mirror of https://gitee.com/openkylin/linux.git
203 lines
8.5 KiB
XML
203 lines
8.5 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 buffer applications set the <structfield>type</structfield>
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field of a &v4l2-buffer; to the same buffer type as was previously used
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with &v4l2-format; <structfield>type</structfield> and &v4l2-requestbuffers;
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<structfield>type</structfield>. Applications must also set 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>,
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<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</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.
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Applications must also set <structfield>flags</structfield> to 0.
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The <structfield>reserved2</structfield> and
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<structfield>reserved</structfield> fields must be set to 0. When using
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the <link linkend="planar-apis">multi-planar API</link>, the
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<structfield>m.planes</structfield> field must contain a userspace pointer
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to a filled-in array of &v4l2-plane; and the <structfield>length</structfield>
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field must be set to the number of elements in that array.
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</para>
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<para>To enqueue a <link linkend="mmap">memory mapped</link>
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buffer applications set the <structfield>memory</structfield>
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field to <constant>V4L2_MEMORY_MMAP</constant>. 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>memory</structfield>
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field to <constant>V4L2_MEMORY_USERPTR</constant>, 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 multi-planar
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API is used, <structfield>m.userptr</structfield> and
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<structfield>length</structfield> members of the passed array of &v4l2-plane;
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have to be used instead. When <constant>VIDIOC_QBUF</constant> is called with
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a pointer to this structure the driver sets the
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<constant>V4L2_BUF_FLAG_QUEUED</constant> flag and clears the
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<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 is
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called, or until the device is closed.</para>
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<para>To enqueue a <link linkend="dmabuf">DMABUF</link> buffer applications
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set the <structfield>memory</structfield> field to
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<constant>V4L2_MEMORY_DMABUF</constant> and the <structfield>m.fd</structfield>
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field to a file descriptor associated with a DMABUF buffer. When the
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multi-planar API is used the <structfield>m.fd</structfield> fields of the
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passed array of &v4l2-plane; have to be used instead. When
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<constant>VIDIOC_QBUF</constant> is called with a pointer to this structure the
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driver sets the <constant>V4L2_BUF_FLAG_QUEUED</constant> flag and clears the
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<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. This
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ioctl locks the buffer. Locking a buffer means passing it to a driver for a
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hardware access (usually DMA). If an application accesses (reads/writes) a
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locked buffer then the result is undefined. Buffers remain locked until
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dequeued, until the &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl is called, or
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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>, <structfield>memory</structfield>
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and <structfield>reserved</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. The driver may also set
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<constant>V4L2_BUF_FLAG_ERROR</constant> in the <structfield>flags</structfield>
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field. It indicates a non-critical (recoverable) streaming error. In such case
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the application may continue as normal, but should be aware that data in the
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dequeued buffer might be corrupted. When using the multi-planar API, the
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planes array must be passed in as well.</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>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. Reusing such buffer may be unsafe
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though and its details (e.g. <structfield>index</structfield>) may not be
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returned either. It is recommended that drivers indicate recoverable errors
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by setting the <constant>V4L2_BUF_FLAG_ERROR</constant> and returning 0 instead.
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In that case the application should be able to safely reuse the buffer and
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continue streaming.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EPIPE</errorcode></term>
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<listitem>
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<para><constant>VIDIOC_DQBUF</constant> returns this on an empty
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capture queue for mem2mem codecs if a buffer with the
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<constant>V4L2_BUF_FLAG_LAST</constant> was already dequeued and no new buffers
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are expected to become available.
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</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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