric Woudstra <ericwouds@...il.com>,
        René van Dorst <opensource@...rst.com>,
        Frank Wunderlich <frank-w@...lic-files.de>
Subject: Re: [RFC net-next v2 1/4] net: dsa: mt7530: enable assisted learning
 on CPU port

On Sun, Aug 01, 2021 at 03:10:19AM +0800, DENG Qingfang wrote:
> Consider the following bridge configuration, where bond0 is not
> offloaded:
> 
>          +-- br0 --+
>         / /   |     \
>        / /    |      \
>       /  |    |     bond0
>      /   |    |     /   \
>    swp0 swp1 swp2 swp3 swp4
>      .        .       .
>      .        .       .
>      A        B       C
> 
> Address learning is enabled on offloaded ports (swp0~2) and the CPU
> port, so when client A sends a packet to C, the following will happen:
> 
> 1. The switch learns that client A can be reached at swp0.
> 2. The switch probably already knows that client C can be reached at the
>    CPU port, so it forwards the packet to the CPU.
> 3. The bridge core knows client C can be reached at bond0, so it
>    forwards the packet back to the switch.
> 4. The switch learns that client A can be reached at the CPU port.
> 5. The switch forwards the packet to either swp3 or swp4, according to
>    the packet's tag.
> 
> That makes client A's MAC address flap between swp0 and the CPU port. If
> client B sends a packet to A, it is possible that the packet is
> forwarded to the CPU. With offload_fwd_mark = 1, the bridge core won't
> forward it back to the switch, resulting in packet loss.
> 
> As we have the assisted_learning_on_cpu_port in DSA core now, enable
> that and disable hardware learning on the CPU port.
> 
> Signed-off-by: DENG Qingfang <dqfext@...il.com>
> ---

Reviewed-by: Vladimir Oltean <oltean@...il.com>