FAILED: patch "[PATCH] net: airoha: Fix skb->priority underflow in" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 86e51aa24686cc95bb35613059e8b94b9b81e3f0
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072154-partly-effective-6632@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 86e51aa24686cc95bb35613059e8b94b9b81e3f0 Mon Sep 17 00:00:00 2001
From: Wayen Yan <[email protected]>
Date: Sat, 20 Jun 2026 16:17:44 +0800
Subject: [PATCH] net: airoha: Fix skb->priority underflow in
airoha_dev_select_queue()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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.

index 64dde6464f3f..3370c3df7c10 100644
@@ -2110,7 +2110,7 @@ static u16 airoha_dev_select_queue(struct net_device *netdev,
 	 */
 	channel = netdev_uses_dsa(netdev) ? 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 < netdev->num_tx_queues ? queue : 0;

Queued for 6.12 (the driver still lives under ethernet/mediatek there, so only the path needed adjusting), thanks.

Thanks,
Sasha