linux-kernel - [PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

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Message-Id: <20251017-qbv-fixes-v3-2-d3a42e32646a@altera.com>
Date: Fri, 17 Oct 2025 14:11:20 +0800
From: Rohan G Thomas via B4 Relay <devnull+rohan.g.thomas.altera.com@...nel.org>
To: Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Maxime Coquelin <mcoquelin.stm32@...il.com>, 
 Alexandre Torgue <alexandre.torgue@...s.st.com>, 
 Jose Abreu <Jose.Abreu@...opsys.com>, 
 Rohan G Thomas <rohan.g.thomas@...el.com>, 
 Boon Khai Ng <boon.khai.ng@...era.com>
Cc: netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, 
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Rohan G Thomas <rohan.g.thomas@...era.com>, 
 Matthew Gerlach <matthew.gerlach@...era.com>
Subject: [PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag
 length for maxSDU

From: Rohan G Thomas <rohan.g.thomas@...era.com>

On hardware with Tx VLAN offload enabled, add the VLAN tag length to
the skb length before checking the Qbv maxSDU if Tx VLAN offload is
requested for the packet. Add 4 bytes for 802.1Q tag.

Fixes: c5c3e1bfc9e0 ("net: stmmac: Offload queueMaxSDU from tc-taprio")
Signed-off-by: Rohan G Thomas <rohan.g.thomas@...era.com>
Reviewed-by: Matthew Gerlach <matthew.gerlach@...era.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index dedaaef3208bfadc105961029f79d0d26c3289d8..23bf4a3d324b7f8e8c3067ed4d47b436a89c97d3 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4500,6 +4500,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	bool has_vlan, set_ic;
 	int entry, first_tx;
 	dma_addr_t des;
+	u32 sdu_len;

 	tx_q = &priv->dma_conf.tx_queue[queue];
 	txq_stats = &priv->xstats.txq_stats[queue];
@@ -4516,13 +4517,6 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 			return stmmac_tso_xmit(skb, dev);
 	}

-	if (priv->est && priv->est->enable &&
-	    priv->est->max_sdu[queue] &&
-	    skb->len > priv->est->max_sdu[queue]){
-		priv->xstats.max_sdu_txq_drop[queue]++;
-		goto max_sdu_err;
-	}
-
 	if (unlikely(stmmac_tx_avail(priv, queue) < nfrags + 1)) {
 		if (!netif_tx_queue_stopped(netdev_get_tx_queue(dev, queue))) {
 			netif_tx_stop_queue(netdev_get_tx_queue(priv->dev,
@@ -4535,8 +4529,18 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_BUSY;
 	}

+	sdu_len = skb->len;
 	/* Check if VLAN can be inserted by HW */
 	has_vlan = stmmac_vlan_insert(priv, skb, tx_q);
+	if (has_vlan)
+		sdu_len += VLAN_HLEN;
+
+	if (priv->est && priv->est->enable &&
+	    priv->est->max_sdu[queue] &&
+	    skb->len > priv->est->max_sdu[queue]){
+		priv->xstats.max_sdu_txq_drop[queue]++;
+		goto max_sdu_err;
+	}

 	entry = tx_q->cur_tx;
 	first_entry = entry;

-- 
2.43.7