netdev - [PATCH 4/6] can: esd_usb: Add watermark handling for TX jobs

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Message-Id: <20250811210611.3233202-5-stefan.maetje@esd.eu>
Date: Mon, 11 Aug 2025 23:06:09 +0200
From: Stefan Mätje <stefan.maetje@....eu>
To: Marc Kleine-Budde <mkl@...gutronix.de>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>,
	Frank Jungclaus <frank.jungclaus@....eu>,
	linux-can@...r.kernel.org,
	socketcan@....eu
Cc: Simon Horman <horms@...nel.org>,
	Olivier Sobrie <olivier@...rie.be>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	netdev@...r.kernel.org
Subject: [PATCH 4/6] can: esd_usb: Add watermark handling for TX jobs

The driver tried to keep as much CAN frames as possible submitted to the
USB device (ESD_USB_MAX_TX_URBS). This has led to occasional "No free
context" error messages in high load situations like with
"cangen -g 0 -p 10 canX".

This patch now calls netif_stop_queue() already if the number of active
jobs reaches ESD_USB_TX_URBS_HI_WM which is < ESD_USB_MAX_TX_URBS.
The netif_start_queue() is called in esd_usb_tx_done_msg() only if
the number of active jobs is <= ESD_USB_TX_URBS_LO_WM.

This change eliminates the occasional error messages and significantly
reduces the number of calls to netif_start_queue() and
netif_stop_queue().

The watermark limits have been chosen with the CAN-USB/Micro in mind to
not to compromise its TX throughput. This device is running on USB 1.1
only with its 1ms USB polling cycle where a ESD_USB_TX_URBS_LO_WM
value below 9 decreases the TX throughput.

Signed-off-by: Stefan Mätje <stefan.maetje@....eu>
---
 drivers/net/can/usb/esd_usb.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/net/can/usb/esd_usb.c b/drivers/net/can/usb/esd_usb.c
index fc28fb52564c..41ff453f87b8 100644
--- a/drivers/net/can/usb/esd_usb.c
+++ b/drivers/net/can/usb/esd_usb.c
@@ -98,6 +98,8 @@ MODULE_LICENSE("GPL v2");
 #define ESD_USB_RX_BUFFER_SIZE		1024
 #define ESD_USB_MAX_RX_URBS		4
 #define ESD_USB_MAX_TX_URBS		16 /* must be power of 2 */
+#define ESD_USB_TX_URBS_HI_WM		((15 * ESD_USB_MAX_TX_URBS) / 16)
+#define ESD_USB_TX_URBS_LO_WM		((9 * ESD_USB_MAX_TX_URBS) / 16)
 #define ESD_USB_DRAIN_TIMEOUT_MS	100
 
 /* Modes for CAN-USB/3, to be used for esd_usb_3_set_baudrate_msg_x.mode */
@@ -478,7 +480,8 @@ static void esd_usb_tx_done_msg(struct esd_usb_net_priv *priv,
 	if (!netif_device_present(netdev))
 		return;
 
-	netif_wake_queue(netdev);
+	if (atomic_read(&priv->active_tx_jobs) <= ESD_USB_TX_URBS_LO_WM)
+		netif_wake_queue(netdev);
 }
 
 static void esd_usb_read_bulk_callback(struct urb *urb)
@@ -973,7 +976,7 @@ static netdev_tx_t esd_usb_start_xmit(struct sk_buff *skb,
 	context->priv = priv;
 	context->echo_index = i;
 
-	/* hnd must not be 0 - MSB is stripped in txdone handling */
+	/* hnd must not be 0 - MSB is stripped in TX done handling */
 	msg->tx.hnd = BIT(31) | i; /* returned in TX done message */
 
 	usb_fill_bulk_urb(urb, dev->udev, usb_sndbulkpipe(dev->udev, 2), buf,
@@ -988,8 +991,8 @@ static netdev_tx_t esd_usb_start_xmit(struct sk_buff *skb,
 
 	atomic_inc(&priv->active_tx_jobs);
 
-	/* Slow down tx path */
-	if (atomic_read(&priv->active_tx_jobs) >= ESD_USB_MAX_TX_URBS)
+	/* Slow down TX path */
+	if (atomic_read(&priv->active_tx_jobs) >= ESD_USB_TX_URBS_HI_WM)
 		netif_stop_queue(netdev);
 
 	err = usb_submit_urb(urb, GFP_ATOMIC);
-- 
2.34.1