On 2019-06-18 14:55, Arnd Bergmann wrote:
On Tue, Jun 18, 2019 at 10:36 PM Johannes Berg
<johannes@xxxxxxxxxxxxxxxx> 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.
There is a n:1 relationship between rmnet and IPA.
rmnet does the de-muxing to multiple netdevs based on the mux id
in the MAP header for RX packets and vice versa.
[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.
Hardware may flow control specific PDNs (rmnet interfaces) based on QoS
-
not necessarily only in case of hardware queue full.
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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