[PATCH v3] net: stmmac: Modify the judgment condition of "tx_avail" from 1 to 2

[PATCH v3] net: stmmac: Modify the judgment condition of "tx_avail" from 1 to 2

[2694439648]

From: "hailong.fan" <hailong.fan@siengine.com>

Under certain conditions, a WARN_ON will be triggered
if avail equals 1.

For example, when a VLAN packet is to send,
stmmac_vlan_insert consumes one unit of space,
and the data itself consumes another.
actually requiring 2 units of space in total.

Changes from v3:
        - format commit message
Changes from v2:
        - add fixes tag
        - Add stmmac_extra_space to count the additional required space
Changes from v1:
        - Stop their queues earlier

Fixes: 30d932279dc2 ("net: stmmac: Add support for VLAN Insertion Offload")
Signed-off-by: hailong.fan <hailong.fan@siengine.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 7b90ecd3a..9a665a3b2 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4476,6 +4476,15 @@ static bool stmmac_has_ip_ethertype(struct sk_buff *skb)
 		(proto == htons(ETH_P_IP) || proto == htons(ETH_P_IPV6));
 }
 
+static inline int stmmac_extra_space(struct stmmac_priv *priv,
+				     struct sk_buff *skb)
+{
+	if (!priv->dma_cap.vlins || !skb_vlan_tag_present(skb))
+		return 0;
+
+	return 1;
+}
+
 /**
  *  stmmac_xmit - Tx entry point of the driver
  *  @skb : the socket buffer
@@ -4529,7 +4538,8 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	}
 
-	if (unlikely(stmmac_tx_avail(priv, queue) < nfrags + 1)) {
+	if (unlikely(stmmac_tx_avail(priv, queue) <
+		nfrags + 1 + stmmac_extra_space(priv, skb))) {
 		if (!netif_tx_queue_stopped(netdev_get_tx_queue(dev, queue))) {
 			netif_tx_stop_queue(netdev_get_tx_queue(priv->dev,
 								queue));
@@ -4675,7 +4685,7 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 		print_pkt(skb->data, skb->len);
 	}
 
-	if (unlikely(stmmac_tx_avail(priv, queue) <= (MAX_SKB_FRAGS + 1))) {
+	if (unlikely(stmmac_tx_avail(priv, queue) <= (MAX_SKB_FRAGS + 2))) {
 		netif_dbg(priv, hw, priv->dev, "%s: stop transmitted packets\n",
 			  __func__);
 		netif_tx_stop_queue(netdev_get_tx_queue(priv->dev, queue));
-- 
2.34.1

Re: [PATCH v3] net: stmmac: Modify the judgment condition of "tx_avail" from 1 to 2

[Jakub Kicinski]

On Wed,  3 Dec 2025 09:31:52 +0800 2694439648@qq.com wrote:
> -	if (unlikely(stmmac_tx_avail(priv, queue) <= (MAX_SKB_FRAGS + 1))) {
> +	if (unlikely(stmmac_tx_avail(priv, queue) <= (MAX_SKB_FRAGS + 2))) {

Please add a constant for "(MAX_SKB_FRAGS + 2)" and a comment next to
it explaining why the literal 2 is used.