[media] Documentation: media: description of DMABUF importing in V4L2
This patch adds description and usage examples for importing DMABUF file descriptor in V4L2. Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com> Acked-by: Hans Verkuil <hans.verkuil@cisco.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Tomasz Stanislawski
Mauro Carvalho Chehab
parent
051c7788bc
commit
4b9c1cb641
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@ -2586,6 +2586,10 @@ ioctls.</para>
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<para>Vendor and device specific media bus pixel formats.
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<xref linkend="v4l2-mbus-vendor-spec-fmts" />.</para>
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</listitem>
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<listitem>
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<para>Importing DMABUF file descriptors as a new IO method described
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in <xref linkend="dmabuf" />.</para>
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</listitem>
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</itemizedlist>
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</section>
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@ -331,7 +331,7 @@ application until one or more buffers can be dequeued. By default
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outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
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given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
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returns immediately with an &EAGAIN; when no buffer is available. The
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&func-select; or &func-poll; function are always available.</para>
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&func-select; or &func-poll; functions are always available.</para>
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<para>To start and stop capturing or output applications call the
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&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
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@ -472,6 +472,162 @@ rest should be evident.</para>
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</footnote></para>
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</section>
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<section id="dmabuf">
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<title>Streaming I/O (DMA buffer importing)</title>
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<note>
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<title>Experimental</title>
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<para>This is an <link linkend="experimental"> experimental </link>
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interface and may change in the future.</para>
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</note>
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<para>The DMABUF framework provides a generic method for sharing buffers
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between multiple devices. Device drivers that support DMABUF can export a DMA
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buffer to userspace as a file descriptor (known as the exporter role), import a
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DMA buffer from userspace using a file descriptor previously exported for a
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different or the same device (known as the importer role), or both. This
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section describes the DMABUF importer role API in V4L2.</para>
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<para>Input and output devices support the streaming I/O method when the
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<constant>V4L2_CAP_STREAMING</constant> flag in the
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<structfield>capabilities</structfield> field of &v4l2-capability; returned by
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the &VIDIOC-QUERYCAP; ioctl is set. Whether importing DMA buffers through
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DMABUF file descriptors is supported is determined by calling the
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&VIDIOC-REQBUFS; ioctl with the memory type set to
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<constant>V4L2_MEMORY_DMABUF</constant>.</para>
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<para>This I/O method is dedicated to sharing DMA buffers between different
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devices, which may be V4L devices or other video-related devices (e.g. DRM).
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Buffers (planes) are allocated by a driver on behalf of an application. Next,
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these buffers are exported to the application as file descriptors using an API
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which is specific for an allocator driver. Only such file descriptor are
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exchanged. The descriptors and meta-information are passed in &v4l2-buffer; (or
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in &v4l2-plane; in the multi-planar API case). The driver must be switched
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into DMABUF I/O mode by calling the &VIDIOC-REQBUFS; with the desired buffer
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type.</para>
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<example>
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<title>Initiating streaming I/O with DMABUF file descriptors</title>
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<programlisting>
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&v4l2-requestbuffers; reqbuf;
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memset(&reqbuf, 0, sizeof (reqbuf));
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reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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reqbuf.memory = V4L2_MEMORY_DMABUF;
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reqbuf.count = 1;
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if (ioctl(fd, &VIDIOC-REQBUFS;, &reqbuf) == -1) {
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if (errno == EINVAL)
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printf("Video capturing or DMABUF streaming is not supported\n");
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else
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perror("VIDIOC_REQBUFS");
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exit(EXIT_FAILURE);
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}
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</programlisting>
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</example>
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<para>The buffer (plane) file descriptor is passed on the fly with the
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&VIDIOC-QBUF; ioctl. In case of multiplanar buffers, every plane can be
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associated with a different DMABUF descriptor. Although buffers are commonly
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cycled, applications can pass a different DMABUF descriptor at each
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<constant>VIDIOC_QBUF</constant> call.</para>
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<example>
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<title>Queueing DMABUF using single plane API</title>
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<programlisting>
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int buffer_queue(int v4lfd, int index, int dmafd)
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{
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&v4l2-buffer; buf;
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memset(&buf, 0, sizeof buf);
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buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
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buf.memory = V4L2_MEMORY_DMABUF;
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buf.index = index;
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buf.m.fd = dmafd;
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if (ioctl(v4lfd, &VIDIOC-QBUF;, &buf) == -1) {
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perror("VIDIOC_QBUF");
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return -1;
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}
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return 0;
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}
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</programlisting>
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</example>
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<example>
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<title>Queueing DMABUF using multi plane API</title>
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<programlisting>
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int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
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{
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&v4l2-buffer; buf;
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&v4l2-plane; planes[VIDEO_MAX_PLANES];
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int i;
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memset(&buf, 0, sizeof buf);
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buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
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buf.memory = V4L2_MEMORY_DMABUF;
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buf.index = index;
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buf.m.planes = planes;
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buf.length = n_planes;
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memset(&planes, 0, sizeof planes);
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for (i = 0; i < n_planes; ++i)
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buf.m.planes[i].m.fd = dmafd[i];
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if (ioctl(v4lfd, &VIDIOC-QBUF;, &buf) == -1) {
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perror("VIDIOC_QBUF");
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return -1;
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}
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return 0;
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}
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</programlisting>
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</example>
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<para>Captured or displayed buffers are dequeued with the
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&VIDIOC-DQBUF; ioctl. The driver can unlock the buffer at any
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time between the completion of the DMA and this ioctl. The memory is
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also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
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when the device is closed.</para>
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<para>For capturing applications it is customary to enqueue a
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number of empty buffers, to start capturing and enter the read loop.
