Date:   Tue, 29 Mar 2022 10:33:32 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Corbet <corbet@....net>, linux-iio@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linaro-mm-sig@...ts.linaro.org,
        Alexandru Ardelean <ardeleanalex@...il.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 00/12] iio: buffer-dma: write() and new DMABUF based
 API

On Tue, Feb 15, 2022 at 05:43:35PM +0000, Paul Cercueil wrote:
> Hi Jonathan,
> 
> Le dim., févr. 13 2022 at 18:46:16 +0000, Jonathan Cameron
> <jic23@...nel.org> a écrit :
> > On Mon,  7 Feb 2022 12:59:21 +0000
> > Paul Cercueil <paul@...pouillou.net> wrote:
> > 
> > >  Hi Jonathan,
> > > 
> > >  This is the V2 of my patchset that introduces a new userspace
> > > interface
> > >  based on DMABUF objects to complement the fileio API, and adds
> > > write()
> > >  support to the existing fileio API.
> > 
> > Hi Paul,
> > 
> > It's been a little while. Perhaps you could summarize the various view
> > points around the appropriateness of using DMABUF for this?
> > I appreciate it is a tricky topic to distil into a brief summary but
> > I know I would find it useful even if no one else does!
> 
> So we want to have a high-speed interface where buffers of samples are
> passed around between IIO devices and other devices (e.g. USB or network),
> or made available to userspace without copying the data.
> 
> DMABUF is, at least in theory, exactly what we need. Quoting the
> documentation
> (https://www.kernel.org/doc/html/v5.15/driver-api/dma-buf.html):
> "The dma-buf subsystem provides the framework for sharing buffers for
> hardware (DMA) access across multiple device drivers and subsystems, and for
> synchronizing asynchronous hardware access. This is used, for example, by
> drm “prime” multi-GPU support, but is of course not limited to GPU use
> cases."
> 
> The problem is that right now DMABUF is only really used by DRM, and to
> quote Daniel, "dma-buf looks like something super generic and useful, until
> you realize that there's a metric ton of gpu/accelerator bagage piled in".
> 
> Still, it seems to be the only viable option. We could add a custom
> buffer-passing interface, but that would mean implementing the same
> buffer-passing interface on the network and USB stacks, and before we know
> it we re-invented DMABUFs.

dma-buf also doesn't support sharing with network and usb stacks, so I'm a
bit confused why exactly this is useful?

So yeah unless there's some sharing going on with gpu stuff (for data
processing maybe) I'm not sure this makes a lot of sense really. Or at
least some zero-copy sharing between drivers, but even that would
minimally require a dma-buf import ioctl of some sorts. Which I either
missed or doesn't exist.

If there's none of that then just hand-roll your buffer handling code
(xarray is cheap to use in terms of code for this), you can always add
dma-buf import/export later on when the need arises.

Scrolling through patches you only have dma-buf export, but no importing,
so the use-case that works is with one of the existing subsystems that
supporting dma-buf importing.

I think minimally we need the use-case (in form of code) that needs the
buffer sharing here.
-Daniel

> Cheers,
> -Paul
> 
> 
> > > 
> > >  Changes since v1:
> > > 
> > >  - the patches that were merged in v1 have been (obviously) dropped
> > > from
> > >    this patchset;
> > >  - the patch that was setting the write-combine cache setting has
> > > been
> > >    dropped as well, as it was simply not useful.
> > >  - [01/12]:
> > >      * Only remove the outgoing queue, and keep the incoming queue,
> > > as we
> > >        want the buffer to start streaming data as soon as it is
> > > enabled.
> > >      * Remove IIO_BLOCK_STATE_DEQUEUED, since it is now functionally
> > > the
> > >        same as IIO_BLOCK_STATE_DONE.
> > >  - [02/12]:
> > >      * Fix block->state not being reset in
> > >        iio_dma_buffer_request_update() for output buffers.
> > >      * Only update block->bytes_used once and add a comment about
> > > why we
> > >        update it.
> > >      * Add a comment about why we're setting a different state for
> > > output
> > >        buffers in iio_dma_buffer_request_update()
> > >      * Remove useless cast to bool (!!) in iio_dma_buffer_io()
> > >  - [05/12]:
> > >      Only allow the new IOCTLs on the buffer FD created with
> > >      IIO_BUFFER_GET_FD_IOCTL().
> > >  - [12/12]:
> > >      * Explicitly state that the new interface is optional and is
> > >        not implemented by all drivers.
> > >      * The IOCTLs can now only be called on the buffer FD returned by
> > >        IIO_BUFFER_GET_FD_IOCTL.
> > >      * Move the page up a bit in the index since it is core stuff
> > > and not
> > >        driver-specific.
> > > 
> > >  The patches not listed here have not been modified since v1.
> > > 
> > >  Cheers,
> > >  -Paul
> > > 
> > >  Alexandru Ardelean (1):
> > >    iio: buffer-dma: split iio_dma_buffer_fileio_free() function
> > > 
> > >  Paul Cercueil (11):
> > >    iio: buffer-dma: Get rid of outgoing queue
> > >    iio: buffer-dma: Enable buffer write support
> > >    iio: buffer-dmaengine: Support specifying buffer direction
> > >    iio: buffer-dmaengine: Enable write support
> > >    iio: core: Add new DMABUF interface infrastructure
> > >    iio: buffer-dma: Use DMABUFs instead of custom solution
> > >    iio: buffer-dma: Implement new DMABUF based userspace API
> > >    iio: buffer-dmaengine: Support new DMABUF based userspace API
> > >    iio: core: Add support for cyclic buffers
> > >    iio: buffer-dmaengine: Add support for cyclic buffers
> > >    Documentation: iio: Document high-speed DMABUF based API
> > > 
> > >   Documentation/driver-api/dma-buf.rst          |   2 +
> > >   Documentation/iio/dmabuf_api.rst              |  94 +++
> > >   Documentation/iio/index.rst                   |   2 +
> > >   drivers/iio/adc/adi-axi-adc.c                 |   3 +-
> > >   drivers/iio/buffer/industrialio-buffer-dma.c  | 610
> > > ++++++++++++++----
> > >   .../buffer/industrialio-buffer-dmaengine.c    |  42 +-
> > >   drivers/iio/industrialio-buffer.c             |  60 ++
> > >   include/linux/iio/buffer-dma.h                |  38 +-
> > >   include/linux/iio/buffer-dmaengine.h          |   5 +-
> > >   include/linux/iio/buffer_impl.h               |   8 +
> > >   include/uapi/linux/iio/buffer.h               |  30 +
> > >   11 files changed, 749 insertions(+), 145 deletions(-)
> > >   create mode 100644 Documentation/iio/dmabuf_api.rst
> > > 
> > 
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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