[PATCH net] net: bcmgenet: cap the Tx queue threshold at the ring size

[Nicolai Buchwitz]

A Tx queue stops when MAX_SKB_FRAGS + 1 or fewer descriptors are free and
wakes once there are more. Queues 1 to 4 hold 32 descriptors, so with
CONFIG_MAX_SKB_FRAGS at 31 they would need 33 to wake. As a result the
first packet stops the queue and it stays stopped:

  eth0: NETDEV WATCHDOG: CPU: 0: transmit queue 3 timed out 2004 ms
  eth0: NETDEV WATCHDOG: CPU: 1: transmit queue 4 timed out 2004 ms

On a Raspberry Pi CM4 only queue 0 carried traffic afterwards. The other
four counted an error on nearly every packet.

Fix this and cap the threshold at what the ring can free.

Fixes: 3948b05950fd ("net: introduce a config option to tweak MAX_SKB_FRAGS")
Signed-off-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>
---
Found this while fixing the clashiko findings on v6 of my MTU series. After
those fixes this bug would be easier to hit. At the moment it needs a
larger CONFIG_MAX_SKB_FRAGS than the default.

Everything was tested on a Raspberry Pi CM4 (BCM2711, genet v5).
---
 drivers/net/ethernet/broadcom/genet/bcmgenet.c | 6 ++++--
 drivers/net/ethernet/broadcom/genet/bcmgenet.h | 1 +
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
index 4d2be2a21ed1b..186d17781b890 100644
--- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
+++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
@@ -2030,7 +2030,7 @@ static int bcmgenet_tx_poll(struct napi_struct *napi, int budget)
 
 	spin_lock(&ring->lock);
 	__bcmgenet_tx_reclaim(ring->priv->dev, ring);
-	if (ring->free_bds > (MAX_SKB_FRAGS + 1)) {
+	if (ring->free_bds > ring->stop_thld) {
 		txq = netdev_get_tx_queue(ring->priv->dev, ring->index);
 		netif_tx_wake_queue(txq);
 	}
@@ -2235,7 +2235,7 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	netdev_tx_sent_queue(txq, GENET_CB(skb)->bytes_sent);
 
-	if (ring->free_bds <= (MAX_SKB_FRAGS + 1))
+	if (ring->free_bds <= ring->stop_thld)
 		netif_tx_stop_queue(txq);
 
 	if (!netdev_xmit_more() || netif_xmit_stopped(txq))
@@ -2742,6 +2742,8 @@ static void bcmgenet_init_tx_ring(struct bcmgenet_priv *priv,
 	ring->index = index;
 	ring->cbs = priv->tx_cbs + start_ptr;
 	ring->size = size;
+	/* A ring smaller than that would never wake */
+	ring->stop_thld = min_t(unsigned int, MAX_SKB_FRAGS + 1, size - 1);
 	ring->clean_ptr = start_ptr;
 	ring->c_index = 0;
 	ring->free_bds = size;
diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.h b/drivers/net/ethernet/broadcom/genet/bcmgenet.h
index e48ce39d3e7c8..6ab3f73475550 100644
--- a/drivers/net/ethernet/broadcom/genet/bcmgenet.h
+++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.h
@@ -546,6 +546,7 @@ struct bcmgenet_tx_ring {
 	unsigned int	index;		/* ring index */
 	struct enet_cb	*cbs;		/* tx ring buffer control block*/
 	unsigned int	size;		/* size of each tx ring */
+	unsigned int	stop_thld;	/* queue stops at this many free bds or fewer */
 	unsigned int    clean_ptr;      /* Tx ring clean pointer */
 	unsigned int	c_index;	/* last consumer index of each ring*/
 	unsigned int	free_bds;	/* # of free bds for each ring */

---
base-commit: 19a227903e65c7a9c179156485660a4e017575fe
change-id: 20261106-nb-genet-txq-thld-net-e0864f235d22

Best regards,
--  
Nicolai Buchwitz <nb@xxxxxxxxxxx>