linux-kernel - [PATCH] net: usb: lan78xx: Enforce a minimum interrupt polling period

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Message-ID: <20250310165932.1201702-1-fiona.klute@gmx.de>
Date: Mon, 10 Mar 2025 17:59:31 +0100
From: Fiona Klute <fiona.klute@....de>
To: netdev@...r.kernel.org
Cc: Thangaraj Samynathan <Thangaraj.S@...rochip.com>,
	Rengarajan Sundararajan <Rengarajan.S@...rochip.com>,
	UNGLinuxDriver@...rochip.com,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Fiona Klute <fiona.klute@....de>,
	kernel-list@...pberrypi.com,
	stable@...r.kernel.org
Subject: [PATCH] net: usb: lan78xx: Enforce a minimum interrupt polling period

If a new reset event appears before the previous one has been
processed, the device can get stuck into a reset loop. This happens
rarely, but blocks the device when it does, and floods the log with
messages like the following:

  lan78xx 2-3:1.0 enp1s0u3: kevent 4 may have been dropped

The only bit that the driver pays attention to in the interrupt data
is "link was reset". If there's a flapping status bit in that endpoint
data (such as if PHY negotiation needs a few tries to get a stable
link), polling at a slower rate allows the state to settle.

This is a simplified version of a patch that's been in the Raspberry
Pi downstream kernel since their 4.14 branch, see also:
https://github.com/raspberrypi/linux/issues/2447

Signed-off-by: Fiona Klute <fiona.klute@....de>
Cc: kernel-list@...pberrypi.com
Cc: stable@...r.kernel.org
---
For the stable crew: I've *tested* the patch with 6.12.7 and 6.13.5 on
a Revolution Pi Connect 4 (Raspberry Pi CM4 based device with built-in
LAN7800 as second ethernet port), according to the linked issue for
the RPi downstream kernel the problem should be present in all
maintained longterm kernel versions, too (based on how long they've
carried a patch).

 drivers/net/usb/lan78xx.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c
index a91bf9c7e31d..7bf01a31a932 100644
--- a/drivers/net/usb/lan78xx.c
+++ b/drivers/net/usb/lan78xx.c
@@ -173,6 +173,12 @@
 #define INT_EP_GPIO_1			(1)
 #define INT_EP_GPIO_0			(0)
 
+/* highspeed device, so polling interval is in microframes (eight per
+ * millisecond)
+ */
+#define INT_URB_MICROFRAMES_PER_MS	8
+#define MIN_INT_URB_INTERVAL_MS		8
+
 static const char lan78xx_gstrings[][ETH_GSTRING_LEN] = {
 	"RX FCS Errors",
 	"RX Alignment Errors",
@@ -4527,7 +4533,11 @@ static int lan78xx_probe(struct usb_interface *intf,
 	if (ret < 0)
 		goto out4;
 
-	period = ep_intr->desc.bInterval;
+	period = max(ep_intr->desc.bInterval,
+		     MIN_INT_URB_INTERVAL_MS * INT_URB_MICROFRAMES_PER_MS);
+	dev_info(&intf->dev,
+		 "interrupt urb period set to %d, bInterval is %d\n",
+		 period, ep_intr->desc.bInterval);
 	maxp = usb_maxpacket(dev->udev, dev->pipe_intr);
 
 	dev->urb_intr = usb_alloc_urb(0, GFP_KERNEL);

base-commit: dd83757f6e686a2188997cb58b5975f744bb7786
-- 
2.47.2