linux-kernel - Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets

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Message-ID: <5959946d-1d34-49b9-1abe-9f9299cc194e@nbd.name>
Date:   Tue, 10 May 2022 16:52:16 +0200
From:   Felix Fietkau <nbd@....name>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        DENG Qingfang <dqfext@...il.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        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
Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets


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