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.