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

[Daniel Vetter <daniel@ffwll.ch>]

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.

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).
-Daniel

> +
> +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