[PATCH v6 0/6] iio: new DMABUF based API, v6

[Paul Cercueil]

Hi Jonathan,

This is the v6 of my patchset that introduces a new interface based on
DMABUF objects.

The code was updated quite a bit, using the feedback on the list for
this patchset but also the feedback I received on the FunctionFS
patchset that I'm working on upstreaming in parallel [1] where the
DMABUF handling code is very similar.

See below for the full changelog.

I decided to drop the scope-based memory management for dma_buf and
I hope you are OK with that. Christian wants the patch(es) to support
scope-based memory management in dma-buf as a separate patchset; once
it's in, I will gladly send a follow-up patch to use __free() where it
makes sense.

For performance numbers, I'll point you to the cover letter for my v5
patchset [2].

This patchset was based on next-20240129.

Cheers,
-Paul

[1] https://lore.kernel.org/all/20230322092118.9213-1-paul@xxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linux-iio/219abc43b4fdd4a13b307ed2efaa0e6869e68e3f.camel@xxxxxxxxx/T/

---

Changelog:
* [2/6]:
    - Use new prototype for axi_dmac_alloc_desc() as it changed upstream
* [3/6]:
    - Remove dead code in iio_dma_resv_lock()
    - Fix non-block actually blocking
    - Cache dma_buf_attachment instead of mapping/unmapping it for every
      transfer
    - Return -EINVAL instead of IIO_IOCTL_UNHANDLED for unknown ioctl
    - Make .block_enqueue() callback take a dma_fence pointer, which
      will be passed to iio_buffer_signal_dmabuf_done() instead of the
      dma_buf_attachment; and remove the backpointer from the priv
      structure to the dma_fence.
    - Use dma_fence_begin/end_signalling in the dma_fence critical
      sections
    - Unref dma_fence and dma_buf_attachment in worker, because they
      might try to lock the dma_resv, which would deadlock.
    - Add buffer ops to lock/unlock the queue. This is motivated by the
      fact that once the dma_fence has been installed, we cannot lock
      anything anymore - so the queue must be locked before the
      dma_fence is installed.
    - Use 'long retl' variable to handle the return value of
      dma_resv_wait_timeout()
    - Protect dmabufs list access with a mutex
    - Rework iio_buffer_find_attachment() to use the internal dmabufs
      list, instead of messing with dmabufs private data.
    - Add an atomically-increasing sequence number for fences
* [4/6]:
    - Update iio_dma_buffer_enqueue_dmabuf() to take a dma_fence pointer
    - Pass that dma_fence pointer along to
      iio_buffer_signal_dmabuf_done()
    - Add iio_dma_buffer_lock_queue() / iio_dma_buffer_unlock_queue()
    - Do not lock the queue in iio_dma_buffer_enqueue_dmabuf().
      The caller will ensure that it has been locked already.
    - Replace "int += bool;" by "if (bool) int++;"
    - Use dma_fence_begin/end_signalling in the dma_fence critical
      sections
    - Use one "num_dmabufs" fields instead of one "num_blocks" and one
      "num_fileio_blocks". Make it an atomic_t, which makes it possible
      to decrement it atomically in iio_buffer_block_release() without
      having to lock the queue mutex; and in turn, it means that we
      don't need to use iio_buffer_block_put_atomic() everywhere to
      avoid locking the queue mutex twice.
    - Use cleanup.h guard(mutex) when possible
    - Explicitely list all states in the switch in
      iio_dma_can_enqueue_block()
    - Rename iio_dma_buffer_fileio_mode() to
      iio_dma_buffer_can_use_fileio(), and add a comment explaining why
      it cannot race vs. DMABUF.
* [5/6]:
    - Populate .lock_queue / .unlock_queue callbacks
    - Switch to atomic memory allocations in .submit_queue, because of
      the dma_fence critical section
    - Make sure that the size of the scatterlist is enough

---
Paul Cercueil (6):
  dmaengine: Add API function dmaengine_prep_slave_dma_vec()
  dmaengine: dma-axi-dmac: Implement device_prep_slave_dma_vec
  iio: core: Add new DMABUF interface infrastructure
  iio: buffer-dma: Enable support for DMABUFs
  iio: buffer-dmaengine: Support new DMABUF based userspace API
  Documentation: iio: Document high-speed DMABUF based API

 Documentation/iio/dmabuf_api.rst              |  54 ++
 Documentation/iio/index.rst                   |   2 +
 drivers/dma/dma-axi-dmac.c                    |  40 ++
 drivers/iio/buffer/industrialio-buffer-dma.c  | 181 ++++++-
 .../buffer/industrialio-buffer-dmaengine.c    |  58 ++-
 drivers/iio/industrialio-buffer.c             | 462 ++++++++++++++++++
 include/linux/dmaengine.h                     |  25 +
 include/linux/iio/buffer-dma.h                |  31 ++
 include/linux/iio/buffer_impl.h               |  33 ++
 include/uapi/linux/iio/buffer.h               |  22 +
 10 files changed, 891 insertions(+), 17 deletions(-)
 create mode 100644 Documentation/iio/dmabuf_api.rst

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2.43.0