Re: commit 662dc80a5e86 breaks rmnet over usb

[Daniele Palmas]

Hello,

Il giorno gio 26 feb 2026 alle ore 19:18 Daniele Palmas
<dnlplm@xxxxxxxxx> ha scritto:
>
> Il giorno gio 26 feb 2026 alle ore 10:09 Laurent Vivier
> <lvivier@xxxxxxxxxx> ha scritto:
> >
> > On 2/26/26 09:26, Daniele Palmas wrote:
> > > Hello Jakub,
> > >
> > > Il giorno gio 26 feb 2026 alle ore 02:01 Jakub Kicinski
> > > <kuba@xxxxxxxxxx> ha scritto:
> > >>
> > >> On Wed, 25 Feb 2026 08:19:46 +0100 Daniele Palmas wrote:
> > >>>> Could you try something like:
> > >>>>
> > >>>> diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
> > >>>> index 3a4985b582cb..6b4796fac692 100644
> > >>>> --- a/drivers/net/usb/qmi_wwan.c
> > >>>> +++ b/drivers/net/usb/qmi_wwan.c
> > >>>> @@ -788,6 +788,8 @@ static int qmi_wwan_bind(struct usbnet *dev, struct usb_interface *intf)
> > >>>>                   usbnet_get_ethernet_addr(dev, cdc_ether->iMACAddress);
> > >>>>           }
> > >>>>
> > >>>> +       dev->rx_urb_size = 32768;
> > >>>> +
> > >>>
> > >>> So far userspace tools (e.g. also the most important one which is
> > >>> ModemManager) rely on changing the rx_urb_size by changing the MTU: I
> > >>> know this is ugly, but it is a behavior that has been there since a
> > >>> lot of time, not sure how many tools based on this assumption could
> > >>> break.
> > >>
> > >> What's the policy in ModemManager to change the rx_urb_size?
> > >> Increase it to make sure it can hold some specific cmd / packet?
> > >>
> > >> I wonder if having rx_urb_size max of (mtu, 32k) would break anything.
> > >>
> > >
> > > Typically the host sends a QMI request to the modem for setting the
> > > size of the maximum QMAP aggregated packets block. Then the modem
> > > replies with the maximum size it supports and that one is then set by
> > > the host through changing the MTU of the netdevice. Low-cat modems
> > > usually support 4-8 KB, while 5G 16-32KB.
> > >
> > > On ModemManager side this is currently fixed at 16KB, but one can use
> > > other tools e.g. qmicli to set custom values as far as they are
> > > supported by the modem.
> > >
> > >> Since we're talking about rx buffer config the right API to configure
> > >> it is likely ethtool -G rx-buf-len :(
> > >>
> > >
> > > Thanks for the hint, I'll try to have a look at that to improve qmi_wwan.
> > >
> > >>> There's also the chance that there are modems which require a higher
> > >>> rx_urb_size, so having this fixed could not work well.
> > >>>
> > >>> Unfortunately usbnet serves many drivers, I agree with Koen that a
> > >>> revert is the safest option.
> > >>
> > >> Then again the usbnet driver is brittle enough as is, if it's just qmi
> > >> that needs this workaround we would be making common code less robust
> > >> for a benefit of a single "hack" (for lack of a better term)
> > >
> > > Makes sense, also Laurent proposed a solution to keep his fix in
> > > usbnet and make qmi_wwan the exception.
> >
> > I was thinking to something like that (see below), but I'm not really able to test it. If
> > everyone thinks it's the path to follow, I can send a patch.
>
> I think I can test this by the end of the week and let you know.
>

I've verified that without this patch I'm not able to set mtu > 1500
and ip -d link shows:

3: wwan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode
DEFAULT group default qlen 1000
    link/ether 16:7f:28:3c:c4:3e brd ff:ff:ff:ff:ff:ff promiscuity 0
allmulti 0 minmtu 0 maxmtu 1500 addrgenmode eui64 numtxqueues 1
numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536
tso_max_segs 65535 gro_max_size 65536 parentbus usb parentdev 3-8:1.0

while with the patch I'm able to set mtu > 1500 and ip -d link shows:

3: wwan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode
DEFAULT group default qlen 1000
    link/ether 2e:56:3a:37:75:e2 brd ff:ff:ff:ff:ff:ff promiscuity 0
allmulti 0 minmtu 0 maxmtu 65535 addrgenmode eui64 numtxqueues 1
numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 tso_max_size 65536
tso_max_segs 65535 gro_max_size 65536 parentbus usb parentdev 3-8:1.0

