Re: [PATCH net v3] net: airoha: Fix skb->priority underflow in airoha_dev_select_queue()

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Sun, 14 Jun 2026 07:30:54 +0800 Wayen Yan wrote:
> > In airoha_dev_select_queue(), the expression:
> > 
> >   queue = (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES;
> > 
> > implicitly converts to unsigned arithmetic: when skb->priority is 0
> > (the default for unclassified traffic), (0u - 1u) wraps to UINT_MAX,
> > and UINT_MAX % 8 = 7, routing default best-effort packets to the
> > highest-priority QoS queue. This causes QoS inversion where the
> > majority of traffic on a PON gateway starves actual high-priority
> > flows (VoIP, gaming, etc.).
> > 
> > Fix by guarding the subtraction: when priority is 0, map to queue 0
> > (lowest priority), otherwise apply the original (priority - 1) % 8
> > mapping.
> > 
> > Fixes: 2b288b81560b ("net: airoha: Introduce ndo_select_queue callback")
> > Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > Reviewed-by: Joe Damato <joe@dama.to>
> > Signed-off-by: Wayen Yan <win847@gmail.com>
> > ---
> >  drivers/net/ethernet/airoha/airoha_eth.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
> > index 31cdb11cd7..d476ef83c3 100644
> > --- a/drivers/net/ethernet/airoha/airoha_eth.c
> > +++ b/drivers/net/ethernet/airoha/airoha_eth.c
> > @@ -1933,7 +1933,7 @@ static u16 airoha_dev_select_queue(struct net_device *dev, struct sk_buff *skb,
> >  	 */
> >  	channel = netdev_uses_dsa(dev) ? skb_get_queue_mapping(skb) : port->id;
> >  	channel = channel % AIROHA_NUM_QOS_CHANNELS;
> > -	queue = (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES; /* QoS queue */
> > +	queue = skb->priority ? (skb->priority - 1) % AIROHA_NUM_QOS_QUEUES : 0;
> 
> Hi Lorenzo, is there a reason we're subtracting 1 here in the first
> place? Could be just me, but may be worth adding a comment here.
> 
> Intuitively if we are "narrowing" 16 prios to 8 queues it'd make most
> sense to group the adjacent ones -- divide by two.
> 
> Please respin with some sort of an explanation..

IIRC this is a leftover of the ETS offload support.
I agree it is righ to just do:

	queue = skb->priority % AIROHA_NUM_QOS_QUEUES; /* QoS queue */

@Wayen: can you please respin fixing the issue? Please add even my Acked-by:

Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>

Regards,
Lorenzo

> 
> >  	queue = channel * AIROHA_NUM_QOS_QUEUES + queue;
> >  
> >  	return queue < dev->num_tx_queues ? queue : 0;
> -- 
> pw-bot: cr

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