net: mctp: usb: Allow multiple urbs in flight

Currently, we stop tx queues when we have one urb submitted. This means
we will immediately hit dev_hard_start_xmit's tx-queues-off ->
NETDEV_TX_BUSY case, and revert to the requeue -> gso_skb single-dequeue
path, and no longer be able to pack skbs without an xmit_more
indication.

Instead, allow a few urbs to be in-flight, with a limit of 16kB of data
outstanding (after which we will disable queues). With this, the tx path
will cause fewer requeues (and therefore non-packed transfers) under
normal loads.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20260724-dev-mctp-usb-1-1-v5-12-e66bbba0dbdc@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 29 insertions, 4 deletions

diff --git a/drivers/net/mctp/mctp-usb.c b/drivers/net/mctp/mctp-usb.c
index c4ff9e91a768..542a570c76cc 100644
--- a/drivers/net/mctp/mctp-usb.c
+++ b/drivers/net/mctp/mctp-usb.c
@@ -42,6 +42,9 @@ struct mctp_usb {
 
 	struct mctp_usblib_tx tx;
 	struct usb_anchor tx_anchor;
+	/* serialises tx_qmem updates to netdev queue states */
+	spinlock_t tx_qmem_lock;
+	int tx_qmem;
 };
 
 enum {
@@ -49,23 +52,41 @@ enum {
 	MCTP_USB_SUBCLASS_SPAN = 0x02,
 };
 
+/* We use a total-size limit for outstanding URBs, as the transfer counts
+ * may vary a lot between spanning- and non-spanning modes. In spanning mode,
+ * this will allow for a couple of max-sized transfers to be in flight. In
+ * non-spanning mode, 32.
+ *
+ * We want to avoid disabling the tx queue if possible; doing so will end up
+ * requeueing to gso_skb, and we only dequeue from that one skb at a time,
+ * so can no longer perform transfer packing.
+ */
+static const unsigned int TX_QMEM_MAX = 16384;
+
 static void mctp_usb_out_complete(struct urb *urb)
 {
 	struct mctp_usblib_tx_ctx *tx_ctx = urb->context;
 	struct mctp_usb *mctp_usb = mctp_usblib_tx_ctx_priv(tx_ctx);
+	unsigned int len = urb->transfer_buffer_length;
 	struct net_device *netdev = mctp_usb->netdev;
+	unsigned long flags;
 
 	mctp_usblib_tx_send_complete(tx_ctx, netdev, urb->status == 0);
 
 	usb_free_urb(urb);
 
-	netif_wake_queue(netdev);
+	spin_lock_irqsave(&mctp_usb->tx_qmem_lock, flags);
+	mctp_usb->tx_qmem -= len;
+	if (mctp_usb->tx_qmem < TX_QMEM_MAX && netif_running(netdev))
+		netif_wake_queue(netdev);
+	spin_unlock_irqrestore(&mctp_usb->tx_qmem_lock, flags);
 }
 
 static int mctp_usb_tx_send(struct mctp_usblib_tx_ctx *tx_ctx,
 			    void *data, size_t len)
 {
 	struct mctp_usb *mctp_usb = mctp_usblib_tx_ctx_priv(tx_ctx);
+	unsigned long flags;
 	struct urb *urb;
 	int rc;
 
@@ -80,8 +101,6 @@ static int mctp_usb_tx_send(struct mctp_usblib_tx_ctx *tx_ctx,
 	if (mctp_usb->span)
 		urb->transfer_flags |= URB_ZERO_PACKET;
 
-	netif_stop_queue(mctp_usb->netdev);
-
 	usb_anchor_urb(urb, &mctp_usb->tx_anchor);
 
 	rc = usb_submit_urb(urb, GFP_ATOMIC);
@@ -89,7 +108,12 @@ static int mctp_usb_tx_send(struct mctp_usblib_tx_ctx *tx_ctx,
 		netdev_dbg(mctp_usb->netdev, "TX urb submit failed, %d\n", rc);
 		usb_unanchor_urb(urb);
 		usb_free_urb(urb);
-		netif_start_queue(mctp_usb->netdev);
+	} else {
+		spin_lock_irqsave(&mctp_usb->tx_qmem_lock, flags);
+		mctp_usb->tx_qmem += len;
+		if (mctp_usb->tx_qmem >= TX_QMEM_MAX)
+			netif_stop_queue(mctp_usb->netdev);
+		spin_unlock_irqrestore(&mctp_usb->tx_qmem_lock, flags);
 	}
 
 	return rc;
@@ -353,6 +377,7 @@ static int mctp_usb_probe(struct usb_interface *intf,
 	spin_lock_init(&dev->rx_lock);
 	if (dev->span)
 		netdev->max_mtu = MCTP_USB_1_1_MTU_MAX;
+	spin_lock_init(&dev->tx_qmem_lock);
 	usb_set_intfdata(intf, dev);
 
 	rc = mctp_usblib_rx_init(&dev->rx, le16_to_cpu(ep_in->wMaxPacketSize),