RM500Q slow with qmi_wwan, fast with proprietary

[Bjørn Mork]

Nick <mips171 at icloud.com> writes:

> Hey,
>
> I am testing a Quectel RM500Q on OpenWrt master, and have noticed to
> my surprise that the speed is much slower when using the qmi_wwan with
> MM than it is when using qmi_wwan_q and quectel-CM (Quectel’s
> proprietary driver and connection manager).

This is sort of expected since the qmi_wwan driver will use one USB
transaction per IP packet whereas the qmi_wwan_q will buffer a number of
packets per transaction.

There is some built-in support for MAP (RMNET muxing, which implies
buffering) in qmi_wwan.  But I recommend using the more recent rmnet
driver for that, with qmi_wwan in pass-throuh mode.  This is supported
by recent ModemManager/libqmi.  Ref
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/447

> Under good signal conditions the speed tops out at around 100Mbps on
> qmi_wwan + MM (and is a little bit faster when in MBIM mode with MM),
> but switching to qmi_wwan_q and quectel_CM it gets the expected
> 700Mbps+ where I am. Is there an easy explanation for this? Any
> suggestions as to what I can change to get speeds equivalent to the
> proprietary stack?

I'm a little surprised that you don't get better numbers in MBIM mode.
It should have the same advantages as qmi_wwan_q or qmi_wwan+rmnet. I
must admit that I haven't done any seriuos testing of this theory myself
though.  But "A little bit faster than 100Mbps" is unexpectedly slow.
I'm pretty sure we can do much better than that in MBIM mode.

What kind of hardware is the host running?  Maybe we have some alignment
issue punishing this hardware?  Or maybe the buffers we use are
sub-optimal for thise host+device combo?  You could try to adjust some
of the writable settings in /sys/class/net/wwan0/cdc_ncm/ (replace wwan0
with your interface name)



Bjørn