[PATCH RFC net 6/6] net: macb: keep best-effort traffic on TX queue 0

[Steffen Dirkwinkel <lists@steffen.cc>]

From: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>

Add an ndo_select_queue() that puts every frame on queue 0 unless
traffic classes are configured (taprio, or software mqprio).

The GEM serves its transmit queues in strict priority order, the
highest queue first, while the stack hashes flows over all of them. On
a saturated link the flows on the lower queues starve until the netdev
watchdog fires on them, and the timeout path cannot help as nothing is
wrong with the queue.

XPS settings, the socket queue cache and the queue choice of
PACKET_QDISC_BYPASS sockets no longer take effect on multi-queue GEMs
without traffic classes; queues 1 and up stay reachable through tc.

Fixes: 02c958dd3446 ("net/macb: add TX multiqueue support for gem")
Assisted-by: LLM
Signed-off-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 3dfc7ffdd598..eae43490c5f7 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -4596,10 +4596,21 @@ static void macb_tx_timeout(struct net_device *netdev, unsigned int q)
 	macb_tx_restart(&bp->queues[q]);
 }
 
+/* GEM serves TX queues in strict priority order, the highest queue first */
+static u16 macb_select_queue(struct net_device *netdev, struct sk_buff *skb,
+			     struct net_device *sb_dev)
+{
+	if (netdev_get_num_tc(netdev))
+		return netdev_pick_tx(netdev, skb, sb_dev);
+
+	return 0;
+}
+
 static const struct net_device_ops macb_netdev_ops = {
 	.ndo_open		= macb_open,
 	.ndo_stop		= macb_close,
 	.ndo_start_xmit		= macb_start_xmit,
+	.ndo_select_queue	= macb_select_queue,
 	.ndo_set_rx_mode	= macb_set_rx_mode,
 	.ndo_get_stats64	= macb_get_stats,
 	.ndo_eth_ioctl		= macb_ioctl,

-- 
2.56.0