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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Stephan Gerhold <stephan@gerhold.net> writes:

> Hi Loic,
>
> On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
>> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.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@gerhold.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(...).
>
> But having some sort of workqueue also sounds like it could work quite
> well, thanks for the suggestion! Will think about it some more, or
> I might post what I have right now so you can take a look.

How big are those hardware TXQs? Just pushing packets to the hardware
until it overflows sounds like a recipe for absolutely terrible
bufferbloat... That would be bad!

-Toke