linux-kernel - Re: [PATCH v3 11/11] Documentation: iio: Document high-speed DMABUF based API

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Message-ID: <87zg7p7xz4.fsf@meer.lwn.net>
Date:   Mon, 03 Apr 2023 10:05:19 -0600
From:   Jonathan Corbet <corbet@....net>
To:     Paul Cercueil <paul@...pouillou.net>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Vinod Koul <vkoul@...nel.org>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Nuno Sá <noname.nuno@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>
Cc:     linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-iio@...r.kernel.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        Paul Cercueil <paul@...pouillou.net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 11/11] Documentation: iio: Document high-speed DMABUF
 based API

Paul Cercueil <paul@...pouillou.net> writes:

One nit:

> Document the new DMABUF based API.
>
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: linux-doc@...r.kernel.org
>
> ---
> 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.
> v3: Update the documentation to reflect the new API.
> ---
>  Documentation/iio/dmabuf_api.rst | 59 ++++++++++++++++++++++++++++++++
>  Documentation/iio/index.rst      |  2 ++
>  2 files changed, 61 insertions(+)
>  create mode 100644 Documentation/iio/dmabuf_api.rst
>
> diff --git a/Documentation/iio/dmabuf_api.rst b/Documentation/iio/dmabuf_api.rst
> new file mode 100644
> index 000000000000..4d70372c7ebd
> --- /dev/null
> +++ b/Documentation/iio/dmabuf_api.rst
> @@ -0,0 +1,59 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===================================
> +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
> +can attach DMABUF objects (externally created) to a IIO buffer, and
> +subsequently use them for data transfers.
> +
> +A userspace application can then use this interface to share DMABUF
> +objects between several interfaces, allowing it to transfer data in a
> +zero-copy fashion, for instance between IIO and the USB stack.
> +
> +The userspace application can also memory-map the DMABUF objects, and
> +access the sample data directly. The advantage of doing this 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.
> +It does however increase the userspace-kernelspace synchronization
> +overhead, as the DMA_BUF_SYNC_START and DMA_BUF_SYNC_END IOCTLs have to
> +be used for data integrity.
> +
> +2. User API
> +===========
> +
> +As part of this interface, three new IOCTLs have been added. These three
> +IOCTLs have to be performed on the IIO buffer's file descriptor,
> +obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
> +
> +``IIO_BUFFER_DMABUF_ATTACH_IOCTL(int)``
> +----------------------------------------------------------------
> +
> +Attach the DMABUF object, identified by its file descriptor, to the IIO
> +buffer. Returns zero on success, and a negative errno value on error.

Rather than abusing subsections, this would be better done as a
description list:

  IIO_BUFFER_DMABUF_ATTACH_IOCTL(int)
      Attach the DMABUF object, identified by its file descriptor, to
      the IIO buffer. Returns zero on success, and a negative errno
      value on error.

Thanks,

jon