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@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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. > > > > But I think you're right that it's probably easiest if I start with > > that, see if I can get anything working at all ... > > > > > And for sure you can propose changes in the WWAN framework if you > > > think something is missing to support your specific case. > > > > > > > ... and then we can discuss that further on a RFC PATCH or something > > like that. Does that sound good to you? > > Yes, you can submit the series, no need to be RFC IMHO, this thread is > already your RFC. > I kind of see "RFC" like a "I'm not sure if the approach taken here is really a good idea" and my current patch set currently still fits that criteria. But at the end it's just a strange prefix for the mail subject so it shouldn't matter too much. :) Thanks! Stephan
