Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Loic Poulain <loic.poulain@linaro.org>]

Hi Stephan,

On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> Hi Loic, Hi Bjorn,
>
> I've been thinking about creating some sort of "RPMSG" driver for the
> new WWAN subsystem; this would be used as a QMI/AT channel to the
> integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
>
> It's easy to confuse all the different approaches that Qualcomm has to
> talk to their modems, so I will first try to briefly give an overview
> about those that I'm familiar with:
>
> ---
> There is USB and MHI that are mainly used to talk to "external" modems.
>
> For the integrated modems in many Qualcomm SoCs there is typically
> a separate control and data path. They are not really related to each
> other (e.g. currently no common parent device in sysfs).
>
> For the data path (network interface) there is "IPA" (drivers/net/ipa)
> on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> I have a driver for BAM-DMUX that I hope to finish up and submit soon.
>
> The connection is set up via QMI. The messages are either sent via
> a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
>
> This gives a lot of possible combinations like BAM-DMUX+RPMSG
> (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
>
> Simply put, supporting all these in userspace like ModemManager
> is a mess (Aleksander can probably confirm).
> It would be nice if this could be simplified through the WWAN subsystem.
>
> It's not clear to me if or how well QRTR sockets can be mapped to a char
> device for the WWAN subsystem, so for now I'm trying to focus on the
> standalone RPMSG approach (for MSM8916, MSM8974, ...).
> ---
>
> Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> I just took that over from someone else. Realistically speaking, the
> current approach isn't too different from the UCI "backdoor interface"
> approach that was rejected for MHI...
>
> I kind of expected that I can just trivially copy some code from
> rpmsg_char.c into a WWAN driver since they both end up as a simple char
> device. But it looks like the abstractions in wwan_core are kind of
> getting in the way here... As far as I can tell, they don't really fit
> together with the RPMSG interface.
>
> For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> get notified when the TX queue is full or no longer full so I can call
> wwan_port_txon/off().
>
> Any suggestions or other thoughts?

It would be indeed nice to get this in the WWAN framework.
I don't know much about rpmsg but I think it is straightforward for
the RX path, the ept_cb can simply forward the buffers to
wwan_port_rx. For tx, simply call rpmsg_trysend() in the wwan tx
callback and don't use the txon/off helpers. In short, keep it simple
and check if you observe any issues.

And for sure you can propose changes in the WWAN framework if you
think something is missing to support your specific case.

Regards,
Loic