Hi Stephan, On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@xxxxxxxxxxx> wrote: > > Hi Loic, > > 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. > > 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. Regards, Loic
