1 files changed, 137 insertions, 0 deletions
diff --git a/Documentation/userspace-api/media/v4l/func-mmap.rst b/Documentation/userspace-api/media/v4l/func-mmap.rst
new file mode 100644
index 000000000..e3e5e64eb
--- /dev/null
+++ b/Documentation/userspace-api/media/v4l/func-mmap.rst
@@ -0,0 +1,137 @@
+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _func-mmap:
+
+***********
+V4L2 mmap()
+***********
+
+Name
+====
+
+v4l2-mmap - Map device memory into application address space
+
+Synopsis
+========
+
+.. code-block:: c
+
+    #include <unistd.h>
+    #include <sys/mman.h>
+
+.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
+
+Arguments
+=========
+
+``start``
+    Map the buffer to this address in the application's address space.
+    When the ``MAP_FIXED`` flag is specified, ``start`` must be a
+    multiple of the pagesize and mmap will fail when the specified
+    address cannot be used. Use of this option is discouraged;
+    applications should just specify a ``NULL`` pointer here.
+
+``length``
+    Length of the memory area to map. This must be the same value as
+    returned by the driver in the struct
+    :c:type:`v4l2_buffer` ``length`` field for the
+    single-planar API, and the same value as returned by the driver in
+    the struct :c:type:`v4l2_plane` ``length`` field for
+    the multi-planar API.
+
+``prot``
+    The ``prot`` argument describes the desired memory protection.
+    Regardless of the device type and the direction of data exchange it
+    should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
+    and write access to image buffers. Drivers should support at least
+    this combination of flags.
+
+    .. note::
+
+      #. The Linux ``videobuf`` kernel module, which is used by some
+	 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
+	 driver does not support the desired protection, the
+	 :c:func:`mmap()` function fails.
+
+      #. Device memory accesses (e. g. the memory on a graphics card
+	 with video capturing hardware) may incur a performance penalty
+	 compared to main memory accesses, or reads may be significantly
+	 slower than writes or vice versa. Other I/O methods may be more
+	 efficient in such case.
+
+``flags``
+    The ``flags`` parameter specifies the type of the mapped object,
+    mapping options and whether modifications made to the mapped copy of
+    the page are private to the process or are to be shared with other
+    references.
+
+    ``MAP_FIXED`` requests that the driver selects no other address than
+    the one specified. If the specified address cannot be used,
+    :c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified,
+    ``start`` must be a multiple of the pagesize. Use of this option is
+    discouraged.
+
+    One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
+    ``MAP_SHARED`` allows applications to share the mapped memory with
+    other (e. g. child-) processes.
+
+    .. note::
+
+       The Linux ``videobuf`` module  which is used by some
+       drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
+       copy-on-write semantics. V4L2 applications should not set the
+       ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
+       flags.
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``offset``
+    Offset of the buffer in device memory. This must be the same value
+    as returned by the driver in the struct
+    :c:type:`v4l2_buffer` ``m`` union ``offset`` field for
+    the single-planar API, and the same value as returned by the driver
+    in the struct :c:type:`v4l2_plane` ``m`` union
+    ``mem_offset`` field for the multi-planar API.
+
+Description
+===========
+
+The :c:func:`mmap()` function asks to map ``length`` bytes starting at
+``offset`` in the memory of the device specified by ``fd`` into the
+application address space, preferably at address ``start``. This latter
+address is a hint only, and is usually specified as 0.
+
+Suitable length and offset parameters are queried with the
+:ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
+allocated with the :ref:`VIDIOC_REQBUFS` ioctl
+before they can be queried.
+
+To unmap buffers the :c:func:`munmap()` function is used.
+
+Return Value
+============
+
+On success :c:func:`mmap()` returns a pointer to the mapped buffer. On
+error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
+appropriately. Possible error codes are:
+
+EBADF
+    ``fd`` is not a valid file descriptor.
+
+EACCES
+    ``fd`` is not open for reading and writing.
+
+EINVAL
+    The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
+    they are too large, or not aligned on a ``PAGESIZE`` boundary.)
+
+    The ``flags`` or ``prot`` value is not supported.
+
+    No buffers have been allocated with the
+    :ref:`VIDIOC_REQBUFS` ioctl.
+
+ENOMEM
+    Not enough physical or virtual memory was available to complete the
+    request.