linux-kernel - Re: [PATCH v10 6/6] Documentation: iio: Document high-speed DMABUF based API

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Date: Sat, 8 Jun 2024 16:44:24 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Paul Cercueil <paul@...pouillou.net>, Jonathan Cameron
 <jic23@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>,
 Vinod Koul <vkoul@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>
Cc: Jonathan Corbet <corbet@....net>, Nuno Sa <nuno.sa@...log.com>,
 linux-iio@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v10 6/6] Documentation: iio: Document high-speed DMABUF
 based API

Hi Paul.

On 6/7/24 12:44 AM, Paul Cercueil wrote:
> Hi Randy,
> 
> Le jeudi 06 juin 2024 à 10:32 -0700, Randy Dunlap a écrit :
>> Hi,
>>
>> On 6/5/24 4:08 AM, Paul Cercueil wrote:
>>> Document the new DMABUF based API.
>>>
>>> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
>>> Signed-off-by: Nuno Sa <nuno.sa@...log.com>
>>>
>>> ---
>>> 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.
>>>
>>> v5: Use description lists for the documentation of the three new
>>> IOCTLs
>>>     instead of abusing subsections.
>>>
>>> v8: Renamed dmabuf_api.rst -> iio_dmabuf_api.rst, and updated
>>> index.rst
>>>     whose format changed in iio/togreg.
>>> ---
>>>  Documentation/iio/iio_dmabuf_api.rst | 54
>>> ++++++++++++++++++++++++++++
>>>  Documentation/iio/index.rst          |  1 +
>>>  2 files changed, 55 insertions(+)
>>>  create mode 100644 Documentation/iio/iio_dmabuf_api.rst
>>>
>>> diff --git a/Documentation/iio/iio_dmabuf_api.rst
>>> b/Documentation/iio/iio_dmabuf_api.rst
>>> new file mode 100644
>>> index 000000000000..1cd6cd51a582
>>> --- /dev/null
>>> +++ b/Documentation/iio/iio_dmabuf_api.rst
>>> @@ -0,0 +1,54 @@
>>> +.. 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
>>
>> I would say/write:                                to an IIO buffer,
> 
> Right.
> 
>>> +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)``
>>
>>                                      (int fd)
>> ?
> 
> Yes, I can change that. Although it's very obvious what the "int" is
> for, given the text above.
> 

Yes. This is just to be consistent with the text below:

+  ``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *iio_dmabuf)``

>>
>>> +    Attach the DMABUF object, identified by its file descriptor,
>>> to the
>>> +    IIO buffer. Returns zero on success, and a negative errno
>>> value on
>>> +    error.
>>> +
>>> +  ``IIO_BUFFER_DMABUF_DETACH_IOCTL(int)``
>>
>> ditto.
>>
>>> +    Detach the given DMABUF object, identified by its file
>>> descriptor,
>>> +    from the IIO buffer. Returns zero on success, and a negative
>>> errno
>>> +    value on error.
>>> +
>>> +    Note that closing the IIO buffer's file descriptor will
>>> +    automatically detach all previously attached DMABUF objects.
>>> +
>>> +  ``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf
>>> *iio_dmabuf)``
>>> +    Enqueue a previously attached DMABUF object to the buffer
>>> queue.
>>> +    Enqueued DMABUFs will be read from (if output buffer) or
>>> written to
>>> +    (if input buffer) as long as the buffer is enabled.
>>
>> thanks.
> 
> Cheers,
> -Paul

thanks.
-- 
#Randy
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