Interesting enough, in the failing kernel ModemManager is still able
to change the MTU of wwan and properly create the QMAP data call with
the correct rx_urb size, resulting in a netdevice like:

3: wwan0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 16384 qdisc
fq_codel state UNKNOWN mode DEFAULT group default qlen 1000
    link/none  promiscuity 0  allmulti 0 minmtu 0 maxmtu 65535
addrgenmode random numtxqueues 1 numrxqueues 1 gso_max_size 65536
gso_max_segs 65535 tso_max_size 65536 tso_max_segs 65535 gro_max_size
65536 parentbus usb parentdev 3-8:1.0

Still need to check the reason for this, but it seems to me that your
fix is working properly.

Thanks,
Daniele

> Thanks,
> Daniele
>
> >
> > Thanks,
> > Laurent
> >
> > diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
> > index 3a4985b582cb..05acac10cd2b 100644
> > --- a/drivers/net/usb/qmi_wwan.c
> > +++ b/drivers/net/usb/qmi_wwan.c
> > @@ -928,7 +928,7 @@ static int qmi_wwan_resume(struct usb_interface *intf)
> >
> >   static const struct driver_info       qmi_wwan_info = {
> >         .description    = "WWAN/QMI device",
> > -       .flags          = FLAG_WWAN | FLAG_SEND_ZLP,
> > +       .flags          = FLAG_WWAN | FLAG_NOMAXMTU | FLAG_SEND_ZLP,
> >         .bind           = qmi_wwan_bind,
> >         .unbind         = qmi_wwan_unbind,
> >         .manage_power   = qmi_wwan_manage_power,
> > @@ -937,7 +937,7 @@ static const struct driver_info     qmi_wwan_info = {
> >
> >   static const struct driver_info       qmi_wwan_info_quirk_dtr = {
> >         .description    = "WWAN/QMI device",
> > -       .flags          = FLAG_WWAN | FLAG_SEND_ZLP,
> > +       .flags          = FLAG_WWAN | FLAG_NOMAXMTU | FLAG_SEND_ZLP,
> >         .bind           = qmi_wwan_bind,
> >         .unbind         = qmi_wwan_unbind,
> >         .manage_power   = qmi_wwan_manage_power,
> > diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> > index ed86ba87ca4e..b72ba0803392 100644
> > --- a/drivers/net/usb/usbnet.c
> > +++ b/drivers/net/usb/usbnet.c
> > @@ -1829,11 +1829,12 @@ usbnet_probe(struct usb_interface *udev, const struct
> > usb_device_id *prod)
> >                 if ((dev->driver_info->flags & FLAG_NOARP) != 0)
> >                         net->flags |= IFF_NOARP;
> >
> > -               if (net->max_mtu > (dev->hard_mtu - net->hard_header_len))
> > +               if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 &&
> > +                   net->max_mtu > (dev->hard_mtu - net->hard_header_len))
> >                         net->max_mtu = dev->hard_mtu - net->hard_header_len;
> >
> > -               if (net->mtu > net->max_mtu)
> > -                       net->mtu = net->max_mtu;
> > +               if (net->mtu > (dev->hard_mtu - net->hard_header_len))
> > +                       net->mtu = dev->hard_mtu - net->hard_header_len;
> >
> >         } else if (!info->in || !info->out)
> >                 status = usbnet_get_endpoints(dev, udev);
> > diff --git a/include/linux/usb/usbnet.h b/include/linux/usb/usbnet.h
> > index b0e84896e6ac..bbf799ccf3b3 100644
> > --- a/include/linux/usb/usbnet.h
> > +++ b/include/linux/usb/usbnet.h
> > @@ -132,6 +132,7 @@ struct driver_info {
> >   #define FLAG_MULTI_PACKET     0x2000
> >   #define FLAG_RX_ASSEMBLE      0x4000  /* rx packets may span >1 frames */
> >   #define FLAG_NOARP            0x8000  /* device can't do ARP */
> > +#define FLAG_NOMAXMTU          0x10000 /* allow max_mtu above hard_mtu */
> >
> >         /* init device ... can sleep, or cause probe() failure */
> >         int     (*bind)(struct usbnet *, struct usb_interface *);
> >
> >