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Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets
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To:     Vladimir Oltean <olteanv@gmail.com>
Cc:     Sean Wang <sean.wang@mediatek.com>,
        Landen Chao <Landen.Chao@mediatek.com>,
        DENG Qingfang <dqfext@gmail.com>, Andrew Lunn <andrew@lunn.ch>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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References: <20220510094014.68440-1-nbd@nbd.name>
 <20220510123724.i2xqepc56z4eouh2@skbuf>
From:   Felix Fietkau <nbd@nbd.name>
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On 10.05.22 14:37, Vladimir Oltean wrote:
> On Tue, May 10, 2022 at 11:40:13AM +0200, Felix Fietkau wrote:
>> Padding for transmitted packets needs to account for the special tag.
>> With not enough padding, garbage bytes are inserted by the switch at the
>> end of small packets.
> 
> I don't think padding bytes are guaranteed to be zeroes. Aren't they
> discarded? What is the issue?
With the broken padding, ARP requests are silently discarded on the 
receiver side in my test. Adding the padding explicitly fixes the issue.

- Felix