Re: WWAN Controller Framework (was IPA [PATCH v2 00/17])

[Alex Elder <elder@linaro.org>]

On 6/25/19 9:34 AM, Johannes Berg wrote:
> On Mon, 2019-06-24 at 12:06 -0500, Alex Elder wrote:
> 
>>> OK I want to try to organize a little more concisely some of the
>>> discussion on this, because there is a very large amount of volume
>>> to date and I think we need to try to narrow the focus back down
>>> again.
> 
> Sounds good to me!

. . .

>>> - A WWAN unit shall implement a *WWAN control function*, used to
>>>   manage the use of other WWAN functions, as well as the WWAN unit
>>>   itself.
> 
> I think here we need to be more careful. I don't know how you want to
> call it, but we actually have multiple levels of control here.

I completely agree with you.  From what I understand there exists
a control channel (or even more than one?) that serves a very
specific purpose in modem management.  The main reason I mention
the WWAN control function is that someone (maybe you) indicated
that a control channel automatically gets created.

But I agree, we need to be careful to avoid confusion here.

> You have
>  * hardware control, to control how you actually use the (multiple or
>    not) physical communication channel(s) to the WWAN unit
>  * this is partially exposed to userspace via the WWAN netlink family or
>    something like that, so userspace can create new netdevs to tx/rx
>    with the "data function" and to the network; note that it could be
>    one or multiple
>  * WWAN control, which is typically userspace communicating with the
>    WWAN control function in the WWAN unit, but this can take different
>    forms (as I mentioned earlier, e.g. AT commands, MBIM, QMI)
> 
>>> - The AP communicates with a WWAN function using a WWAN protocol.
> 
> Right, that's just device specific (IPA vs. Intel vs. ...)
> 
>>> - A WWAN physical channel can be *multiplexed*, in which case it
>>>   carries the data for one or more *WWAN logical channels*.
> 
> This ... depends a bit on how you exactly define a physical channel
> here. Is that, to you, the PCIe/USB link? In that case, yes, obviously
> you have only one physical channel for each WWAN unit.

I think that was what I was trying to capture.  There exists
one or more "physical" communication paths between the AP
and WWAN unit/modem.  And while one path *could* carry just
one type of traffic, it could also carry multiple logical
channels of traffic by multiplexing.

> However, I'd probably see this slightly differently, because e.g. the
> Intel modem has multiple DMA engines, and so you actually have multiple
> DMA rings to talk to the WWAN unit, and I'd have called each DMA ring a
> physical channel. And then, you just have a 1:1 from physical to logical
> channel since it doesn't actually carry a multiplexing protocol.

Understood.

. . .

> I only disagree slightly on the control planes (there are multiple, and
> multiple options for the "Control function" one), and on the whole
> notion of physical link/logical link/multiplexing which is device
> specific.
> 
>>> And if I understand it right, the purpose of the generic framework
>>> being discussed is to define a common mechanism for managing (i.e.,
>>> discovering, creating, destroying, querying, configuring, enabling,
>>> disabling, etc.) WWAN units and the functions they implement, along
>>> with the communication and logical channels used to communicate with
>>> them.
> 
> Well, some subset of that matrix, the framework won't actually destroy
> WWAN units I hope ;-)

Hardware self-destruct would be an optional behavior.

> But yes. I'd probably captured it in layers, and say that we have a
> 
> WWAN framework layer
>  - discover, query, configure WWAN units
>  - enable, disable channels to the functions inside the WWAN units
> 
> WWAN device driver
>  - implement (partial) API offered by WWAN framework layer to allow
>    these things
>    (sometimes may not allow creating more control or data channels for
>    example, and fixed function channels are precreated, but then can
>    still discover data about the device and configure the channels
>  - implement the device-specific protocols etc. necessary to achieve
>    this
> 
> Userspace
>  - uses control function channel (e.g. TTY) to talk directly to the WWAN
>    unit's control function
>  - uses WWAN framework APIs to create/configure/... (other) function
>    channels
>    (it may be necessary to create a control channel even, before being
>    able to use it, since different options (AT/MBIM/QMI) may be there
>  - configures netdevs (data function channels) after their creation

I don't think I have any argument with this.  I'm going to try to
put together something that goes beyond what I wrote in this message,
to try to capture what I think we agree on in a sort of loose design
document.

Thanks Johannes.

					-Alex