Re: [PATCH 00/15] iio: buffer-dma: write() and new DMABUF based API

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Hi Daniel,

Le lun., nov. 15 2021 at 15:37:16 +0100, Daniel Vetter <daniel@xxxxxxxx> a écrit :
On Mon, Nov 15, 2021 at 02:19:10PM +0000, Paul Cercueil wrote:
 Hi Jonathan,

 This patchset introduces a new userspace interface based on DMABUF
 objects, to complement the existing fileio based API.

 The advantage of this DMABUF based interface vs. the fileio
 interface, is that it avoids an extra copy of the data between the
 kernel and userspace. This is particularly userful for high-speed
devices which produce several megabytes or even gigabytes of data per
 second.

 The first few patches [01/15] to [03/15] are not really related, but
 allow to reduce the size of the patches that introduce the new API.

 Patch [04/15] to [06/15] enables write() support to the buffer-dma
 implementation of the buffer API, to continue the work done by
 Mihail Chindris.

 Patches [07/15] to [12/15] introduce the new DMABUF based API.

Patches [13/15] and [14/15] add support for cyclic buffers, only through the new API. A cyclic buffer will be repeated on the output until the
 buffer is disabled.

 Patch [15/15] adds documentation about the new API.

For now, the API allows you to alloc DMABUF objects and mmap() them to
 read or write the samples. It does not yet allow to import DMABUFs
 parented to other subsystems, but that should eventually be possible
 once it's wired.

This patchset is inspired by the "mmap interface" that was previously submitted by Alexandru Ardelean and Lars-Peter Clausen, so it would be
 great if I could get a review from you guys. Alexandru's commit was
signed with his @analog.com address but he doesn't work at ADI anymore,
 so I believe I'll need him to sign with a new email.

Why dma-buf? dma-buf looks like something super generic and useful, until you realize that there's a metric ton of gpu/accelerator bagage piled in.
So unless buffer sharing with a gpu/video/accel/whatever device is the
goal here, and it's just for a convenient way to get at buffer handles,
this doesn't sound like a good idea.

Good question. The first reason is that a somewhat similar API was intented before[1], but refused upstream as it was kind of re-inventing the wheel.

The second reason, is that we want to be able to share buffers too, not with gpu/video but with the network (zctap) and in the future with USB (functionFS) too.

[1]: https://lore.kernel.org/linux-iio/20210217073638.21681-1-alexandru.ardelean@xxxxxxxxxx/T/

Also if the idea is to this with gpus/accelerators then I'd really like to
see the full thing, since most likely at that point you also want
dma_fence. And once we talk dma_fence things get truly horrible from a
locking pov :-( Or well, just highly constrained and I get to review what
iio is doing with these buffers to make sure it all fits.

There is some dma_fence action in patch #10, which is enough for the userspace apps to use the API.

What "horribleness" are we talking about here? It doesn't look that scary to me, but I certainly don't have the complete picture.

Cheers,
-Paul

Cheers, Daniel


 Cheers,
 -Paul

 Alexandru Ardelean (1):
   iio: buffer-dma: split iio_dma_buffer_fileio_free() function

 Paul Cercueil (14):
   iio: buffer-dma: Get rid of incoming/outgoing queues
   iio: buffer-dma: Remove unused iio_buffer_block struct
   iio: buffer-dma: Use round_down() instead of rounddown()
   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-dma: Boost performance using write-combine cache setting
   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 | 670 ++++++++++++++----
  .../buffer/industrialio-buffer-dmaengine.c    |  42 +-
  drivers/iio/industrialio-buffer.c             |  49 ++
  include/linux/iio/buffer-dma.h                |  43 +-
  include/linux/iio/buffer-dmaengine.h          |   5 +-
  include/linux/iio/buffer_impl.h               |   8 +
  include/uapi/linux/iio/buffer.h               |  30 +
  11 files changed, 783 insertions(+), 165 deletions(-)
  create mode 100644 Documentation/iio/dmabuf_api.rst

 --
 2.33.0


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






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