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

Le mar., nov. 16 2021 at 17:02:25 +0100, Daniel Vetter
<daniel@ffwll.ch> a écrit :
> On Mon, Nov 15, 2021 at 02:57:37PM +0000, Paul Cercueil wrote:
>> 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/
>
> Hm is that code merged already in upstream already?

No, it was never merged.

> I know that dma-buf looks really generic, but like I said if there's
> no
> need ever to interface with any of the gpu buffer sharing it might be
> better to use something else (like get_user_pages maybe, would that
> work?).

If it was such a bad idea, why didn't you say it in the first place
when you replied to my request for feedback? [1]

I don't think we have any other solution. We can design a custom API to
pass buffers between IIO and user space, but that won't allow us to
share these buffers with other subsystems. If dma-buf is not a generic
solution, then we need a generic solution.

[1]:
https://x-lore.kernel.org/io-uring/b0a336c0-ae2f-e77f-3c5f-51fdb3fc51fe@amd.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.
>
> You need to annotate all the code involved in signalling that
> dma_fence
> using dma_fence_begin/end_signalling, and then enable full lockdep and
> everything.

Doesn't dma_fence_signal() do it for me? Looking at the code, it does
call dma_fence_begin/end_signalling.

Cheers,
-Paul

> You can safely assume you'll find bugs, because we even have bugs
> about
> this in gpu drivers (where that annotation isn't fully rolled out
> yet).
>
> The tldr is that you can allocate memory in there. And a pile of other
> restrictions, but not being able to allocate memory (well GFP_ATOMIC
> is
> ok, but that can fail) is a very serious restriction.
> -Daniel
>
>
>>
>> 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
>>
>>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch


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