Hello Seija, Il giorno mar 13 dic 2022 alle ore 18:44 Seija K. <doremylover123@xxxxxxxxx> ha scritto: > > 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@xxxxxxxxx> > > Co-Authored-By: TarAldarion <gildeap@xxxxxx> > --- > 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; Did you test this change with QMAP? To support qmap dl aggregated blocks qmi_wwan relies on the usbnet_change_mtu behavior of changing the rx_urb_size. Thanks, Daniele > err: > return status; > } > -- > 2.38.2
