Re: [PATCH v2 00/12] iio: buffer-dma: write() and new DMABUF based API

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Hi Jonathan,

Le dim., févr. 13 2022 at 18:46:16 +0000, Jonathan Cameron <jic23@xxxxxxxxxx> a écrit :
On Mon,  7 Feb 2022 12:59:21 +0000
Paul Cercueil <paul@xxxxxxxxxxxxxxx> 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.

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








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