Event ring full using qmi_wwan but not cdc_mbim

[Bjørn Mork]

Nick B <nickberry17 at icloud.com> writes:

> I have noticed that when using qmi_wwan when I run a speed test I will
> start to see a fair amount (more than ten per test) of these...
> [  643.225139] xhci-hcd xhci-hcd.0.auto: ERROR unknown event type 37
>
> I have searched around and found that it is probably an "even ring
> full” error generated by the xhci module.  But I do not see this error
> when running the same test while running the cdc_mbim driver.  Just
> curious if anyone knows why that might be?
>
> I’m on OpenWrt and have seen the same behaviour on two boards with two
> modems, MC7430 with MT7621-based board (about a year ago with
> then-current MM and friends) and more recently with an EM7565 on an
> IPQ4019-based board.

I guess this can be a positive effect of the MBIM aggregation.

QMI/rmnet transports one IP packet per USB transaction. Each transaction
will be between 0 and 2 full USB packets, followed by a short USB
packet.  MBIM collects as many IP packets as will fit into a fixed size
frame. Each USB transaction will always be for example 32 or 64 full USB
packets.  There is no short USB packet.  This reduces the workload on
both the device controller and the host controller.

You are probably seeing a host CPU not being able to keep up with the
number of USB transactions on the bus.


Bjørn