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