From: Paul Cercueil <paul@crapouillou.net>
To: Jonathan Cameron <jic23@kernel.org>
Cc: "Alexandru Ardelean" <ardeleanalex@gmail.com>,
"Lars-Peter Clausen" <lars@metafoo.de>,
"Michael Hennerich" <Michael.Hennerich@analog.com>,
"Sumit Semwal" <sumit.semwal@linaro.org>,
"Christian König" <christian.koenig@amd.com>,
linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
linaro-mm-sig@lists.linaro.org,
"Paul Cercueil" <paul@crapouillou.net>
Subject: [PATCH 15/15] Documentation: iio: Document high-speed DMABUF based API
Date: Mon, 15 Nov 2021 14:22:43 +0000 [thread overview]
Message-ID: <20211115142243.60605-4-paul@crapouillou.net> (raw)
In-Reply-To: <20211115142243.60605-1-paul@crapouillou.net>
Document the new DMABUF based API.
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..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.
+
+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
--
2.33.0
next prev parent reply other threads:[~2021-11-15 14:29 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-15 14:19 [PATCH 00/15] iio: buffer-dma: write() and new DMABUF based API Paul Cercueil
2021-11-15 14:19 ` [PATCH 01/15] iio: buffer-dma: Get rid of incoming/outgoing queues Paul Cercueil
2021-11-16 8:23 ` Alexandru Ardelean
2021-11-21 14:05 ` Jonathan Cameron
2021-11-21 16:23 ` Lars-Peter Clausen
2021-11-21 17:52 ` Paul Cercueil
2021-11-21 18:49 ` Lars-Peter Clausen
2021-11-21 20:08 ` Paul Cercueil
2021-11-22 15:08 ` Lars-Peter Clausen
2021-11-22 15:16 ` Paul Cercueil
2021-11-22 15:17 ` Lars-Peter Clausen
2021-11-22 15:27 ` Paul Cercueil
2021-11-15 14:19 ` [PATCH 02/15] iio: buffer-dma: Remove unused iio_buffer_block struct Paul Cercueil
2021-11-16 8:22 ` Alexandru Ardelean
2021-11-15 14:19 ` [PATCH 03/15] iio: buffer-dma: Use round_down() instead of rounddown() Paul Cercueil
2021-11-16 8:26 ` Alexandru Ardelean
2021-11-21 14:08 ` Jonathan Cameron
2021-11-22 10:00 ` Paul Cercueil
2021-11-27 15:15 ` Jonathan Cameron
2021-11-15 14:19 ` [PATCH 04/15] iio: buffer-dma: Enable buffer write support Paul Cercueil
2021-11-16 8:52 ` Alexandru Ardelean
2021-11-21 14:20 ` Jonathan Cameron
2021-11-21 17:19 ` Paul Cercueil
2021-11-27 15:17 ` Jonathan Cameron
2021-11-15 14:19 ` [PATCH 05/15] iio: buffer-dmaengine: Support specifying buffer direction Paul Cercueil
2021-11-16 8:53 ` Alexandru Ardelean
2021-11-15 14:19 ` [PATCH 06/15] iio: buffer-dmaengine: Enable write support Paul Cercueil
2021-11-16 8:55 ` Alexandru Ardelean
2021-11-15 14:19 ` [PATCH 07/15] iio: core: Add new DMABUF interface infrastructure Paul Cercueil
2021-11-21 14:31 ` Jonathan Cameron
2021-11-15 14:19 ` [PATCH 08/15] iio: buffer-dma: split iio_dma_buffer_fileio_free() function Paul Cercueil
2021-11-16 10:59 ` Alexandru Ardelean
2021-11-21 13:49 ` Jonathan Cameron
2021-11-15 14:19 ` [PATCH 09/15] iio: buffer-dma: Use DMABUFs instead of custom solution Paul Cercueil
2021-11-15 14:19 ` [PATCH 10/15] iio: buffer-dma: Implement new DMABUF based userspace API Paul Cercueil
2021-11-15 14:19 ` [PATCH 11/15] iio: buffer-dma: Boost performance using write-combine cache setting Paul Cercueil
2021-11-18 11:45 ` Paul Cercueil
2021-11-21 15:00 ` Jonathan Cameron
2021-11-21 17:43 ` Paul Cercueil
2021-11-25 17:29 ` Paul Cercueil
2021-11-27 16:05 ` Jonathan Cameron
2021-11-28 13:25 ` Lars-Peter Clausen
2021-11-27 15:20 ` Jonathan Cameron
2021-11-15 14:22 ` [PATCH 12/15] iio: buffer-dmaengine: Support new DMABUF based userspace API Paul Cercueil
2021-11-15 14:22 ` [PATCH 13/15] iio: core: Add support for cyclic buffers Paul Cercueil
2021-11-16 9:50 ` Alexandru Ardelean
2021-11-15 14:22 ` [PATCH 14/15] iio: buffer-dmaengine: " Paul Cercueil
2021-11-16 9:50 ` Alexandru Ardelean
2021-11-15 14:22 ` Paul Cercueil [this message]
2021-11-21 15:10 ` [PATCH 15/15] Documentation: iio: Document high-speed DMABUF based API Jonathan Cameron
2021-11-21 17:46 ` Paul Cercueil
2021-11-15 14:37 ` [PATCH 00/15] iio: buffer-dma: write() and new " Daniel Vetter
2021-11-15 14:57 ` Paul Cercueil
2021-11-16 16:02 ` Daniel Vetter
2021-11-16 16:31 ` Laurent Pinchart
2021-11-17 8:48 ` Christian König
2021-11-17 12:50 ` Paul Cercueil
2021-11-17 13:42 ` Hennerich, Michael
2021-11-21 13:57 ` Jonathan Cameron
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