Re: commit 662dc80a5e86 breaks rmnet over usb

[Daniele Palmas]

Hello,

Il giorno lun 23 feb 2026 alle ore 15:08 Laurent Vivier
<lvivier@xxxxxxxxxx> ha scritto:
>
> On 2/23/26 13:04, Koen Vandeputte wrote:
> > Hi Laurent,
>
> Hi Koen,
>
> > I'm testing the latest openwrt state and found an issue probably
> > caused by your usb mtu limit patch :-)
> >
> > I'm talking about this one:
> > 662dc80a5e86 ("usbnet: limit max_mtu based on device's hard_mtu")
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v6.12.74&id=662dc80a5e86b35bbf339e0b87b7cc3f07c09de1
> >
> >
> > When using wwan0 iface normally, this makes sense, but the problem is
> > when using QMI modems combined with the rmnet driver and aggregated
> > frames.
> >
> > - The modem is configured to frame sizes of 16383 or 32767 using QMI
> > - wwan0 (using qmi_wwan) is configured to match this frame size by
> > setting it's MTU to the same value
> > - Frames of this size are sent over to qmi_wwan driver (containing
> > multiple data packets)
> > - Frames are then forwarded to the rmnet driver
> > - Frames get de-aggregated here and sent to the network stack for processing.
> >
> > The reason here is to reduce USB transfers heavily.
> >
> >
> > As you see, it's perfectly possible here to use very large MTU sizes
> > as the aggregation feature by rmnet relies on this.
> > Also the modem can be perfectly configured to send very large aggregated frames.
> >
> > After your patch, wwan0 is limited to 1500 bytes it seems, effectively
> > breaking aggregation.
> >
> > On my tests, download speeds are reduced from >300Mbps to ~.8Mbps
> >
> > I also made a build reverting this patch and all works well again.
> >
> >
> > Is there any other solution to fix this?
> > I guess it should be reverted otherwise :-)
>
> It's weird to be able to set an MTU that is not supported by the hardware.
>
> To restore performance I think the rx_usb_size should be decoupled from MTU max in qmi_wwan.
>
> 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.

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.

Regards,
Daniele

>          /* claim data interface and set it up */
>          if (info->control != info->data) {
>                  status = usb_driver_claim_interface(driver, info->data, dev);
>
> Thanks,
> Laurent
>
>