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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: DTV.dmx
+
+.. _DMX_REQBUFS:
+
+*****************
+ioctl DMX_REQBUFS
+*****************
+
+Name
+====
+
+DMX_REQBUFS - Initiate Memory Mapping and/or DMA buffer I/O
+
+.. warning:: this API is still experimental
+
+Synopsis
+========
+
+.. c:macro:: DMX_REQBUFS
+
+``int ioctl(int fd, DMX_REQBUFS, struct dmx_requestbuffers *argp)``
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``argp``
+    Pointer to struct :c:type:`dmx_requestbuffers`.
+
+Description
+===========
+
+This ioctl is used to initiate a memory mapped or DMABUF based demux I/O.
+
+Memory mapped buffers are located in device memory and must be allocated
+with this ioctl before they can be mapped into the application's address
+space. User buffers are allocated by applications themselves, and this
+ioctl is merely used to switch the driver into user pointer I/O mode and
+to setup some internal structures. Similarly, DMABUF buffers are
+allocated by applications through a device driver, and this ioctl only
+configures the driver into DMABUF I/O mode without performing any direct
+allocation.
+
+To allocate device buffers applications initialize all fields of the
+struct :c:type:`dmx_requestbuffers` structure. They set the  ``count`` field
+to the desired number of buffers,  and ``size`` to the size of each
+buffer.
+
+When the ioctl is called with a pointer to this structure, the driver will
+attempt to allocate the requested number of buffers and it stores the actual
+number allocated in the ``count`` field. The ``count`` can be smaller than the number requested, even zero, when the driver runs out of free memory. A larger
+number is also possible when the driver requires more buffers to
+function correctly. The actual allocated buffer size can is returned
+at ``size``, and can be smaller than what's requested.
+
+When this I/O method is not supported, the ioctl returns an ``EOPNOTSUPP``
+error code.
+
+Applications can call :ref:`DMX_REQBUFS` again to change the number of
+buffers, however this cannot succeed when any buffers are still mapped.
+A ``count`` value of zero frees all buffers, after aborting or finishing
+any DMA in progress.
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.
+
+EOPNOTSUPP
+    The  the requested I/O method is not supported.