Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver

[Arnd Bergmann]

On Tue, Jun 18, 2019 at 10:36 PM Johannes Berg
<johannes@xxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 2019-06-18 at 21:59 +0200, Arnd Bergmann wrote:
> >
> > From my understanding, the ioctl interface would create the lower
> > netdev after talking to the firmware, and then user space would use
> > the rmnet interface to create a matching upper-level device for that.
> > This is an artifact of the strong separation of ipa and rmnet in the
> > code.
>
> Huh. But if rmnet has muxing, and IPA supports that, why would you ever
> need multiple lower netdevs?

>From my reading of the code, there is always exactly a 1:1 relationship
between an rmnet netdev an an ipa netdev. rmnet does the encapsulation/
decapsulation of the qmap data and forwards it to the ipa netdev,
which then just passes data through between a hardware queue and
its netdevice.

[side note: on top of that, rmnet also does "aggregation", which may
 be a confusing term that only means transferring multiple frames
 at once]

> > ipa definitely has multiple hardware queues, and the Alex'
> > driver does implement  the data path on those, just not the
> > configuration to enable them.
>
> OK, but perhaps you don't actually have enough to use one for each
> session?

I'm lacking the terminology here, but what I understood was that
the netdev and queue again map to a session.

> > Guessing once more, I suspect the the XON/XOFF flow control
> > was a workaround for the fact that rmnet and ipa have separate
> > queues. The hardware channel on IPA may fill up, but user space
> > talks to rmnet and still add more frames to it because it doesn't
> > know IPA is busy.
> >
> > Another possible explanation would be that this is actually
> > forwarding state from the base station to tell the driver to
> > stop sending data over the air.
>
> Yeah, but if you actually have a hardware queue per upper netdev then
> you don't really need this - you just stop the netdev queue when the
> hardware queue is full, and you have flow control automatically.
>
> So I really don't see any reason to have these messages going back and
> forth unless you plan to have multiple sessions muxed on a single
> hardware queue.

Sure, I definitely understand what you mean, and I agree that would
be the right way to do it. All I said is that this is not how it was done
in rmnet (this was again my main concern about the rmnet design
after I learned it was required for ipa) ;-)

     Arnd