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

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.


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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