Re: [PATCH v2] net: stmmac: Modify the judgment condition of "tx_avail" from 1 to 2

[Jakub Kicinski <kuba@kernel.org>]

On Tue,  2 Dec 2025 15:43:59 +0800 2694439648@qq.com wrote:
> 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.
> 
>     ---
>     V0-V1:
>        1. Stop their queues earlier
>     V2-V1:
>        1. add fixes tag
>        2. Add stmmac_extra_space to count the additional required space

Why is the commit message indented ? Please looks around the mailing
list and try to follow the format others are using. Or read the
documentation. Either of the two will do.
https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#changes-requested

> 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))) {

extra logic is likely not worth the single descriptor saving, can you
use the same + 2 as in the condition for stopping the queue, please?

>  		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))) {