1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _DMX_REQBUFS:
4
5*****************
6ioctl DMX_REQBUFS
7*****************
8
9Name
10====
11
12DMX_REQBUFS - Initiate Memory Mapping and/or DMA buffer I/O
13
14.. warning:: this API is still experimental
15
16
17Synopsis
18========
19
20.. c:function:: int ioctl( int fd, DMX_REQBUFS, struct dmx_requestbuffers *argp )
21    :name: DMX_REQBUFS
22
23
24Arguments
25=========
26
27``fd``
28    File descriptor returned by :ref:`open() <dmx_fopen>`.
29
30``argp``
31    Pointer to struct :c:type:`dmx_requestbuffers`.
32
33Description
34===========
35
36This ioctl is used to initiate a memory mapped or DMABUF based demux I/O.
37
38Memory mapped buffers are located in device memory and must be allocated
39with this ioctl before they can be mapped into the application's address
40space. User buffers are allocated by applications themselves, and this
41ioctl is merely used to switch the driver into user pointer I/O mode and
42to setup some internal structures. Similarly, DMABUF buffers are
43allocated by applications through a device driver, and this ioctl only
44configures the driver into DMABUF I/O mode without performing any direct
45allocation.
46
47To allocate device buffers applications initialize all fields of the
48struct :c:type:`dmx_requestbuffers` structure. They set the  ``count`` field
49to the desired number of buffers,  and ``size`` to the size of each
50buffer.
51
52When the ioctl is called with a pointer to this structure, the driver will
53attempt to allocate the requested number of buffers and it stores the actual
54number 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
55number is also possible when the driver requires more buffers to
56function correctly. The actual allocated buffer size can is returned
57at ``size``, and can be smaller than what's requested.
58
59When this I/O method is not supported, the ioctl returns an ``EOPNOTSUPP``
60error code.
61
62Applications can call :ref:`DMX_REQBUFS` again to change the number of
63buffers, however this cannot succeed when any buffers are still mapped.
64A ``count`` value of zero frees all buffers, after aborting or finishing
65any DMA in progress.
66
67
68Return Value
69============
70
71On success 0 is returned, on error -1 and the ``errno`` variable is set
72appropriately. The generic error codes are described at the
73:ref:`Generic Error Codes <gen-errors>` chapter.
74
75EOPNOTSUPP
76    The  the requested I/O method is not supported.
77