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

[Daniel Vetter]

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.

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.

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