[deliverable/linux.git] / Documentation / media / uapi / v4l / vidioc-reqbufs.rst

.. -*- coding: utf-8; mode: rst -*-

.. _VIDIOC_REQBUFS:

********************
ioctl VIDIOC_REQBUFS
********************

Name
====

VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp )
    :name: VIDIOC_REQBUFS


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``


Description
===========

This ioctl is used to initiate :ref:`memory mapped <mmap>`,
:ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based 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:`v4l2_requestbuffers` structure. They set the ``type``
field to the respective stream or buffer type, the ``count`` field to
the desired number of buffers, ``memory`` must be set to the requested
I/O method and the ``reserved`` array must be zeroed. 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. It 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. For example video output requires at least two
buffers, one displayed and one filled by the application.

When the I/O method is not supported the ioctl returns an ``EINVAL`` error
code.

Applications can call :ref:`VIDIOC_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, an implicit
:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.


.. c:type:: v4l2_requestbuffers

.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|

.. flat-table:: struct v4l2_requestbuffers
    :header-rows:  0
    :stub-columns: 0
    :widths:       1 1 2


    -  .. row 1

       -  __u32

       -  ``count``

       -  The number of buffers requested or granted.

    -  .. row 2

       -  __u32

       -  ``type``

       -  Type of the stream or buffers, this is the same as the struct
	  :c:type:`v4l2_format` ``type`` field. See
	  :c:type:`v4l2_buf_type` for valid values.

    -  .. row 3

       -  __u32

       -  ``memory``

       -  Applications set this field to ``V4L2_MEMORY_MMAP``,
	  ``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
	  :c:type:`v4l2_memory`.

    -  .. row 4

       -  __u32

       -  ``reserved``\ [2]

       -  A place holder for future extensions. Drivers and applications
	  must set the array to zero.


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.

EINVAL
    The buffer type (``type`` field) or the requested I/O method
    (``memory``) is not supported.
Commit	Line	Data
	1	.. -- coding: utf-8; mode: rst --
	2
	3	.. _VIDIOC_REQBUFS:
	4
	5	********************
	6	ioctl VIDIOC_REQBUFS
	7	********************
	8
	9	Name
	10	====
	11
	12	VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
	13
	14
	15	Synopsis
	16	========
	17
	18	.. c:function:: int ioctl( int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp )
	19	:name: VIDIOC_REQBUFS
	20
	21
	22	Arguments
	23	=========
	24
	25	``fd``
	26	File descriptor returned by :ref:`open() <func-open>`.
	27
	28	``argp``
	29
	30
	31	Description
	32	===========
	33
	34	This ioctl is used to initiate :ref:`memory mapped <mmap>`,
	35	:ref:`user pointer <userp>` or :ref:`DMABUF <dmabuf>` based I/O.
	36	Memory mapped buffers are located in device memory and must be allocated
	37	with this ioctl before they can be mapped into the application's address
	38	space. User buffers are allocated by applications themselves, and this
	39	ioctl is merely used to switch the driver into user pointer I/O mode and
	40	to setup some internal structures. Similarly, DMABUF buffers are
	41	allocated by applications through a device driver, and this ioctl only
	42	configures the driver into DMABUF I/O mode without performing any direct
	43	allocation.
	44
	45	To allocate device buffers applications initialize all fields of the
	46	struct :c:type:`v4l2_requestbuffers` structure. They set the ``type``
	47	field to the respective stream or buffer type, the ``count`` field to
	48	the desired number of buffers, ``memory`` must be set to the requested
	49	I/O method and the ``reserved`` array must be zeroed. When the ioctl is
	50	called with a pointer to this structure the driver will attempt to
	51	allocate the requested number of buffers and it stores the actual number
	52	allocated in the ``count`` field. It can be smaller than the number
	53	requested, even zero, when the driver runs out of free memory. A larger
	54	number is also possible when the driver requires more buffers to
	55	function correctly. For example video output requires at least two
	56	buffers, one displayed and one filled by the application.
	57
	58	When the I/O method is not supported the ioctl returns an ``EINVAL`` error
	59	code.
	60
	61	Applications can call :ref:`VIDIOC_REQBUFS` again to change the number of
	62	buffers, however this cannot succeed when any buffers are still mapped.
	63	A ``count`` value of zero frees all buffers, after aborting or finishing
	64	any DMA in progress, an implicit
	65	:ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`.
	66
	67
	68	.. c:type:: v4l2_requestbuffers
	69
	70	.. tabularcolumns:: \|p{4.4cm}\|p{4.4cm}\|p{8.7cm}\|
	71
	72	.. flat-table:: struct v4l2_requestbuffers
	73	:header-rows: 0
	74	:stub-columns: 0
	75	:widths: 1 1 2
	76
	77
	78	- .. row 1
	79
	80	- __u32
	81
	82	- ``count``
	83
	84	- The number of buffers requested or granted.
	85
	86	- .. row 2
	87
	88	- __u32
	89
	90	- ``type``
	91
	92	- Type of the stream or buffers, this is the same as the struct
	93	:c:type:`v4l2_format` ``type`` field. See
	94	:c:type:`v4l2_buf_type` for valid values.
	95
	96	- .. row 3
	97
	98	- __u32
	99
	100	- ``memory``
	101
	102	- Applications set this field to ``V4L2_MEMORY_MMAP``,
	103	``V4L2_MEMORY_DMABUF`` or ``V4L2_MEMORY_USERPTR``. See
	104	:c:type:`v4l2_memory`.
	105
	106	- .. row 4
	107
	108	- __u32
	109
	110	- ``reserved``\ [2]
	111
	112	- A place holder for future extensions. Drivers and applications
	113	must set the array to zero.
	114
	115
	116	Return Value
	117	============
	118
	119	On success 0 is returned, on error -1 and the ``errno`` variable is set
	120	appropriately. The generic error codes are described at the
	121	:ref:`Generic Error Codes <gen-errors>` chapter.
	122
	123	EINVAL
	124	The buffer type (``type`` field) or the requested I/O method
	125	(``memory``) is not supported.