Pileus Git - ~andy/linux/blob - Documentation/DocBook/media/v4l/vidioc-create-bufs.xml

   1 <refentry id="vidioc-create-bufs">
   2   <refmeta>
   3     <refentrytitle>ioctl VIDIOC_CREATE_BUFS</refentrytitle>
   4     &manvol;
   5   </refmeta>
   6
   7   <refnamediv>
   8     <refname>VIDIOC_CREATE_BUFS</refname>
   9     <refpurpose>Create buffers for Memory Mapped 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_create_buffers *<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_CREATE_BUFS</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     <note>
  52       <title>Experimental</title>
  53       <para>This is an <link linkend="experimental"> experimental </link>
  54       interface and may change in the future.</para>
  55     </note>
  56
  57     <para>This ioctl is used to create buffers for <link linkend="mmap">memory
  58 mapped</link> or <link linkend="userp">user pointer</link>
  59 I/O. It can be used as an alternative or in addition to the
  60 <constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter control over buffers
  61 is required. This ioctl can be called multiple times to create buffers of
  62 different sizes.</para>
  63
  64     <para>To allocate device buffers applications initialize relevant fields of
  65 the <structname>v4l2_create_buffers</structname> structure. They set the
  66 <structfield>type</structfield> field in the
  67 &v4l2-format; structure, embedded in this
  68 structure, to the respective stream or buffer type.
  69 <structfield>count</structfield> must be set to the number of required buffers.
  70 <structfield>memory</structfield> specifies the required I/O method. The
  71 <structfield>format</structfield> field shall typically be filled in using
  72 either the <constant>VIDIOC_TRY_FMT</constant> or
  73 <constant>VIDIOC_G_FMT</constant> ioctl(). Additionally, applications can adjust
  74 <structfield>sizeimage</structfield> fields to fit their specific needs. The
  75 <structfield>reserved</structfield> array must be zeroed.</para>
  76
  77     <para>When the ioctl is called with a pointer to this structure the driver
  78 will attempt to allocate up to the requested number of buffers and store the
  79 actual number allocated and the starting index in the
  80 <structfield>count</structfield> and the <structfield>index</structfield> fields
  81 respectively. On return <structfield>count</structfield> can be smaller than
  82 the number requested. The driver may also increase buffer sizes if required,
  83 however, it will not update <structfield>sizeimage</structfield> field values.
  84 The user has to use <constant>VIDIOC_QUERYBUF</constant> to retrieve that
  85 information.</para>
  86
  87     <table pgwide="1" frame="none" id="v4l2-create-buffers">
  88       <title>struct <structname>v4l2_create_buffers</structname></title>
  89       <tgroup cols="3">
  90         &cs-str;
  91         <tbody valign="top">
  92           <row>
  93             <entry>__u32</entry>
  94             <entry><structfield>index</structfield></entry>
  95             <entry>The starting buffer index, returned by the driver.</entry>
  96           </row>
  97           <row>
  98             <entry>__u32</entry>
  99             <entry><structfield>count</structfield></entry>
 100             <entry>The number of buffers requested or granted. If count == 0, then
 101             <constant>VIDIOC_CREATE_BUFS</constant> will set <structfield>index</structfield>
 102             to the current number of created buffers, and it will check the validity of
 103             <structfield>memory</structfield> and <structfield>format.type</structfield>.
 104             If those are invalid -1 is returned and errno is set to &EINVAL;,
 105             otherwise <constant>VIDIOC_CREATE_BUFS</constant> returns 0. It will
 106             never set errno to &EBUSY; in this particular case.</entry>
 107           </row>
 108           <row>
 109             <entry>__u32</entry>
 110             <entry><structfield>memory</structfield></entry>
 111             <entry>Applications set this field to
 112 <constant>V4L2_MEMORY_MMAP</constant> or
 113 <constant>V4L2_MEMORY_USERPTR</constant>. See <xref linkend="v4l2-memory"
 114 /></entry>
 115           </row>
 116           <row>
 117             <entry>&v4l2-format;</entry>
 118             <entry><structfield>format</structfield></entry>
 119             <entry>Filled in by the application, preserved by the driver.</entry>
 120           </row>
 121           <row>
 122             <entry>__u32</entry>
 123             <entry><structfield>reserved</structfield>[8]</entry>
 124             <entry>A place holder for future extensions.</entry>
 125           </row>
 126         </tbody>
 127       </tgroup>
 128     </table>
 129   </refsect1>
 130
 131   <refsect1>
 132     &return-value;
 133
 134     <variablelist>
 135       <varlistentry>
 136         <term><errorcode>ENOMEM</errorcode></term>
 137         <listitem>
 138           <para>No memory to allocate buffers for <link linkend="mmap">memory
 139 mapped</link> I/O.</para>
 140         </listitem>
 141       </varlistentry>
 142       <varlistentry>
 143         <term><errorcode>EINVAL</errorcode></term>
 144         <listitem>
 145           <para>The buffer type (<structfield>type</structfield> field) or the
 146 requested I/O method (<structfield>memory</structfield>) is not
 147 supported.</para>
 148         </listitem>
 149       </varlistentry>
 150     </variablelist>
 151   </refsect1>
 152 </refentry>