Stephan Gerhold <stephan@xxxxxxxxxxx> writes: > On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote: >> Stephan Gerhold <stephan@xxxxxxxxxxx> 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@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. >> >> 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! >> > > For reference, we're not really talking about "hardware" TXQs here. > As far as I understand, the RPMSG channels on Qualcomm devices are > mostly just a firmware convention for communicating between different > CPUs/DSPs via shared memory. > > The packets are copied into some kind of shared FIFO/ring buffer > per channel, with varying sizes. On my test device, the firmware > allcates 1024 bytes for the QMI channel and 8192 bytes > for the AT channel. > > I'm not sure how this would cause any kind of overflow/bufferbloat. > The remote side (e.g. modem DSP) is notified separately for every packet > that is sent. If we're really writing more quickly than the remote side > will read, rpmsg_send() will block and therefore the client will > block as well (since tx was disabled before calling rpmsg_send()). Hmm, okay, if this is just control channel traffic and the buffers are no bigger than that maybe this is not such a huge issue. As long as the client (which I guess is whichever application is trying to control the modem?) can block and back off, so it won't just keep queuing up commands faster than the modem can process them... -Toke
