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SubjectRe: IIO, dmabuf, io_uring
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


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