Date:   Wed, 30 Mar 2022 11:19:23 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Jonathan Cameron <jic23@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        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, Mar 29, 2022 at 06:16:56PM +0100, Paul Cercueil wrote:
> Hi Daniel,
> 
> Le mar., mars 29 2022 at 16:10:44 +0200, Daniel Vetter <daniel@...ll.ch> a
> écrit :
> > On Tue, Mar 29, 2022 at 10:11:14AM +0100, Paul Cercueil wrote:
> > >  Hi Daniel,
> > > 
> > >  Le mar., mars 29 2022 at 10:33:32 +0200, Daniel Vetter
> > > <daniel@...ll.ch> a
> > >  écrit :
> > >  > 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?
> > > 
> > >  There is an attempt to get dma-buf support in the network stack,
> > > called
> > >  "zctap". Last patchset was sent last november. USB stack does not
> > > support
> > >  dma-buf, but we can add it later I guess.
> > > 
> > >  > 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.
> > > 
> > >  We do want zero-copy between drivers, the network stack, and the
> > > USB stack.
> > >  It's not just about having a userspace interface.
> > 
> > I think in that case we need these other pieces too. And we need acks
> > from
> > relevant subsystems that these other pieces are a) ready for upstream
> > merging and also that the dma-buf side of things actually makes sense.
> 
> Ok...
> 
> > >  > 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.
> > > 
> > >  I'll try with zctap and report back.
> > 
> > Do you have a link for this? I just checked dri-devel on lore, and it's
> > not there. Nor anywhere else.
> 
> The code is here: https://github.com/jlemon/zctap_kernel
> 
> I know Jonathan Lemon (Cc'd) was working on upstreaming it, I saw a few
> patchsets.

Yeah if the goal here is to zero-copy from iio to network sockets, then I
think we really need the full picture first, at least as a prototype.

And also a rough consensus among all involved subsystems that this is the
right approach and that there's no fundamental issues. I really have no
clue about network to make a call there.

I'm bringing this up because a few folks wanted to look into zero-copy
between gpu and nvme, using dma-buf. And after lots of
head-banging-against-solid-concrete-walls, at least my conclusion is that
due to locking issues it's really not possible without huge changes to the
block i/o. And those are not on the table.
-Daniel

> 
> Cheers,
> -Paul
> 
> > We really need all the pieces, and if block layer reaction is anything
> > to
> > judge by, dma-buf wont happen for networking either. There's some really
> > nasty and fairly fundamental issues with locking and memory reclaim that
> > make this utter pain or outright impossible.
> > -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
> > > 
> > > 
> > 
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
> 
> 

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

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