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

[Subash Abhinov Kasiviswanathan]

On 2019-05-31 17:33, Bjorn Andersson wrote:
> On Fri 31 May 13:47 PDT 2019, Alex Elder wrote:
>
> > On 5/31/19 2:19 PM, Arnd Bergmann wrote:
> > > On Fri, May 31, 2019 at 6:36 PM Alex Elder <elder@xxxxxxxxxx> wrote:
> > >> On 5/31/19 9:58 AM, Dan Williams wrote:
> > >>> On Thu, 2019-05-30 at 22:53 -0500, Alex Elder wrote:
> > >>>
> > >>> My question from the Nov 2018 IPA rmnet driver still stands; how does
> > >>> this relate to net/ethernet/qualcomm/rmnet/ if at all? And if this is
> > >>> really just a netdev talking to the IPA itself and unrelated to
> > >>> net/ethernet/qualcomm/rmnet, let's call it "ipa%d" and stop cargo-
> > >>> culting rmnet around just because it happens to be a net driver for a
> > >>> QC SoC.
> > >>
> > >> First, the relationship between the IPA driver and the rmnet driver
> > >> is that the IPA driver is assumed to sit between the rmnet driver
> > >> and the hardware.
> > >
> > > 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.
>
> But any such changes would either be years into the future or for
> specific devices and as such not applicable to any/most of devices on
> the market now or in the coming years.
>
>
> But as Arnd points out, if the software split between IPA and rmnet is
> suboptimal your are encouraged to fix that.
>
> Regards,
> Bjorn

The split rmnet design was chosen because we could place rmnet
over any transport - IPA, PCIe (https://lkml.org/lkml/2018/4/26/1159)
or USB.

rmnet registers a rx handler, so the rmnet packet processing itself
happens in the same softirq when packets are queued to network stack
by IPA.

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