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

[Subash Abhinov Kasiviswanathan]

On 2019-06-18 14:55, Arnd Bergmann wrote:
> 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.

There is a n:1 relationship between rmnet and IPA.
rmnet does the de-muxing to multiple netdevs based on the mux id
in the MAP header for RX packets and vice versa.

> [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.

Hardware may flow control specific PDNs (rmnet interfaces) based on QoS 
-
not necessarily only in case of hardware queue full.

> 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

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