Re: commit 662dc80a5e86 breaks rmnet over usb

[Laurent Vivier <lvivier@redhat.com>]

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

Thanks,
Laurent