Re: [PATCH v2 12/12] Documentation: iio: Document high-speed DMABUF based API

Paul Cercueil
2022-03-29T17:35:16Z

Le mar., mars 29 2022 at 16:07:21 +0200, Daniel Vetter
<daniel@ffwll.ch> a écrit :
> On Tue, Mar 29, 2022 at 10:47:23AM +0100, Paul Cercueil wrote:
>> Hi Daniel,
>>
>> Le mar., mars 29 2022 at 10:54:43 +0200, Daniel Vetter
>> <daniel@ffwll.ch> a
>> écrit :
>> > On Mon, Feb 07, 2022 at 01:01:40PM +0000, Paul Cercueil wrote:
>> > > Document the new DMABUF based API.
>> > >
>> > > v2: - Explicitly state that the new interface is optional and
>> is
>> > > not implemented by all drivers.
>> > > - The IOCTLs can now only be called on the buffer FD
>> returned by
>> > > IIO_BUFFER_GET_FD_IOCTL.
>> > > - Move the page up a bit in the index since it is core
>> stuff
>> > > and not
>> > > driver-specific.
>> > >
>> > > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>> > > ---
>> > > Documentation/driver-api/dma-buf.rst | 2 +
>> > > Documentation/iio/dmabuf_api.rst | 94
>> > > ++++++++++++++++++++++++++++
>> > > Documentation/iio/index.rst | 2 +
>> > > 3 files changed, 98 insertions(+)
>> > > create mode 100644 Documentation/iio/dmabuf_api.rst
>> > >
>> > > diff --git a/Documentation/driver-api/dma-buf.rst
>> > > b/Documentation/driver-api/dma-buf.rst
>> > > index 2cd7db82d9fe..d3c9b58d2706 100644
>> > > --- a/Documentation/driver-api/dma-buf.rst
>> > > +++ b/Documentation/driver-api/dma-buf.rst
>> > > @@ -1,3 +1,5 @@
>> > > +.. _dma-buf:
>> > > +
>> > > Buffer Sharing and Synchronization
>> > > ==================================
>> > >
>> > > diff --git a/Documentation/iio/dmabuf_api.rst
>> > > b/Documentation/iio/dmabuf_api.rst
>> > > new file mode 100644
>> > > index 000000000000..43bb2c1b9fdc
>> > > --- /dev/null
>> > > +++ b/Documentation/iio/dmabuf_api.rst
>> > > @@ -0,0 +1,94 @@
>> > > +===================================
>> > > +High-speed DMABUF interface for IIO
>> > > +===================================
>> > > +
>> > > +1. Overview
>> > > +===========
>> > > +
>> > > +The Industrial I/O subsystem supports access to buffers
>> through a
>> > > file-based
>> > > +interface, with read() and write() access calls through the
>> IIO
>> > > device's dev
>> > > +node.
>> > > +
>> > > +It additionally supports a DMABUF based interface, where the
>> > > userspace
>> > > +application can allocate and append DMABUF objects to the
>> buffer's
>> > > queue.
>> > > +This interface is however optional and is not available in all
>> > > drivers.
>> > > +
>> > > +The advantage of this DMABUF based interface vs. the read()
>> > > +interface, is that it avoids an extra copy of the data
>> between the
>> > > +kernel and userspace. This is particularly useful for
>> high-speed
>> > > +devices which produce several megabytes or even gigabytes of
>> data
>> > > per
>> > > +second.
>> > > +
>> > > +The data in this DMABUF interface is managed at the
>> granularity of
>> > > +DMABUF objects. Reducing the granularity from byte level to
>> block
>> > > level
>> > > +is done to reduce the userspace-kernelspace synchronization
>> > > overhead
>> > > +since performing syscalls for each byte at a few Mbps is just
>> not
>> > > +feasible.
>> > > +
>> > > +This of course leads to a slightly increased latency. For this
>> > > reason an
>> > > +application can choose the size of the DMABUFs as well as how
>> many
>> > > it
>> > > +allocates. E.g. two DMABUFs would be a traditional double
>> buffering
>> > > +scheme. But using a higher number might be necessary to avoid
>> > > +underflow/overflow situations in the presence of scheduling
>> > > latencies.
>> >
>> > So this reads a lot like reinventing io-uring with pre-registered
>> > O_DIRECT
>> > memory ranges. Except it's using dma-buf and hand-rolling a lot of
>> > pieces
>> > instead of io-uring and O_DIRECT.
>>
>> I don't see how io_uring would help us. It's an async I/O
>> framework, does it
>> allow us to access a kernel buffer without copying the data? Does
>> it allow
>> us to zero-copy the data to a network interface?
>
> With networking, do you mean rdma, or some other kind of networking?
> Anything else than rdma doesn't support dma-buf, and I don't think it
> will
> likely ever do so. Similar it's really tricky to glue dma-buf support
> into
> the block layer.

By networking I mean standard sockets. If I'm not mistaken, Jonathan
Lemon's work on zctap was to add dma-buf import/export support to
standard sockets.

