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

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Message-ID: <16d25a4e-e678-407f-8ee3-59986bcbbc28@altera.com>
Date: Mon, 27 Oct 2025 14:33:54 +0530
From: "G Thomas, Rohan" <rohan.g.thomas@...era.com>
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,
 Matthew Gerlach <matthew.gerlach@...era.com>
Subject: Re: [PATCH net v3 2/3] net: stmmac: Consider Tx VLAN offload tag
 length for maxSDU

Hi All,

I've noticed one more issue with this commit. Need to drop the packet
before inserting the context descriptor with VLAN. So I think I have to
keep the max_sdu check in the original place itself, but add VLAN length 
if priv->dma_cap.vlins && skb_vlan_tag_present(skb) are true. Will 
change it accordingly in the next version.

Apologies for the oversight in the initial patch.

On 10/17/2025 11:41 AM, Rohan G Thomas via B4 Relay wrote:
> 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;
> 

Best Regards,
Rohan