Message-ID: <CAMZdPi-91y+t1bHb+6NY5Dh-xV_yvJTzG65efaSKzyTNsEGBvA@mail.gmail.com>
Date:   Mon, 7 Jun 2021 14:16:10 +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 Setphan,

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.


Loic

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