Hi Christoph,
Le sam., août 14 2021 at 09:30:19 +0200, Christoph Hellwig
<hch@xxxxxx> 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
