netdev - Re: RMNET QMAP data aggregation with size greater than 16384

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Date:   Thu, 5 Aug 2021 23:01:26 +0200
From:   Aleksander Morgado <aleksander@...ksander.es>
To:     Daniele Palmas <dnlplm@...il.com>
Cc:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Bjørn Mork <bjorn@...k.no>,
        Network Development <netdev@...r.kernel.org>,
        Sean Tranchetti <stranche@...eaurora.org>
Subject: Re: RMNET QMAP data aggregation with size greater than 16384

>> > > I'm playing with the whole QMAP data aggregation setup with a USB
>> > > connected Fibocom FM150-AE module (SDX55).
>> > > See https://gitlab.freedesktop.org/mobile-broadband/libqmi/-/issues/71
>> > > for some details on how I tested all this.
>> > >
>> > > This module reports a "Downlink Data Aggregation Max Size" of 32768
>> > > via the "QMI WDA Get Data Format" request/response, and therefore I
>> > > configured the MTU of the master wwan0 interface with that same value
>> > > (while in 802.3 mode, before switching to raw-ip and enabling
>> > > qmap-pass-through in qmi_wwan).
>> > >
>> > > When attempting to create a new link using netlink, the operation
>> > > fails with -EINVAL, and following the code path in the kernel driver,
>> > > it looks like there is a check in rmnet_vnd_change_mtu() where the
>> > > master interface MTU is checked against the RMNET_MAX_PACKET_SIZE
>> > > value, defined as 16384.
>> > >
>> > > If I setup the master interface with MTU 16384 before creating the
>> > > links with netlink, there's no error reported anywhere. The FM150
>> > > module crashes as soon as I connect it with data aggregation enabled,
>> > > but that's a different story...
>> > >
>> > > Is this limitation imposed by the RMNET_MAX_PACKET_SIZE value still a
>> > > valid one in this case? Should changing the max packet size to 32768
>> > > be a reasonable approach? Am I doing something wrong? :)
>> > >
>> > > This previous discussion for the qmi_wwan add_mux/del_mux case is
>> > > relevant:
>> > > https://patchwork.ozlabs.org/project/netdev/patch/20200909091302.20992-1-dnlplm@gmail.com/..
>> > > The suggested patch was not included yet in the qmi_wwan driver and
>> > > therefore the user still needs to manually configure the MTU of the
>> > > master interface before setting up all the links, but at least there
>> > > seems to be no maximum hardcoded limit.
>> > >
>> > > Cheers!
>> >
>> > Hi Aleksander
>> >
>> > The downlink data aggregation size shouldn't affect the MTU.
>> > MTU applies for uplink only and there is no correlation with the
>> > downlink path.
>> > Ideally, you should be able to use standard 1500 bytes (+ additional
>> > size for MAP header)
>> > for the master device. Is there some specific network which is using
>> > greater than 1500 for the IP packet itself in uplink.
>> >
>>
>> I may be mistaken then in how this should be setup when using rmnet.
>> For the qmi_wwan case using add_mux/del_mux (Daniele correct me if
>> wrong!), we do need to configure the MTU of the master interface to be
>> equal to the aggregation data size reported via QMI WDA before
>> creating any mux link; see
>> http://paldan.altervista.org/linux-qmap-qmi_wwan-multiple-pdn-setup/
>>
>
> Right: it's not for the MTU itself, but for changing the rx_urb_size, since usbnet_change_mtu has that side effect.
>

I knew there was a reason even if not obvious. Should we fix that rx
urb size value to 16384 to avoid needing that extra step? Was that
what you were suggesting in that patch that was never merged?

-- 
Aleksander
https://aleksander.es