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+<refentry id="vidioc-reqbufs">
+  <refmeta>
+    <refentrytitle>ioctl VIDIOC_REQBUFS</refentrytitle>
+    &manvol;
+  </refmeta>
+
+  <refnamediv>
+    <refname>VIDIOC_REQBUFS</refname>
+    <refpurpose>Initiate Memory Mapping or User Pointer I/O</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcprototype>
+        <funcdef>int <function>ioctl</function></funcdef>
+        <paramdef>int <parameter>fd</parameter></paramdef>
+        <paramdef>int <parameter>request</parameter></paramdef>
+        <paramdef>struct v4l2_requestbuffers *<parameter>argp</parameter></paramdef>
+      </funcprototype>
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Arguments</title>
+
+    <variablelist>
+      <varlistentry>
+        <term><parameter>fd</parameter></term>
+        <listitem>
+          <para>&fd;</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><parameter>request</parameter></term>
+        <listitem>
+          <para>VIDIOC_REQBUFS</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><parameter>argp</parameter></term>
+        <listitem>
+          <para></para>
+        </listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para>This ioctl is used to initiate <link linkend="mmap">memory
+mapped</link> or <link linkend="userp">user pointer</link>
+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.</para>
+
+    <para>To allocate device buffers applications initialize three
+fields of a <structname>v4l2_requestbuffers</structname> structure.
+They set the <structfield>type</structfield> field to the respective
+stream or buffer type, the <structfield>count</structfield> field to
+the desired number of buffers, and <structfield>memory</structfield>
+must be set to <constant>V4L2_MEMORY_MMAP</constant>. When the ioctl
+is called with a pointer to this structure the driver attempts to
+allocate the requested number of buffers and stores the actual number
+allocated in the <structfield>count</structfield> field. It can be
+smaller than the number requested, even zero, when the driver runs out
+of free memory. A larger number is possible when the driver requires
+more buffers to function correctly.<footnote>
+        <para>For example video output requires at least two buffers,
+one displayed and one filled by the application.</para>
+        </footnote> When memory mapping I/O is not supported the ioctl
+returns an &EINVAL;.</para>
+
+    <para>Applications can call <constant>VIDIOC_REQBUFS</constant>
+again to change the number of buffers, however this cannot succeed
+when any buffers are still mapped. A <structfield>count</structfield>
+value of zero frees all buffers, after aborting or finishing any DMA
+in progress, an implicit &VIDIOC-STREAMOFF;. <!-- mhs: I see no
+reason why munmap()ping one or even all buffers must imply
+streamoff.--></para>
+
+    <para>To negotiate user pointer I/O, applications initialize only
+the <structfield>type</structfield> field and set
+<structfield>memory</structfield> to
+<constant>V4L2_MEMORY_USERPTR</constant>. When the ioctl is called
+with a pointer to this structure the driver prepares for user pointer
+I/O, when this I/O method is not supported the ioctl returns an
+&EINVAL;.</para>
+
+    <table pgwide="1" frame="none" id="v4l2-requestbuffers">
+      <title>struct <structname>v4l2_requestbuffers</structname></title>
+      <tgroup cols="3">
+        &cs-str;
+        <tbody valign="top">
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>count</structfield></entry>
+            <entry>The number of buffers requested or granted. This
+field is only used when <structfield>memory</structfield> is set to
+<constant>V4L2_MEMORY_MMAP</constant>.</entry>
+          </row>
+          <row>
+            <entry>&v4l2-buf-type;</entry>
+            <entry><structfield>type</structfield></entry>
+            <entry>Type of the stream or buffers, this is the same
+as the &v4l2-format; <structfield>type</structfield> field. See <xref
+                linkend="v4l2-buf-type" /> for valid values.</entry>
+          </row>
+          <row>
+            <entry>&v4l2-memory;</entry>
+            <entry><structfield>memory</structfield></entry>
+            <entry>Applications set this field to
+<constant>V4L2_MEMORY_MMAP</constant> or
+<constant>V4L2_MEMORY_USERPTR</constant>.</entry>
+          </row>
+          <row>
+            <entry>__u32</entry>
+            <entry><structfield>reserved</structfield>[2]</entry>
+            <entry>A place holder for future extensions and custom
+(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
+higher.</entry>
+          </row>
+        </tbody>
+      </tgroup>
+    </table>
+  </refsect1>
+
+  <refsect1>
+    &return-value;
+
+    <variablelist>
+      <varlistentry>
+        <term><errorcode>EBUSY</errorcode></term>
+        <listitem>
+          <para>The driver supports multiple opens and I/O is already
+in progress, or reallocation of buffers was attempted although one or
+more are still mapped.</para>
+        </listitem>
+      </varlistentry>
+      <varlistentry>
+        <term><errorcode>EINVAL</errorcode></term>
+        <listitem>
+          <para>The buffer type (<structfield>type</structfield> field) or the
+requested I/O method (<structfield>memory</structfield>) is not
+supported.</para>
+        </listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+</refentry>
+
+<!--
+Local Variables:
+mode: sgml
+sgml-parent-document: "v4l2.sgml"
+indent-tabs-mode: nil
+End:
+-->