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

From: Arnd Bergmann <arnd@arndb.de>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: DTML <devicetree@vger.kernel.org>,
	syadagir@codeaurora.org, Eric Caruso <ejcaruso@google.com>,
	Dan Williams <dcbw@redhat.com>,
	linux-arm-msm@vger.kernel.org, abhishek.esse@gmail.com,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	evgreen@chromium.org,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Ilias Apalodimas <ilias.apalodimas@linaro.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Alex Elder <elder@linaro.org>,
	Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>,
	Networking <netdev@vger.kernel.org>,
	linux-soc@vger.kernel.org, David Miller <davem@davemloft.net>,
	cpratapa@codeaurora.org, Ben Chan <benchan@google.com>
Subject: Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver
Date: Tue, 18 Jun 2019 22:55:23 +0200	[thread overview]
Message-ID: <CAK8P3a2onXpxiE4y9PzRwuPM2dh=h_BKz7Eb0=LLPgBbZoK1bQ@mail.gmail.com> (raw)
In-Reply-To: <613cdfde488eb23d7207c7ba6258662702d04840.camel@sipsolutions.net>

On Tue, Jun 18, 2019 at 10:36 PM Johannes Berg
<johannes@sipsolutions.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.

[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

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