netdev - RMNET QMAP data aggregation with size greater than 16384

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Date:   Sun, 1 Aug 2021 00:45:26 +0200
From:   Aleksander Morgado <aleksander@...ksander.es>
To:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
Cc:     Bjørn Mork <bjorn@...k.no>,
        Daniele Palmas <dnlplm@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: RMNET QMAP data aggregation with size greater than 16384

Hey Subash,

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!

-- 
Aleksander
https://aleksander.es