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

[Alex Elder <elder@linaro.org>]

On 6/25/19 9:14 AM, Johannes Berg wrote:
> Hi Alex,
> 
> I'll just pick a few or your messages and reply there - some other
> subthreads seem to have pretty much completed.
> 

. . .

>>> Linux usually tries to keep drivers generic and focused; each driver is
>>> written for a specific function. For example, a USB device usually
>>> provides multiple USB interfaces which will be bound to different Linux
>>> drivers like a TTY, cdc-ether, QMI (via qmi_wwan), cdc-acm, etc.
>>
>> So USB has some attributes similar to what we're talking about
>> here.  But if I'm not mistaken we want some sort of an overall
>> management scheme as well.
> 
> Yes. For the record, I think the part about "keep drivers generic and
> focused" really only works for USB devices that expose different pieces
> that look like any other network device or a TTY device on the USB
> level, just the combination of these things (and knowing about that)
> really makes them a modem.
> 
> For things like IPA or the (hypothetical) Intel driver we're talking
> about, it's still all managed by a single (PCIe) driver. For the Intel
> device in particular, even all the control channels are over exactly the
> same transport mechanism as the data channels.

Actually the setup for IPA requires certain things to be done via
QMI by something outside the IPA driver, and it uses a separate
communication path.  But I understand what you're saying.

. . .

>> I don't like the "maybe" API unless there's no other way to do it.
>>
>> Instead I think it would be better for the probing driver to register
>> with a whatever the WWAN core is, and then have the WWAN core be
>> responsible for pulling things all together when it receives a
>> request to do so.  I.e., something in user space should request
>> that a registered data interface be brought up, and at that
>> time everything "knows" it's implemented as part of a WWAN
>> device.
> 
> Right, but then we just punt to userspace. Mostly we *do* (eventually!)
> know that it's a WWAN device, just not every component can detect it.
> Some components typically can.

We need to identify the existence of a WWAN device (which is I
guess--typically? always?--a modem).  Perhaps that can be
discovered in some cases but I think it means a node described
by Device Tree.

> So for example, you might have a USB multi-function device with a
> network function (looks just like ethernet pretty much) but another TTY
> control channel that actually has some specific WWAN IDs, so that part
> can know it's a WWAN.
> 
> Here, the ethernet function would need "maybe" attach, and the control
> channel would "definitively" attach, pulling it together as a WWAN
> device without requiring any more action or information.

So you're saying you have a single Ethernet driver, and it can
drive an Ethernet device connected to a WWAN, or not connected
to a WWAN, without any changes.  The only distinction is that
if the device is part of a WWAN it needs to register with the
WWAN framework.  Is that right?

>> So maybe:
>> - Hardware probe detects a WWAN device
>> - The drivers that detect the WWAN device register it with the
>>   WWAN core code.
>> - A control channel is instantiated at/before the time the WWAN
>>   device is registered
>> - Something in user space should manage the bring-up of any
>>   other things on the WWAN device thereafter
> 
> But those things need to actually get connected first :-)

What I meant is that the registering with the "WWAN core code"
is what does that "connecting."  The WWAN code has the information
about what got registered.  But as I said above, this WWAN device
needs to be identified, and I think (at least for IPA) that will
require something in Device Tree.  That will "connect" them.

Or I might be misunderstanding your point.

> In IPA/Intel case this is easy since it's a single driver. But if
> there's multi-function device with ethernet being a completely separate
> driver, the control channel cannot even reach that to tell it to create
> a new data channel.

There's a lot more to talk about with control.  I think
you discuss this in another message, and I'll get to that
shortly.  But I think understand your point, and again
I think it comes back to having an abstraction that
represents the modem, distinct from (but "connected" to)
the functions it implements/includes.

>>> userspace should probably always create the netdevices (since they are
>>> always useless until userspace coordinates with the firmware about
>>> them) but that's not how things are yet.
>>
>> That's too bad.  How hard would that be to change?
> 
> Depends, but as I said above it's probably orthogonal to the question.
> The data channel driver would still need to attach to the WWAN device
> somehow so it becomes reachable by the control plane (note this isn't
> the same as "control channel" since the latter talks to the modem, the
> control plane talks to the kernel drivers).
> 
>>>> - What causes a created channel to be removed?
>>>
>>> Driver removal, userspace WWAN daemon terminating the packet data
>>> connection which the channel represents, the modem terminating the
>>> packet data connection (eg network initiated disconnect), etc.
>>
>> OK this is as I expected.  Driver (or device) removal is somewhat
>> obvious, but you're confirming user space might request it as well.
> 
> If userspace actually had the ability to create (data) channels, then it
> would have the ability to also remove them. Right now, this may or may
> not be supported by the drivers that act together to form the interfaces
> to a WWAN device.

I think this (user space control) needs to be an option, but
it doesn't have to be the only way.

. . .

You made some other good clarifications in this message but I'm
going to try to capture them elsewhere rather than respond here.

					-Alex