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

[Vladimir Oltean]

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 at 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
>