media: dvb uAPI docs: document demux mmap/munmap syscalls

.. _dmx-mmap:

*****************

Digital TV mmap()

*****************

Name

====

dmx-mmap - Map device memory into application address space

.. warning:: this API is still experimental

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 )

    :name: dmx-mmap

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 a multiple of the

    DVB packet length (188, on most drivers).

``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.

``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,

    :ref:`mmap() <dmx-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 Digital TV applications should not set the

       ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``

       flags.

``fd``

    File descriptor returned by :ref:`open() <dmx_fopen>`.

``offset``

    Offset of the buffer in device memory, as returned by

    :ref:`DMX_QUERYBUF` ioctl.

Description

===========

The :ref:`mmap() <dmx-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:`DMX_QUERYBUF` ioctl. Buffers must be allocated with the

:ref:`DMX_REQBUFS` ioctl before they can be queried.

To unmap buffers the :ref:`munmap() <dmx-munmap>` function is used.

Return Value

============

On success :ref:`mmap() <dmx-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:`DMX_REQBUFS` ioctl.

ENOMEM

    Not enough physical or virtual memory was available to complete the

    request.

.. _dmx-munmap:

************

DVB munmap()

************

Name

====

dmx-munmap - Unmap device memory

.. warning:: This API is still experimental.

Synopsis

========

.. code-block:: c

    #include <unistd.h>

    #include <sys/mman.h>

.. c:function:: int munmap( void *start, size_t length )

    :name: dmx-munmap

Arguments

=========

``start``

    Address of the mapped buffer as returned by the

    :ref:`mmap() <dmx-mmap>` function.

``length``

    Length of the mapped buffer. This must be the same value as given to

    :ref:`mmap() <dmx-mmap>`.

Description

===========

Unmaps a previously with the :ref:`mmap() <dmx-mmap>` function mapped

buffer and frees it, if possible.

Return Value

============

On success :ref:`munmap() <dmx-munmap>` returns 0, on failure -1 and the

``errno`` variable is set appropriately:

EINVAL

    The ``start`` or ``length`` is incorrect, or no buffers have been

    mapped yet.

    dmx-fclose

    dmx-fread

    dmx-fwrite

    dmx-mmap

    dmx-munmap

    dmx-start

    dmx-stop

    dmx-set-filter