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From: "Christian König" <>
To: Paul Cercueil <>,
	Jonathan Cameron <>,
	Sumit Semwal <>,
	Christoph Hellwig <>
	Michael Hennerich <>,
	Alexandru Ardelean <>
Subject: Re: IIO, dmabuf, io_uring
Date: Sun, 15 Aug 2021 20:02:02 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

Hi Paul,

Am 13.08.21 um 13:41 schrieb Paul Cercueil:
> 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.

Yeah, that sounds a lot like a DMABUF use case.

> 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?

Well it's pretty close. Doing the map/unmap dance all the time is 
usually a bad idea since flushing the CPU TLB all the time totally kills 
your performance.

What you do instead is to implement the CPU synchronize callbacks in 
your DMA-BUF implementation and flush caches as necessary.

> 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.

Well two possibilities here: Either implement DMA-BUF support in the 
Ethernet/USB subsystem or get splice working efficiently with DMA-BUF 

The first one is certainly a lot of work and no idea if the second is 
even doable and if yes also in a non-hacky way which you can get upstream.

> 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.

As far as I know this is orthogonal to DMA-BUF. Christoph's answer 
sounds like my understand is correct, but there are certainly people 
which know that better than I do.


> Any thoughts? Feedback would be greatly appreciated.
> Cheers,
> -Paul
> [1]: 
> [2]: 

      parent reply	other threads:[~2021-08-15 18:02 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-13 11:41 IIO, dmabuf, io_uring Paul Cercueil
2021-08-13 17:20 ` Pavel Begunkov
2021-08-16  9:20   ` Paul Cercueil
2021-08-14  7:30 ` Christoph Hellwig
2021-08-16  9:27   ` Paul Cercueil
2021-08-16 15:01   ` [Linaro-mm-sig] " Daniel Vetter
2021-08-15 18:02 ` Christian König [this message]

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