/Documentation/DocBook/v4l/vidioc-reqbufs.xml

  1<refentry id="vidioc-reqbufs">
  2  <refmeta>
  3    <refentrytitle>ioctl VIDIOC_REQBUFS</refentrytitle>
  4    &manvol;
  5  </refmeta>
  6
  7  <refnamediv>
  8    <refname>VIDIOC_REQBUFS</refname>
  9    <refpurpose>Initiate Memory Mapping or User Pointer I/O</refpurpose>
 10  </refnamediv>
 11
 12  <refsynopsisdiv>
 13    <funcsynopsis>
 14      <funcprototype>
 15	<funcdef>int <function>ioctl</function></funcdef>
 16	<paramdef>int <parameter>fd</parameter></paramdef>
 17	<paramdef>int <parameter>request</parameter></paramdef>
 18	<paramdef>struct v4l2_requestbuffers *<parameter>argp</parameter></paramdef>
 19      </funcprototype>
 20    </funcsynopsis>
 21  </refsynopsisdiv>
 22
 23  <refsect1>
 24    <title>Arguments</title>
 25
 26    <variablelist>
 27      <varlistentry>
 28	<term><parameter>fd</parameter></term>
 29	<listitem>
 30	  <para>&fd;</para>
 31	</listitem>
 32      </varlistentry>
 33      <varlistentry>
 34	<term><parameter>request</parameter></term>
 35	<listitem>
 36	  <para>VIDIOC_REQBUFS</para>
 37	</listitem>
 38      </varlistentry>
 39      <varlistentry>
 40	<term><parameter>argp</parameter></term>
 41	<listitem>
 42	  <para></para>
 43	</listitem>
 44      </varlistentry>
 45    </variablelist>
 46  </refsect1>
 47
 48  <refsect1>
 49    <title>Description</title>
 50
 51    <para>This ioctl is used to initiate <link linkend="mmap">memory
 52mapped</link> or <link linkend="userp">user pointer</link>
 53I/O. Memory mapped buffers are located in device memory and must be
 54allocated with this ioctl before they can be mapped into the
 55application's address space. User buffers are allocated by
 56applications themselves, and this ioctl is merely used to switch the
 57driver into user pointer I/O mode and to setup some internal structures.</para>
 58
 59    <para>To allocate device buffers applications initialize all
 60fields of the <structname>v4l2_requestbuffers</structname> structure.
 61They set the <structfield>type</structfield> field to the respective
 62stream or buffer type, the <structfield>count</structfield> field to
 63the desired number of buffers, <structfield>memory</structfield>
 64must be set to the requested I/O method and the reserved array
 65must be zeroed. When the ioctl
 66is called with a pointer to this structure the driver will attempt to allocate
 67the requested number of buffers and it stores the actual number
 68allocated in the <structfield>count</structfield> field. It can be
 69smaller than the number requested, even zero, when the driver runs out
 70of free memory. A larger number is also possible when the driver requires
 71more buffers to function correctly. For example video output requires at least two buffers,
 72one displayed and one filled by the application.</para>
 73    <para>When the I/O method is not supported the ioctl
 74returns an &EINVAL;.</para>
 75
 76    <para>Applications can call <constant>VIDIOC_REQBUFS</constant>
 77again to change the number of buffers, however this cannot succeed
 78when any buffers are still mapped. A <structfield>count</structfield>
 79value of zero frees all buffers, after aborting or finishing any DMA
 80in progress, an implicit &VIDIOC-STREAMOFF;. <!-- mhs: I see no
 81reason why munmap()ping one or even all buffers must imply
 82streamoff.--></para>
 83
 84    <table pgwide="1" frame="none" id="v4l2-requestbuffers">
 85      <title>struct <structname>v4l2_requestbuffers</structname></title>
 86      <tgroup cols="3">
 87	&cs-str;
 88	<tbody valign="top">
 89	  <row>
 90	    <entry>__u32</entry>
 91	    <entry><structfield>count</structfield></entry>
 92	    <entry>The number of buffers requested or granted.</entry>
 93	  </row>
 94	  <row>
 95	    <entry>&v4l2-buf-type;</entry>
 96	    <entry><structfield>type</structfield></entry>
 97	    <entry>Type of the stream or buffers, this is the same
 98as the &v4l2-format; <structfield>type</structfield> field. See <xref
 99		linkend="v4l2-buf-type" /> for valid values.</entry>
100	  </row>
101	  <row>
102	    <entry>&v4l2-memory;</entry>
103	    <entry><structfield>memory</structfield></entry>
104	    <entry>Applications set this field to
105<constant>V4L2_MEMORY_MMAP</constant> or
106<constant>V4L2_MEMORY_USERPTR</constant>.</entry>
107	  </row>
108	  <row>
109	    <entry>__u32</entry>
110	    <entry><structfield>reserved</structfield>[2]</entry>
111	    <entry>A place holder for future extensions and custom
112(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
113higher. This array should be zeroed by applications.</entry>
114	  </row>
115	</tbody>
116      </tgroup>
117    </table>
118  </refsect1>
119
120  <refsect1>
121    &return-value;
122
123    <variablelist>
124      <varlistentry>
125	<term><errorcode>EBUSY</errorcode></term>
126	<listitem>
127	  <para>The driver supports multiple opens and I/O is already
128in progress, or reallocation of buffers was attempted although one or
129more are still mapped.</para>
130	</listitem>
131      </varlistentry>
132      <varlistentry>
133	<term><errorcode>EINVAL</errorcode></term>
134	<listitem>
135	  <para>The buffer type (<structfield>type</structfield> field) or the
136requested I/O method (<structfield>memory</structfield>) is not
137supported.</para>
138	</listitem>
139      </varlistentry>
140    </variablelist>
141  </refsect1>
142</refentry>
143
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