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Subject: Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver
From:   Dan Williams <dcbw@redhat.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Alex Elder <elder@linaro.org>,
        Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
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Date:   Tue, 04 Jun 2019 16:29:26 -0500
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On Tue, 2019-06-04 at 22:04 +0200, Arnd Bergmann wrote:
> On Tue, Jun 4, 2019 at 5:18 PM Dan Williams <dcbw@redhat.com> wrote:
> > On Tue, 2019-06-04 at 10:13 +0200, Arnd Bergmann wrote:
> > > Can you describe what kind of multiplexing is actually going on?
> > > I'm still unclear about what we actually use multiple logical
> > > interfaces for here, and how they relate to one another.
> > 
> > Each logical interface represents a different "connection" (PDP/EPS
> > context) to the provider network with a distinct IP address and
> > QoS.
> > VLANs may be a suitable analogy but here they are L3+QoS.
> > 
> > In realistic example the main interface (say rmnet0) would be used
> > for
> > web browsing and have best-effort QoS. A second interface (say
> > rmnet1)
> > would be used for VOIP and have certain QoS guarantees from both
> > the
> > modem and the network itself.
> > 
> > QMAP can also aggregate frames for a given channel
> > (connection/EPS/PDP
> > context/rmnet interface/etc) to better support LTE speeds.
> 
> Thanks, that's a very helpful explanation!
> 
> Is it correct to say then that the concept of having those separate
> connections would be required for any proper LTE modem
> implementation,
> but the QMAP protocol (and based on that, the rmnet implementation)
> is Qualcomm specific and shared only among several generations of
> modems from that one vendor?

Exactly correct.  This is what Johannes is discussing in his "cellular
modem APIs - take 2" thread about how this should all be organized at
the driver level and I think we should figure that out before we commit
to IPA-with-a-useless-netdev that requires rmnets to be created on top.
That may end up being the solution but let's have that discussion.

> > You mentioned the need to have a common user space interface
> for configuration, and if the above is true, I agree that we should
> try
> to achieve that, either by ensuring rmnet is generic enough to
> cover other vendors (and non-QMAP clients), or by creating a
> new user level interface that IPA/rmnet can be adapted to.

I would not suggest making rmnet generic; it's pretty QMAP specific
(but QMAP is spoken by many many modems both SoC, USB stick, and PCIe
minicard).

Instead, I think what Johannes is discussing is a better approach. A
kernel WWAN framework with consistent user API that
rmnet/IPA/qmi_wwan/MBIM/QMI/serial/Sierra can all implement.

That wouldn't affect the core packet processing of IPA/rmnet but
instead:

1) when/how an rmnet device actually gets created on top of the IPA (or
qmi_wwan) device

AND (one of these two)

a) whether IPA creates a netdev on probe

OR

b) whether there is some "WWAN device" kernel object which userspace
interacts with create rmnet channels on top of IPA

Dan