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Diffstat (limited to 'Documentation/DocBook/media/v4l/vidioc-qbuf.xml')
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1 files changed, 177 insertions, 0 deletions
diff --git a/Documentation/DocBook/media/v4l/vidioc-qbuf.xml b/Documentation/DocBook/media/v4l/vidioc-qbuf.xml new file mode 100644 index 0000000..9caa49a --- /dev/null +++ b/Documentation/DocBook/media/v4l/vidioc-qbuf.xml @@ -0,0 +1,177 @@ +<refentry id="vidioc-qbuf"> + <refmeta> + <refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle> + &manvol; + </refmeta> + + <refnamediv> + <refname>VIDIOC_QBUF</refname> + <refname>VIDIOC_DQBUF</refname> + <refpurpose>Exchange a buffer with the driver</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_buffer *<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_QBUF, VIDIOC_DQBUF</para> + </listitem> + </varlistentry> + <varlistentry> + <term><parameter>argp</parameter></term> + <listitem> + <para></para> + </listitem> + </varlistentry> + </variablelist> + </refsect1> + + <refsect1> + <title>Description</title> + + <para>Applications call the <constant>VIDIOC_QBUF</constant> ioctl +to enqueue an empty (capturing) or filled (output) buffer in the +driver's incoming queue. The semantics depend on the selected I/O +method.</para> + + <para>To enqueue a buffer applications set the <structfield>type</structfield> +field of a &v4l2-buffer; to the same buffer type as was previously used +with &v4l2-format; <structfield>type</structfield> and &v4l2-requestbuffers; +<structfield>type</structfield>. Applications must also set the +<structfield>index</structfield> field. Valid index numbers range from +zero to the number of buffers allocated with &VIDIOC-REQBUFS; +(&v4l2-requestbuffers; <structfield>count</structfield>) minus one. The +contents of the struct <structname>v4l2_buffer</structname> returned +by a &VIDIOC-QUERYBUF; ioctl will do as well. When the buffer is +intended for output (<structfield>type</structfield> is +<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant>, +<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE</constant>, or +<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>) applications must also +initialize the <structfield>bytesused</structfield>, +<structfield>field</structfield> and +<structfield>timestamp</structfield> fields, see <xref +linkend="buffer" /> for details. +Applications must also set <structfield>flags</structfield> to 0. If a driver +supports capturing from specific video inputs and you want to specify a video +input, then <structfield>flags</structfield> should be set to +<constant>V4L2_BUF_FLAG_INPUT</constant> and the field +<structfield>input</structfield> must be initialized to the desired input. +The <structfield>reserved</structfield> field must be set to 0. When using +the <link linkend="planar-apis">multi-planar API</link>, the +<structfield>m.planes</structfield> field must contain a userspace pointer +to a filled-in array of &v4l2-plane; and the <structfield>length</structfield> +field must be set to the number of elements in that array. +</para> + + <para>To enqueue a <link linkend="mmap">memory mapped</link> +buffer applications set the <structfield>memory</structfield> +field to <constant>V4L2_MEMORY_MMAP</constant>. When +<constant>VIDIOC_QBUF</constant> is called with a pointer to this +structure the driver sets the +<constant>V4L2_BUF_FLAG_MAPPED</constant> and +<constant>V4L2_BUF_FLAG_QUEUED</constant> flags and clears the +<constant>V4L2_BUF_FLAG_DONE</constant> flag in the +<structfield>flags</structfield> field, or it returns an +&EINVAL;.</para> + + <para>To enqueue a <link linkend="userp">user pointer</link> +buffer applications set the <structfield>memory</structfield> +field to <constant>V4L2_MEMORY_USERPTR</constant>, the +<structfield>m.userptr</structfield> field to the address of the +buffer and <structfield>length</structfield> to its size. When the multi-planar +API is used, <structfield>m.userptr</structfield> and +<structfield>length</structfield> members of the passed array of &v4l2-plane; +have to be used instead. When <constant>VIDIOC_QBUF</constant> is called with +a pointer to this structure the driver sets the +<constant>V4L2_BUF_FLAG_QUEUED</constant> flag and clears the +<constant>V4L2_BUF_FLAG_MAPPED</constant> and +<constant>V4L2_BUF_FLAG_DONE</constant> flags in the +<structfield>flags</structfield> field, or it returns an error code. +This ioctl locks the memory pages of the buffer in physical memory, +they cannot be swapped out to disk. Buffers remain locked until +dequeued, until the &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl is +called, or until the device is closed.</para> + + <para>Applications call the <constant>VIDIOC_DQBUF</constant> +ioctl to dequeue a filled (capturing) or displayed (output) buffer +from the driver's outgoing queue. They just set the +<structfield>type</structfield>, <structfield>memory</structfield> +and <structfield>reserved</structfield> +fields of a &v4l2-buffer; as above, when <constant>VIDIOC_DQBUF</constant> +is called with a pointer to this structure the driver fills the +remaining fields or returns an error code. The driver may also set +<constant>V4L2_BUF_FLAG_ERROR</constant> in the <structfield>flags</structfield> +field. It indicates a non-critical (recoverable) streaming error. In such case +the application may continue as normal, but should be aware that data in the +dequeued buffer might be corrupted. When using the multi-planar API, the +planes array does not have to be passed; the <structfield>m.planes</structfield> +member must be set to NULL in that case.</para> + + <para>By default <constant>VIDIOC_DQBUF</constant> blocks when no +buffer is in the outgoing queue. When the +<constant>O_NONBLOCK</constant> flag was given to the &func-open; +function, <constant>VIDIOC_DQBUF</constant> returns immediately +with an &EAGAIN; when no buffer is available.</para> + + <para>The <structname>v4l2_buffer</structname> structure is +specified in <xref linkend="buffer" />.</para> + </refsect1> + + <refsect1> + &return-value; + + <variablelist> + <varlistentry> + <term><errorcode>EAGAIN</errorcode></term> + <listitem> + <para>Non-blocking I/O has been selected using +<constant>O_NONBLOCK</constant> and no buffer was in the outgoing +queue.</para> + </listitem> + </varlistentry> + <varlistentry> + <term><errorcode>EINVAL</errorcode></term> + <listitem> + <para>The buffer <structfield>type</structfield> is not +supported, or the <structfield>index</structfield> is out of bounds, +or no buffers have been allocated yet, or the +<structfield>userptr</structfield> or +<structfield>length</structfield> are invalid.</para> + </listitem> + <term><errorcode>EIO</errorcode></term> + <listitem> + <para><constant>VIDIOC_DQBUF</constant> failed due to an +internal error. Can also indicate temporary problems like signal +loss. Note the driver might dequeue an (empty) buffer despite +returning an error, or even stop capturing. Reusing such buffer may be unsafe +though and its details (e.g. <structfield>index</structfield>) may not be +returned either. It is recommended that drivers indicate recoverable errors +by setting the <constant>V4L2_BUF_FLAG_ERROR</constant> and returning 0 instead. +In that case the application should be able to safely reuse the buffer and +continue streaming. + </para> + </listitem> + </varlistentry> + </variablelist> + </refsect1> +</refentry> |
