On Fri, Aug 13, 2021 at 01:41:26PM +0200, Paul Cercueil wrote: > Hi, > > A few months ago we (ADI) tried to upstream the interface we use with our > high-speed ADCs and DACs. It is a system with custom ioctls on the iio > device node to dequeue and enqueue buffers (allocated with > dma_alloc_coherent), that can then be mmap'd by userspace applications. > Anyway, it was ultimately denied entry [1]; this API was okay in ~2014 when > it was designed but it feels like re-inventing the wheel in 2021. > > Back to the drawing table, and we'd like to design something that we can > actually upstream. This high-speed interface looks awfully similar to > DMABUF, so we may try to implement a DMABUF interface for IIO, unless > someone has a better idea. To me this does sound a lot like a dma buf use case. The interesting question to me is how to signal arrival of new data, or readyness to consume more data. I suspect that people that are actually using dmabuf heavily at the moment (dri/media folks) might be able to chime in a little more on that. > Our first usecase is, we want userspace applications to be able to dequeue > buffers of samples (from ADCs), and/or enqueue buffers of samples (for > DACs), and to be able to manipulate them (mmapped buffers). With a DMABUF > interface, I guess the userspace application would dequeue a dma buffer > from the driver, mmap it, read/write the data, unmap it, then enqueue it to > the IIO driver again so that it can be disposed of. Does that sound sane? > > Our second usecase is - and that's where things get tricky - to be able to > stream the samples to another computer for processing, over Ethernet or > USB. Our typical setup is a high-speed ADC/DAC on a dev board with a FPGA > and a weak soft-core or low-power CPU; processing the data in-situ is not > an option. Copying the data from one buffer to another is not an option > either (way too slow), so we absolutely want zero-copy. > > Usual userspace zero-copy techniques (vmsplice+splice, MSG_ZEROCOPY etc) > don't really work with mmapped kernel buffers allocated for DMA [2] and/or > have a huge overhead, so the way I see it, we would also need DMABUF > support in both the Ethernet stack and USB (functionfs) stack. However, as > far as I understood, DMABUF is mostly a DRM/V4L2 thing, so I am really not > sure we have the right idea here. > > And finally, there is the new kid in town, io_uring. I am not very literate > about the topic, but it does not seem to be able to handle DMA buffers > (yet?). The idea that we could dequeue a buffer of samples from the IIO > device and send it over the network in one single syscall is appealing, > though. Think of io_uring really just as an async syscall layer. It doesn't replace DMA buffers, but can be used as a different and for some workloads more efficient way to dispatch syscalls.
