8e080c2e6c
The V4L and DVB API's are there for a long time. however, up to now, no efforts were done to merge them to kernel DocBook. This patch adds the current versions of the specs as an unique compendium. Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
169 lines
6.1 KiB
XML
169 lines
6.1 KiB
XML
<refentry id="vidioc-qbuf">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_QBUF</refname>
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<refname>VIDIOC_DQBUF</refname>
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<refpurpose>Exchange a buffer with the driver</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_buffer *<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_QBUF, VIDIOC_DQBUF</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>Applications call the <constant>VIDIOC_QBUF</constant> ioctl
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to enqueue an empty (capturing) or filled (output) buffer in the
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driver's incoming queue. The semantics depend on the selected I/O
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method.</para>
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<para>To enqueue a <link linkend="mmap">memory mapped</link>
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buffer applications set the <structfield>type</structfield> field of a
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&v4l2-buffer; to the same buffer type as previously &v4l2-format;
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<structfield>type</structfield> and &v4l2-requestbuffers;
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<structfield>type</structfield>, the <structfield>memory</structfield>
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field to <constant>V4L2_MEMORY_MMAP</constant> and the
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<structfield>index</structfield> field. Valid index numbers range from
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zero to the number of buffers allocated with &VIDIOC-REQBUFS;
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(&v4l2-requestbuffers; <structfield>count</structfield>) minus one. The
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contents of the struct <structname>v4l2_buffer</structname> returned
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by a &VIDIOC-QUERYBUF; ioctl will do as well. When the buffer is
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intended for output (<structfield>type</structfield> is
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<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> or
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<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>) applications must also
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initialize the <structfield>bytesused</structfield>,
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<structfield>field</structfield> and
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<structfield>timestamp</structfield> fields. See <xref
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linkend="buffer" /> for details. When
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<constant>VIDIOC_QBUF</constant> is called with a pointer to this
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structure the driver sets the
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<constant>V4L2_BUF_FLAG_MAPPED</constant> and
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<constant>V4L2_BUF_FLAG_QUEUED</constant> flags and clears the
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<constant>V4L2_BUF_FLAG_DONE</constant> flag in the
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<structfield>flags</structfield> field, or it returns an
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&EINVAL;.</para>
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<para>To enqueue a <link linkend="userp">user pointer</link>
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buffer applications set the <structfield>type</structfield> field of a
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&v4l2-buffer; to the same buffer type as previously &v4l2-format;
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<structfield>type</structfield> and &v4l2-requestbuffers;
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<structfield>type</structfield>, the <structfield>memory</structfield>
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field to <constant>V4L2_MEMORY_USERPTR</constant> and the
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<structfield>m.userptr</structfield> field to the address of the
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buffer and <structfield>length</structfield> to its size. When the
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buffer is intended for output additional fields must be set as above.
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When <constant>VIDIOC_QBUF</constant> is called with a pointer to this
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structure the driver sets the <constant>V4L2_BUF_FLAG_QUEUED</constant>
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flag and clears the <constant>V4L2_BUF_FLAG_MAPPED</constant> and
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<constant>V4L2_BUF_FLAG_DONE</constant> flags in the
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<structfield>flags</structfield> field, or it returns an error code.
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This ioctl locks the memory pages of the buffer in physical memory,
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they cannot be swapped out to disk. Buffers remain locked until
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dequeued, until the &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl are
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called, or until the device is closed.</para>
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<para>Applications call the <constant>VIDIOC_DQBUF</constant>
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ioctl to dequeue a filled (capturing) or displayed (output) buffer
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from the driver's outgoing queue. They just set the
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<structfield>type</structfield> and <structfield>memory</structfield>
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fields of a &v4l2-buffer; as above, when <constant>VIDIOC_DQBUF</constant>
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is called with a pointer to this structure the driver fills the
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remaining fields or returns an error code.</para>
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<para>By default <constant>VIDIOC_DQBUF</constant> blocks when no
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buffer is in the outgoing queue. When the
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<constant>O_NONBLOCK</constant> flag was given to the &func-open;
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function, <constant>VIDIOC_DQBUF</constant> returns immediately
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with an &EAGAIN; when no buffer is available.</para>
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<para>The <structname>v4l2_buffer</structname> structure is
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specified in <xref linkend="buffer" />.</para>
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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>EAGAIN</errorcode></term>
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<listitem>
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<para>Non-blocking I/O has been selected using
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<constant>O_NONBLOCK</constant> and no buffer was in the outgoing
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queue.</para>
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</listitem>
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</varlistentry>
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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 <structfield>type</structfield> is not
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supported, or the <structfield>index</structfield> is out of bounds,
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or no buffers have been allocated yet, or the
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<structfield>userptr</structfield> or
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<structfield>length</structfield> are invalid.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>ENOMEM</errorcode></term>
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<listitem>
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<para>Not enough physical or virtual memory was available to
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enqueue a user pointer buffer.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EIO</errorcode></term>
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<listitem>
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<para><constant>VIDIOC_DQBUF</constant> failed due to an
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internal error. Can also indicate temporary problems like signal
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loss. Note the driver might dequeue an (empty) buffer despite
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returning an error, or even stop capturing.</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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<!--
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Local Variables:
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mode: sgml
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sgml-parent-document: "v4l2.sgml"
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indent-tabs-mode: nil
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End:
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-->
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