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

[Felix Fietkau <nbd@nbd.name>]

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@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