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From: "Seija K." <doremylover123@gmail.com>
Date: Tue, 13 Dec 2022 12:40:52 -0500
Subject: [PATCH] net: Fix for packets being rejected in the xHCI controller's
ring buffer
When a packet larger than MTU arrives in Linux from the modem, it is
discarded with -EOVERFLOW error (Babble error).
This is seen on USB3.0 and USB2.0 buses.
This is because the MRU (Max Receive Size) is not a separate entity
from the MTU (Max Transmit Size), and the received packets can be
larger than those transmitted.
Following the babble error, there was an endless supply of zero-length
URBs that were rejected with -EPROTO (increasing the rx input error
counter each time).
This is only seen on USB3.0. These continue to come ad infinitum until
the modem is shut down.
There appears to be a bug in the core USB handling code in Linux that
doesn't deal with network MTUs smaller than 1500 bytes well.
By default, the dev->hard_mtu (the real MTU) is in lockstep with
dev->rx_urb_size (essentially an MRU), and the latter is causing
trouble.
This has nothing to do with the modems; the issue can be reproduced by
getting a USB-Ethernet dongle, setting the MTU to 1430, and pinging
with size greater than 1406.
Signed-off-by: Seija Kijin <doremylover123@gmail.com>
Co-Authored-By: TarAldarion <gildeap@tcd.ie>
Upstream-Status: Inappropriate [other]
Workaround for the babble issue, breaks qmap link aggregation that uses
datagram sizes bigger than 1504. qmap link aggregation requires datagram
sizes of 4096, 16384 bytes and so on, depending on the radio.
mPower and mLinux do not use qmap link aggregation, but set the MTU based
on the value reported by the radio (provider-specific). This patch sets
"almost-MRU" to a high and fixed value, and should be enough for our needs.
See GP-2221 for details.
---
drivers/net/usb/qmi_wwan.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
index 554d4e2a84a4..39db53a74b5a 100644
--- a/drivers/net/usb/qmi_wwan.c
+++ b/drivers/net/usb/qmi_wwan.c
@@ -842,6 +842,13 @@ static int qmi_wwan_bind(struct usbnet *dev, struct usb_interface *intf)
}
dev->net->netdev_ops = &qmi_wwan_netdev_ops;
dev->net->sysfs_groups[0] = &qmi_wwan_sysfs_attr_group;
+ /* LTE Networks don't always respect their own MTU on the receiving side;
+ * e.g. AT&T pushes 1430 MTU but still allows 1500 byte packets from
+ * far-end networks. Make the receive buffer large enough to accommodate
+ * them, and add four bytes so MTU does not equal MRU on network
+ * with 1500 MTU. Otherwise, usbnet_change_mtu() will change both.
+ */
+ dev->rx_urb_size = ETH_DATA_LEN + 4;
err:
return status;
}
--
2.38.2
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