On 6/3/19 5:04 AM, Arnd Bergmann wrote:

On Sat, Jun 1, 2019 at 1:59 AM Subash Abhinov Kasiviswanathan
[off-list ref] wrote:

quoted

On 2019-05-31 17:33, Bjorn Andersson wrote:

quoted

On Fri 31 May 13:47 PDT 2019, Alex Elder wrote:

quoted

On 5/31/19 2:19 PM, Arnd Bergmann wrote:

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.

I've read up on the implementation some more, and concluded that
it's mostly a regular protocol wrapper, doing IP over QMAP. There
is nothing wrong with the basic concept I think, and as you describe
this is an abstraction to keep the common bits in one place, and
have them configured consistently.

A few observations on more details here:

- What I'm worried about most here is the flow control handling on the
  transmit side. The IPA driver now uses the modern BQL method to
  control how much data gets submitted to the hardware at any time.
  The rmnet driver also uses flow control using the
  rmnet_map_command() function, that blocks tx on the higher
  level device when the remote side asks us to.
  I fear that doing flow control for a single physical device on two
  separate netdev instances is counterproductive and confuses
  both sides.

I understand what you're saying here, and instinctively I think
you're right.

But BQL manages the *local* interface's ability to get rid of
packets, whereas the QMAP flow control is initiated by the other
end of the connection (the modem in this case).

With multiplexing, it's possible that one of several logical
devices on the modem side has exhausted a resource and must
ask the source of the data on the host side to suspend the
flow.  Meanwhile the other logical devices sharing the physical
link might be fine, and should not be delayed by the first one. 

It is the multiplexing itself that confuses the BQL algorithm.
The abstraction obscures the *real* rates at which individual
logical connections are able to transmit data.

Even if the multiple logical interfaces implemented BQL, they
would not get the feedback they need directly from the IPA
driver, because transmitting over the physical interface might
succeed even if the logical interface on the modem side can't
handle more data.  So I think the flow control commands may be
necessary, given multiplexing.

The rmnet driver could use BQL, and could return NETDEV_TX_BUSY
for a logical interface when its TX flow has been stopped by a
QMAP command.  That way the feedback for BQL on the logical
interfaces would be provided in the right place.

I have no good intuition about the interaction between
two layered BQL managed queues though.

- I was a little confused by the location of the rmnet driver in
  drivers/net/ethernet/... More conventionally, I think as a protocol
  handler it should go into net/qmap/, with the ipa driver going
  into drivers/net/qmap/ipa/, similar to what we have fo ethernet,
  wireless, ppp, appletalk, etc.

- The rx_handler uses gro_cells, which as I understand is meant
  for generic tunnelling setups and takes another loop through
  NAPI to aggregate data from multiple queues, but in case of
  IPA's single-queue receive calling gro directly would be simpler
  and more efficient.

I have been planning to investigate some of the generic GRO
stuff for IPA but was going to wait on that until the basic
code was upstream.

- I'm still not sure I understand the purpose of the layering with
  using an rx_handler as opposed to just using
  EXPORT_SYMBOL(rmnet_rx_handler) and calling that from
  the hardware driver directly.

I think that's a good idea.

  From the overall design and the rmnet Kconfig description, it
  appears as though the intention as that rmnet could be a
  generic wrapper on top of any device, but from the
  implementation it seems that IPA is not actually usable that
  way and would always go through IPA.

As far as I know *nothing* upstream currently uses rmnet; the
IPA driver will be the first, but as Bjorn said others seem to
be on the way.  I'm not sure what you mean by "IPA is not
usable that way."  Currently the IPA driver assumes a fixed
configuration, and that configuration assumes the use of QMAP,
and therefore assumes the rmnet driver is layered above it.
That doesn't preclude rmnet from using a different back end.

And I'll also mention that although QMAP *can* do multiplexed
connections over a single physical link, the IPA code I posted
currently supports only one logical interface.

					-Alex

        Arnd