Hi Christoph,
Le sam., août 14 2021 at 09:30:19 +0200, Christoph Hellwig hch@lst.de a écrit :
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.
Thanks for the feedback.
I haven't looked too much into how dmabuf works; but IIO device nodes right now have a regular stdio interface, so I believe poll() flags can be used to signal arrival of new data.
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.
That was my thought, yes. Thanks.
Cheers, -Paul