[PATCH net v4] net: airoha: Fix skb->priority underflow in airoha_dev_select_queue()

[Wayen Yan <win847@gmail.com>]

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.).

The "- 1" offset was a leftover from the ETS offload implementation
that has since been removed. The correct mapping is a direct modulo:

  queue = skb->priority % AIROHA_NUM_QOS_QUEUES;

This maps priority 0 → queue 0 (lowest), priority 7 → queue 7
(highest), with higher priorities wrapping around. This is the
standard Linux sk_prio → HW queue mapping used by other drivers.

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>
---
Changes in v4:
- Remove the "- 1" offset entirely as suggested by Lorenzo and Jakub.
  The offset was an ETS offload leftover; the correct mapping is a
  direct modulo of skb->priority to AIROHA_NUM_QOS_QUEUES, matching
  the standard Linux sk_prio → HW queue convention. (v3 used a
  ternary guard which addressed the underflow but kept the unneeded
  offset.)

Link: https://lore.kernel.org/netdev/20260617164448.31e189bc@kernel.org/
Link: https://lore.kernel.org/netdev/ajPCgH7E_ke6Fdur@lore-desk/

 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 d0c0c0ec8a..9ec3f22754 100644
--- a/drivers/net/ethernet/airoha/airoha_eth.c
+++ b/drivers/net/ethernet/airoha/airoha_eth.c
@@ -1928,7 +1928,7 @@ 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 % AIROHA_NUM_QOS_QUEUES;
 	queue = channel * AIROHA_NUM_QOS_QUEUES + queue;

 	return queue < dev->num_tx_queues ? queue : 0;
--
2.51.0