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

From: Vladimir Oltean <olteanv@gmail.com>
To: Felix Fietkau <nbd@nbd.name>
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,
	linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] net: dsa: tag_mtk: add padding for tx packets
Date: Tue, 10 May 2022 15:37:24 +0300	[thread overview]
Message-ID: <20220510123724.i2xqepc56z4eouh2@skbuf> (raw)
In-Reply-To: <20220510094014.68440-1-nbd@nbd.name>

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@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
> 

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