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

[Eric Dumazet]

On Sun, Nov 30, 2025 at 6:57 PM <2694439648 at qq.com> wrote:
>
> From: "hailong.fan" <hailong.fan at 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.
>
> Signed-off-by: hailong.fan <hailong.fan at siengine.com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 7b90ecd3a..b575384cd 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -4529,7 +4529,7 @@ 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 + 2)) {
>                 if (!netif_tx_queue_stopped(netdev_get_tx_queue(dev, queue))) {
>                         netif_tx_stop_queue(netdev_get_tx_queue(priv->dev,
>                                                                 queue));

Drivers should stop their queues earlier.

NETDEV_TX_BUSY is almost always wrong.

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 7b90ecd3a55e600458b0c87d6125831626f4683d..6dcc7b84a8759763b6471a48a6c80b1f17cd937c
100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4675,7 +4675,7 @@ 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));