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

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Message-Id: <20220506194344.83702-1-nbd@nbd.name>
Date:   Fri,  6 May 2022 21:43:44 +0200
From:   Felix Fietkau <nbd@....name>
To:     netdev@...r.kernel.org
Subject: [PATCH] net: dsa: tag_mtk: add padding for tx packets

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.

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
-- 
2.35.1