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    SubjectRe: [PATCH 15/15] Documentation: iio: Document high-speed DMABUF based API
    On Mon, 15 Nov 2021 14:22:43 +0000
    Paul Cercueil <paul@crapouillou.net> wrote:

    > Document the new DMABUF based API.
    >
    > Signed-off-by: Paul Cercueil <paul@crapouillou.net>

    Hi Paul,

    A few trivial things inline but looks good to me if we do end up using DMABUF
    anyway.

    Jonathan

    > ---
    > 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:
    > +

    Why this change?

    > Buffer Sharing and Synchronization
    > ==================================
    >
    > diff --git a/Documentation/iio/dmabuf_api.rst b/Documentation/iio/dmabuf_api.rst
    > new file mode 100644
    > index 000000000000..b4e120a4ef0c
    > --- /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.
    I would note somewhere that this interface is optional for a given IIO driver.
    I don't want people to start assuming their i2c ADC will support this and
    wondering why it doesn't work :)

    > +
    > +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 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.
    > +
    > +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 (either opened from the corresponding /dev/iio:deviceX, or
    > +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..9ce799fbf262 100644
    > --- a/Documentation/iio/index.rst
    > +++ b/Documentation/iio/index.rst
    > @@ -10,3 +10,5 @@ Industrial I/O
    > iio_configfs
    >
    > ep93xx_adc
    > +
    > + dmabuf_api

    Given this is core stuff rather than driver specific, perhaps move it up a few lines?


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