Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH 0/9] XSHM: Shared Memory Driver for ST-E Thor M7400 LTE modem | Date | Thu, 22 Mar 2012 16:57:09 +0000 |
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On Thursday 22 March 2012, Sjur BRENDELAND wrote: > Hi Arnd, > > I've got some updates since my last reply from December... > > [Arnd] > >>> Also, to what degree is the protocol design flexible? Is the modem > >>> side fixed, so you have to live with any shortcomings of the interface, > >>> or are you at this point able to improve both sides when something > >>> is found to be lacking? > > I have started working on the next generation shared memory interface for > ST-Ericsson modems. So the interface design is now flexible! This means that > I can be open to input and new ideas, at least for the next month or two. > I'm hoping (if time allows) to prototype and post some patches while working > on the specification.
Ok, very good.
> [Sjur] > >> However for the long term perspective: we expect this interface to evolve > >> for future products, so suggestions and input for improvements is welcome. > >> rpmsg or at least the use of virtio-ring combined with a true end-to-end > >> zero copy is something we definitely are interested to look into for the > >> future. > > My current idea for the new interface design is to use Virtio channels for > transporting CAIF frames. The CAIF interface could be implemented as a > virtio-driver using separate RX and TX rings. > > For uplink traffic (TX) we could (initially) copy SKB into a buffer located in > the shared-memory area, and add the buffer to the Virtio-ring. We will need > to manage a fixed size buffer pool of uplink data buffers. (I don't think we > will be able to access the kernel memory from the modem, so I cannot put SKB > content directly on the virtio-ring as virtio-net does) > > For Downlink payload (RX) I'm planning on using a reversed virtio-ring. > The modem has its own sophisticated memory allocator for payload and > implements mechanisms for efficient buffer handling inside the modem. > The modem could add the payload buffer on the virtual-ring without > copying and kick the host.
Remoteproc and rpmsg are now in the arm-soc tree and will be merged upstream for v3.4, I suggest you discuss with Ohad how to best hook in there.
What is the limitation for the addressing here, i.e. why can't the modem access all of the host memory?
> [Sjur:] > >>>> The driver for the stream channel is implemented as a character device > ... > [Arnd:] > > My feeling is that a character device is not the ideal implementation here, > > but I'm not sure what is. One option I can see is to declare the stream > > interface part of the configuration logic and do everything through netlink, > > packetizing the stream data in netlink frames. Alternatively, a tty port > > device might be more appropriate than a plain chardev. Both of these > > are likely a bit slower than what you have today, but my impression is that > > performance is not the main design goal for the stream interface. If it is, > > the best option would probably be to allow user space to mmap the buffer and > > do the implementation in an application outside of the kernel. > > For the stream interface it's tempting to reuse the ring buffer interface > to the modem from last time. But perhaps the Virtio-console could be as the > user-space interface, with a slim virtual device underneath feeding data > from the ring-buffer into a virtual-ring...?
sounds doable, but again Ohad may have better suggestions as well.
Arnd
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