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

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Date:   Wed, 19 Jun 2019 14:23:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Alex Elder <elder@...aro.org>, abhishek.esse@...il.com,
        Ben Chan <benchan@...gle.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        cpratapa@...eaurora.org, David Miller <davem@...emloft.net>,
        Dan Williams <dcbw@...hat.com>,
        DTML <devicetree@...r.kernel.org>,
        Eric Caruso <ejcaruso@...gle.com>, evgreen@...omium.org,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-soc@...r.kernel.org, Networking <netdev@...r.kernel.org>,
        syadagir@...eaurora.org
Subject: Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver

On Tue, Jun 18, 2019 at 11:15 PM Subash Abhinov Kasiviswanathan
<subashab@...eaurora.org> wrote:
>
> On 2019-06-18 14:55, Arnd Bergmann wrote:
> > On Tue, Jun 18, 2019 at 10:36 PM Johannes Berg
> > <johannes@...solutions.net> 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.

Oh, so you mean that even though IPA supports multiple channels
and multiple netdev instances for a physical device, all the
rmnet devices end up being thrown into a single channel in IPA?

What are the other channels for in IPA? I understand that there
is one channel for commands that is separate, while the others
are for network devices, but that seems to make no sense if
we only use a single channel for rmnet data.

> >> 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.

Right, I guess that makes sense if everything ends up in a
single queue in IPA.

      Arnd