> Wrt io_uring, yes it's async, but that's not the point. The point is
> that
> with io_uring you pre-register ranges for reads and writes to target,
> which in combination with O_DIRECT, makes it effectively (and
> efficient!)
> zero-copy. Plus it has full integration with both networking and
> normal
> file io, which dma-buf just doesn't have.
>
> Like you _cannot_ do zero copy from a dma-buf into a normal file. You
> absolutely can do the same with io_uring.

I believe io_uring does zero-copy the same way as splice(), by
duplicating/moving pages? Because that wouldn't work with DMA coherent
memory, which is contiguous and not backed by pages.

>> > At least if the entire justification for dma-buf support is
>> zero-copy
>> > support between the driver and userspace it's _really_ not the
>> right
>> > tool
>> > for the job. dma-buf is for zero-copy between devices, with cpu
>> access
>> > from userpace (or kernel fwiw) being very much the exception (and
>> often
>> > flat-out not supported at all).
>>
>> We want both. Using dma-bufs for the driver/userspace interface is a
>> convenience as we then have a unique API instead of two distinct
>> ones.
>>
>> Why should CPU access from userspace be the exception? It works
>> fine for IIO
>> dma-bufs. You keep warning about this being a terrible design, but
>> I simply
>> don't see it.
>
> It depends really on what you're trying to do, and there's extremely
> high
> chances it will simply not work.

Well it does work though. The userspace interface is stupidly simple
here - one dma-buf, backed by DMA coherent memory, is enqueued for
processing by the DMA. The userspace calling the "sync" ioctl on the
dma-buf will block until the transfer is complete, and then userspace
can access it again.

> Unless you want to do zero copy with a gpu, or something which is in
> that
> ecosystem of accelerators and devices, then dma-buf is probably not
> what
> you're looking for.
> -Daniel

I want to do zero-copy between a IIO device and the network/USB, and
right now there is absolutely nothing in place that allows me to do
that. So I have to get creative.

Cheers,
-Paul

>>
>> > > +
>> > > +2. User API
>> > > +===========
>> > > +
>> > > +``IIO_BUFFER_DMABUF_ALLOC_IOCTL(struct iio_dmabuf_alloc_req
>> *)``
>> > >
>> +----------------------------------------------------------------
>> > > +
>> > > +Each call will allocate a new DMABUF object. The return value
>> (if
>> > > not
>> > > +a negative errno value as error) will be the file descriptor
>> of
>> > > the new
>> > > +DMABUF.
>> > > +
>> > > +``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *)``
>> > > +--------------------------------------------------------
>> > > +
>> > > +Place the DMABUF object into the queue pending for hardware
>> > > process.
>> > > +
>> > > +These two IOCTLs have to be performed on the IIO buffer's file
>> > > +descriptor, obtained using the `IIO_BUFFER_GET_FD_IOCTL`
>> ioctl.
>> > > +
>> > > +3. Usage
>> > > +========
>> > > +
>> > > +To access the data stored in a block by userspace the block
>> must be
>> > > +mapped to the process's memory. This is done by calling
>> mmap() on
>> > > the
>> > > +DMABUF's file descriptor.
>> > > +
>> > > +Before accessing the data through the map, you must use the
>> > > +DMA_BUF_IOCTL_SYNC(struct dma_buf_sync *) ioctl, with the
>> > > +DMA_BUF_SYNC_START flag, to make sure that the data is
>> available.
>> > > +This call may block until the hardware is done with this
>> block.
>> > > Once
>> > > +you are done reading or writing the data, you must use this
>> ioctl
>> > > again
>> > > +with the DMA_BUF_SYNC_END flag, before enqueueing the DMABUF
>> to the
>> > > +kernel's queue.
>> > > +
>> > > +If you need to know when the hardware is done with a DMABUF,
>> you
>> > > can
>> > > +poll its file descriptor for the EPOLLOUT event.
>> > > +
>> > > +Finally, to destroy a DMABUF object, simply call close() on
>> its
>> > > file
>> > > +descriptor.
>> > > +
>> > > +For more information about manipulating DMABUF objects, see:
>> > > :ref:`dma-buf`.
>> > > +
>> > > +A typical workflow for the new interface is:
>> > > +
>> > > + for block in blocks:
>> > > + DMABUF_ALLOC block
>> > > + mmap block
>> > > +
>> > > + enable buffer
>> > > +
>> > > + while !done
>> > > + for block in blocks:
>> > > + DMABUF_ENQUEUE block
>> > > +
>> > > + DMABUF_SYNC_START block
>> > > + process data
>> > > + DMABUF_SYNC_END block
>> > > +
>> > > + disable buffer
>> > > +
>> > > + for block in blocks:
>> > > + close block
>> > > diff --git a/Documentation/iio/index.rst
>> > > b/Documentation/iio/index.rst
>> > > index 58b7a4ebac51..669deb67ddee 100644
>> > > --- a/Documentation/iio/index.rst
>> > > +++ b/Documentation/iio/index.rst
>> > > @@ -9,4 +9,6 @@ Industrial I/O
>> > >
>> > > iio_configfs
>> > >
>> > > + dmabuf_api
>> > > +
>> > > ep93xx_adc
>> > > --
>> > > 2.34.1
>> > >
>> >
>> > --
>> > Daniel Vetter
>> > Software Engineer, Intel Corporation
>> > http://blog.ffwll.ch
>>
>>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch