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

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Message-ID: <6e7f0fb2f85d5062a9e23d37c47d311e10aa4624.camel@sipsolutions.net>
Date:   Tue, 18 Jun 2019 23:02:18 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     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>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        syadagir@...eaurora.org
Subject: Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver

On Tue, 2019-06-18 at 22:55 +0200, 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.

I'll take your word for it. Seems very odd, given that the whole point
of the QMAP header seems to be ... muxing?

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

Right, but it's not all that much interesting in the context of this
discussion.

> 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) ;-)

:-)

Well, I guess though if the firmware wants us to listen to those on/off
messages we'll have to do that one way or the other.

Oh. Maybe it's just *because* rmnet is layered on top, and thus you
fundamentally cannot do flow control the way I described - not because
you have multiple session on the same hardware ring, but because you
abstracted the hardware ring away too much ...


johannes