Re: [PATCH net v2 2/2] net: stmmac: Consider Tx VLAN offload tag length for maxSDU

["G Thomas, Rohan" <rohan.g.thomas@altera.com>]

Hi Jakub,

Thanks for reviewing the patch.

On 9/18/2025 4:24 AM, Jakub Kicinski wrote:
> On Wed, 17 Sep 2025 15:49:20 -0700 Jakub Kicinski wrote:
>> On Mon, 15 Sep 2025 16:17:19 +0800 Rohan G Thomas via B4 Relay wrote:
>>> From: Rohan G Thomas <rohan.g.thomas@altera.com>
>>>
>>> On hardware with Tx VLAN offload enabled, add the VLAN tag
>>> length to the skb length before checking the Qbv maxSDU.
>>> Add 4 bytes for 802.1Q an add 8 bytes for 802.1AD tagging.
>>>
>>> Fixes: c5c3e1bfc9e0 ("net: stmmac: Offload queueMaxSDU from tc-taprio")
>>> Signed-off-by: Rohan G Thomas <rohan.g.thomas@altera.com>
>>> Reviewed-by: Matthew Gerlach <matthew.gerlach@altera.com>
>>> ---
>>>   drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 25 ++++++++++++++++-------
>>>   1 file changed, 18 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>>> index 8c8ca5999bd8ad369eafa0cd8448a15da55be86b..c06c947ef7764bf40291a556984651f4edd7cb74 100644
>>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>>> @@ -4537,6 +4537,7 @@ 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];
>>> @@ -4553,13 +4554,6 @@ 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,
>>> @@ -4575,6 +4569,23 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
>>>   	/* Check if VLAN can be inserted by HW */
>>>   	has_vlan = stmmac_vlan_insert(priv, skb, tx_q);
>>>   
>>> +	sdu_len = skb->len;
>>> +	if (has_vlan) {
>>> +		/* Add VLAN tag length to sdu length in case of txvlan offload */
>>> +		if (priv->dev->features & NETIF_F_HW_VLAN_CTAG_TX)
>>> +			sdu_len += VLAN_HLEN;
>>> +		if (skb->vlan_proto == htons(ETH_P_8021AD) &&
>>> +		    priv->dev->features & NETIF_F_HW_VLAN_STAG_TX)
>>> +			sdu_len += VLAN_HLEN;
>>
>> Is the device adding the same VLAN tag twice if the proto is 8021AD?
>> It looks like it from the code, but how every strange..
>>
>> In any case, it doesn't look like the driver is doing anything with
>> the NETIF_F_HW_VLAN_* flags right? stmmac_vlan_insert() works purely
>> off of vlan proto. So I think we should do the same thing here?
> 
> I suppose the double tagging depends on the exact SKU but first check
> looks unnecessary. Maybe stmmac_vlan_insert() should return the number
> of vlans it decided to insert?
> 

Agreed, those checks using NETIF_F_HW_VLAN_*_TX flags are redundant, as
stmmac_vlan_insert() already returns true. As you suggested I'll update 
stmmac_vlan_insert() to return the VLAN header length it decides to 
insert, so the logic can be simplified and made more concise.

>>> +	}
>>> +
>>> +	if (priv->est && priv->est->enable &&
>>> +	    priv->est->max_sdu[queue] &&
>>> +	    sdu_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;
>>>   	WARN_ON(tx_q->tx_skbuff[first_entry]);
>>>    
>>
> 

Best Regards,
Rohan