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

[Alex Elder <elder@linaro.org>]

On 6/17/19 7:25 AM, Johannes Berg wrote:
> On Mon, 2019-06-17 at 13:42 +0200, Johannes Berg wrote:
> 
>> But anyway, as I alluded to above, I had something like this in mind:
> 
> I forgot to state this here, but this was *heavily* influenced by
> discussions with Dan - many thanks to him.

Thanks for getting even more concrete with this.  Code is the
most concise way of describing things, once the general ideas
seem to be coming together.

I'm not going to comment on the specific code bits, but I have
some more general questions and comments on the design.  Some
of these are simply due to my lack of knowledge of how WWAN/modem
interactions normally work.

First, a few terms (correct or improve as you like):
- WWAN device is a hardware device (like IPA) that presents a
  connection between AP and modem, and presents an interface
  that allows the use of that connection to be managed.
- WWAN netdevice represents a Linux network interface, with its
  operations and queues, etc., but implements a standardized
  set of WWAN-specific operations.  It represents a logical
' channel whose data is multiplexed over the WWAN device.
- WWAN channel is a user space abstraction that corresponds
  with a WWAN netdevice (but I'm not clear on all the ways
  they differ or interact).
- The WWAN core is kernel code that presents abstractions
  for WWAN devices and netdevices, so they can be managed
  in a generic way.  It is for configuration and communication
  and is not at all involved in the data path.

You're saying that the WWAN driver space calls wwan_add()
to register itself as a new WWAN device.

You're also saying that a WWAN device "attaches" a WWAN
netdevice, which is basically notifying the WWAN core
that the new netdev/channel is available for use.
- I trust that a "tentative" attachement is necessary.  But
  I'm not sure what makes it transition into becoming a
  "real" one, or how that event gets communicated.

Some questions:
- What causes a new channel to be created?  Is it initiated
  by the WWAN device driver?  Does the modem request that
  it get created?  User space?  Both?
- What causes a created channel to be removed?
- You distinguish between attaching a netdevice and (what
  I'll call) activating it.  What causes activation?
- How are the attributes of a WWAN device or channel set,
  or communicated?
- Are there any attributes that are only optionally supported,
  and if so, how are the supported ones communicated?
- Which WWAN channel attributes must be set *before* the
  channel is activated, and can't be changed?  Are there any
  that can be changed dynamically?

And while the whole point of this is to make things generic,
it might be nice to have a way to implement a new feature
before it can be "standardized".

Thanks.

					-Alex

PS  I don't want to exclude anybody but we could probably start
    a different mail chain on this topic...

>> driver_dev
>>   struct device *dev (USB, PCI, ...)
>>   net_device NA
>>   net_device NB
>>   tty TA
>>  ...
>>

. . .