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

[Paul Cercueil <paul@crapouillou.net>]

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.
+
+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