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

[Paul Cercueil <paul@crapouillou.net>]

Hi Jonathan,

Le lundi 03 avril 2023 à 10:05 -0600, Jonathan Corbet a écrit :
> Paul Cercueil <paul@crapouillou.net> writes:
> 
> One nit:
> 
> > Document the new DMABUF based API.
> > 
> > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: linux-doc@vger.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.

Noted, thanks.

Cheers,
-Paul