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

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Date:   Wed, 12 Jun 2019 17:06:17 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Dan Williams <dcbw@...hat.com>
Cc:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        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>,
        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 Wed, Jun 12, 2019 at 4:28 PM Dan Williams <dcbw@...hat.com> wrote:
> On Wed, 2019-06-12 at 10:31 +0200, Arnd Bergmann wrote:
> > On Tue, Jun 11, 2019 at 7:23 PM Dan Williams <dcbw@...hat.com> wrote:
> I was trying to make the point that rmnet doesn't need to care about
> how the QMAP packets get to the device itself; it can be pretty generic
> so that it can be used by IPA/qmi_wwan/rmnet_smd/etc.

rmnet at the moment is completely generic in that regard already,
however it is implemented as a tunnel driver talking to another
device rather than an abstraction layer below that driver.

> > The current rmnet model is different in that by design the upper
> > layer
> > (rmnet) and the lower layer (qmi_wwan, ipa, ...) are kept independent
> > in
> > both directions, i.e. ipa has (almost) no knowledge of rmnet, and
> > just
> > has pointers to the other net_device:
> >
> >        ipa_device
> >            net_device
> >
> >        rmnet_port
> >            net_device
> >
> > I understand that the rmnet model was intended to provide a cleaner
> > abstraction, but it's not how we normally structure subsystems in
> > Linux, and moving to a model more like how wireless_dev works
> > would improve both readability and performance, as you describe
> > it, it would be more like (ignoring for now the need for multiple
> > connections):
> >
> >    ipa_dev
> >         rmnet_dev
> >                wwan_dev
> >                       net_device
>
> Perhaps I'm assuming too much from this diagram but this shows a 1:1
> between wwan_dev and "lower" devices.
>
> What Johannes is proposing (IIRC) is something a bit looser where a
> wwan_dev does not necessarily provide netdev itself, but is instead the
> central point that various channels (control, data, gps, sim card, etc)
> register with. That way the wwan_dev can provide an overall view of the
> WWAN device to userspace, and userspace can talk to the wwan_dev to ask
> the lower drivers (ipa, rmnet, etc) to create new channels (netdev,
> tty, otherwise) when the control channel has told the modem firmware to
> expect one.

Right, as I noted above, I simplified it a bit. We probably want to
have multiple net_device instances for an ipa_dev, so there has
to be a 1:n relationship instead of 1:1 at one of the intermediate
levels, but it's not obvious which level that should be.

In theory we could even have a single net_device instance correspond
to the ipa_dev, but then have multiple IP addresses bound to it,
so each IP address corresponds to a channel/queue/napi_struct,
but the user visible object remains a single device.

I trust that you and Johannes are more qualified than me to make
the call on that point.

       Arnd