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

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Date:   Fri, 31 May 2019 23:12:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alex Elder <elder@...aro.org>
Cc:     Dan Williams <dcbw@...hat.com>, David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        evgreen@...omium.org, Ben Chan <benchan@...gle.com>,
        Eric Caruso <ejcaruso@...gle.com>, cpratapa@...eaurora.org,
        syadagir@...eaurora.org,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        abhishek.esse@...il.com, Networking <netdev@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-soc@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2 00/17] net: introduce Qualcomm IPA driver

On Fri, May 31, 2019 at 10:47 PM Alex Elder <elder@...aro.org> wrote:
> On 5/31/19 2:19 PM, Arnd Bergmann wrote:
> > On Fri, May 31, 2019 at 6:36 PM Alex Elder <elder@...aro.org> wrote:
> >> On 5/31/19 9:58 AM, Dan Williams wrote:
> >>> On Thu, 2019-05-30 at 22:53 -0500, Alex Elder wrote:
> >
> > Does this mean that IPA can only be used to back rmnet, and rmnet
> > can only be used on top of IPA, or can or both of them be combined
> > with another driver to talk to instead?
>
> No it does not mean that.
>
> As I understand it, one reason for the rmnet layer was to abstract
> the back end, which would allow using a modem, or using something
> else (a LAN?), without exposing certain details of the hardware.
> (Perhaps to support multiplexing, etc. without duplicating that
> logic in two "back-end" drivers?)
>
> To be perfectly honest, at first I thought having IPA use rmnet
> was a cargo cult thing like Dan suggested, because I didn't see
> the benefit.  I now see why one would use that pass-through layer
> to handle the QMAP features.
>
> But back to your question.  The other thing is that I see no
> reason the IPA couldn't present a "normal" (non QMAP) interface
> for a modem.  It's something I'd really like to be able to do,
> but I can't do it without having the modem firmware change its
> configuration for these endpoints.  My access to the people who
> implement the modem firmware has been very limited (something
> I hope to improve), and unless and until I can get corresponding
> changes on the modem side to implement connections that don't
> use QMAP, I can't implement such a thing.

Why would that require firmware changes? What I was thinking
here is to turn the bits of the rmnet driver that actually do anything
interesting on the headers into a library module (or a header file
with inline functions) that can be called directly by the ipa driver,
keeping the protocol unchanged.

> > Always passing data from one netdev to another both ways
> > sounds like it introduces both direct CPU overhead, and
> > problems with flow control when data gets buffered inbetween.
>
> My impression is the rmnet driver is a pretty thin layer,
> so the CPU overhead is probably not that great (though
> deaggregating a message is expensive).  I agree with you
> on the flow control.

The CPU overhead I mean is not from executing code in the
rmnet driver, but from passing packets through the network
stack between the two drivers, i.e. adding each frame to
a queue and taking it back out. I'm not sure how this ends
up working in reality but from a first look it seems like
we might bounce in an out of the softirq handler inbetween.

          Arnd