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

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Message-ID: <20220510123724.i2xqepc56z4eouh2@skbuf>
Date:   Tue, 10 May 2022 15:37:24 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Felix Fietkau <nbd@....name>
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 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?

> 
> Fixes: 5cd8985a1909 ("net-next: dsa: add Mediatek tag RX/TX handler")
> Signed-off-by: Felix Fietkau <nbd@....name>
> ---
>  net/dsa/tag_mtk.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/dsa/tag_mtk.c b/net/dsa/tag_mtk.c
> index 415d8ece242a..1d1f9dbd9e93 100644
> --- a/net/dsa/tag_mtk.c
> +++ b/net/dsa/tag_mtk.c
> @@ -25,6 +25,14 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
>  	u8 xmit_tpid;
>  	u8 *mtk_tag;
>  
> +	/* The Ethernet switch we are interfaced with needs packets to be at
> +	 * least 64 bytes (including FCS) otherwise their padding might be
> +	 * corrupted. With tags enabled, we need to make sure that packets are
> +	 * at least 68 bytes (including FCS and tag).
> +	 */
> +	if (__skb_put_padto(skb, ETH_ZLEN + MTK_HDR_LEN, false))
> +		return NULL;
> +
>  	/* Build the special tag after the MAC Source Address. If VLAN header
>  	 * is present, it's required that VLAN header and special tag is
>  	 * being combined. Only in this way we can allow the switch can parse
> -- 
> 2.36.1
>