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SubjectRe: [PATCH 00/15] iio: buffer-dma: write() and new DMABUF based API
Hi Daniel,

Le lun., nov. 15 2021 at 15:37:16 +0100, Daniel Vetter
<daniel@ffwll.ch> 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@analog.com/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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