Hello,

Il giorno gio 26 feb 2026 alle ore 19:18 Daniele Palmas
[off-list ref] ha scritto:

Il giorno gio 26 feb 2026 alle ore 10:09 Laurent Vivier
[off-list ref] ha scritto:

quoted

On 2/26/26 09:26, Daniele Palmas wrote:

quoted

Hello Jakub,

Il giorno gio 26 feb 2026 alle ore 02:01 Jakub Kicinski
[off-list ref] ha scritto:

quoted

On Wed, 25 Feb 2026 08:19:46 +0100 Daniele Palmas wrote:

quoted

quoted

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.

quoted

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.

quoted

quoted

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

quoted

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 *);