Date:   Tue, 15 Jun 2021 11:03:20 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Aleksander Morgado <aleksander@...ksander.es>,
        Network Development <netdev@...r.kernel.org>,
        linux-remoteproc@...r.kernel.org,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

HI Stephan,

On Mon, 7 Jun 2021 at 14:49, Stephan Gerhold <stephan@...hold.net> wrote:
>
> Hi,
>
> On Mon, Jun 07, 2021 at 02:16:10PM +0200, Loic Poulain wrote:
> > On Mon, 7 Jun 2021 at 13:44, Stephan Gerhold <stephan@...hold.net> wrote:
> > >
> > > On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
> > > > Stephan Gerhold <stephan@...hold.net> writes:
> > > > > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> > > > >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@...hold.net> wrote:
> > > > >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > > > >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@...hold.net> wrote:
> > > > >> > > > 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.
> > > > >> >
> > > > >> > Right, that part should be straightforward.
> > > > >> >
> > > > >> > > 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.
> > > > >> > >
> > > > >> >
> > > > >> > I'm not sure that's a good idea. This sounds like exactly the kind of
> > > > >> > thing that might explode later just because I don't manage to get the
> > > > >> > TX queue full in my tests. In that case, writing to the WWAN char dev
> > > > >> > would not block, even if O_NONBLOCK is not set.
> > > > >>
> > > > >> Right, if you think it could be a problem, you can always implement a
> > > > >> more complex solution like calling rpmsg_send from a
> > > > >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> > > > >>
> > > > >
> > > > > I did run into trouble when I tried to stream lots of data into the WWAN
> > > > > char device (e.g. using dd). However, in practice (with ModemManager)
> > > > > I did not manage to cause such issues yet. Personally, I think it's
> > > > > something we should get right, just to avoid trouble later
> > > > > (like "modem suddenly stops working").
> > > > >
> > > > > Right now I extended the WWAN port ops a bit so I tells me if the write
> > > > > should be non-blocking or blocking and so I can call rpmsg_poll(...).
> >
> > OK, but note that poll seems to be an optional rpmsg ops, rpmsg_poll
> > can be a no-op and so would not guarantee that EPOLL_OUT will ever be
> > set.
> >
>
> Wouldn't that be more a limitation of the driver that provides the rpmsg
> ops then? If rpmsg_send(...) may block, it should also be possible to
> implement rpmsg_poll(...). Right now my implementation basically behaves
> mostly like the existing rpmsg-char. That's what has been used on
> several devices for more than a year and it works quite well.
>
> Granted, so far testing was mostly done with the "qcom_smd" RPMSG driver
> (which is likely going to be the primary use case for my driver for now).
> This one also happens to be the only one that implements rpmsg_poll()
> at the moment...
>
> I asked some other people to test it for AT messages with "glink" on
> newer Qualcomm SoCs and that doesn't work that well yet. However, as far
> as I can tell most of the trouble seems to be caused by various strange
> bugs in the "glink" driver.
> (See drivers/rpmsg for the drivers I'm referring to...)
>
> ---
>
> One of the main potential problems I see with some kind of workqueue
> approach is that all writes would seemingly succeed for the client.
> When the extra thread is done with rpmsg_send(...) we will have already
> returned from the char dev write(...) syscall, without a way to report
> the error. Perhaps that's unlikely though and we don't need to worry
> about that too much?
>
> I'm not really sure which one of the solution is best at the end.
> I think it doesn't make too much of a difference at the end, both can
> work well and both have some advantages/disadvantages in certain fairly
> unlikely situations.

Yes, feel free to submit the series for review.

Regards,
Loic

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