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Here the application waits until a filled buffer can be dequeued, and
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re-enqueues the buffer when the data is no longer needed. Output
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applications fill and enqueue buffers, when enough buffers are stacked
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up output is started. In the write loop, when the application
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runs out of free buffers it must wait until an empty buffer can be
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dequeued and reused. Two methods exist to suspend execution of the
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application until one or more buffers can be dequeued. By default
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<constant>VIDIOC_DQBUF</constant> blocks when no buffer is in the
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outgoing queue. When the <constant>O_NONBLOCK</constant> flag was
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given to the &func-open; function, <constant>VIDIOC_DQBUF</constant>
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returns immediately with an &EAGAIN; when no buffer is available. The
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&func-select; and &func-poll; functions are always available.</para>
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<para>To start and stop capturing or displaying applications call the
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&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctls. Note that
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<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both queues and
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unlocks all buffers as a side effect. Since there is no notion of doing
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anything "now" on a multitasking system, if an application needs to synchronize
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with another event it should examine the &v4l2-buffer;
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<structfield>timestamp</structfield> of captured buffers, or set the field
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before enqueuing buffers for output.</para>
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<para>Drivers implementing DMABUF importing I/O must support the
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<constant>VIDIOC_REQBUFS</constant>, <constant>VIDIOC_QBUF</constant>,
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<constant>VIDIOC_DQBUF</constant>, <constant>VIDIOC_STREAMON</constant> and
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<constant>VIDIOC_STREAMOFF</constant> ioctls, and the
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<function>select()</function> and <function>poll()</function> functions.</para>
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</section>
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<section id="async">
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<title>Asynchronous I/O</title>
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@ -672,6 +828,14 @@ memory, set by the application. See <xref linkend="userp" /> for details.
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in the <structfield>length</structfield> field of this
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<structname>v4l2_buffer</structname> structure.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>int</entry>
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<entry><structfield>fd</structfield></entry>
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<entry>For the single-plane API and when
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<structfield>memory</structfield> is <constant>V4L2_MEMORY_DMABUF</constant> this
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is the file descriptor associated with a DMABUF buffer.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>length</structfield></entry>
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@ -743,6 +907,15 @@ should set this to 0.</entry>
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pointer to the memory allocated for this plane by an application.
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</entry>
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</row>
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<row>
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<entry></entry>
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<entry>int</entry>
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<entry><structfield>fd</structfield></entry>
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<entry>When the memory type in the containing &v4l2-buffer; is
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<constant>V4L2_MEMORY_DMABUF</constant>, this is a file
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descriptor associated with a DMABUF buffer, similar to the
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<structfield>fd</structfield> field in &v4l2-buffer;.</entry>
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</row>
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<row>
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<entry>__u32</entry>
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<entry><structfield>data_offset</structfield></entry>
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@ -964,6 +1137,12 @@ pointer</link> I/O.</entry>
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<entry>3</entry>
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<entry>[to do]</entry>
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</row>
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<row>
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<entry><constant>V4L2_MEMORY_DMABUF</constant></entry>
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<entry>4</entry>
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<entry>The buffer is used for <link linkend="dmabuf">DMA shared
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buffer</link> I/O.</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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@ -6,7 +6,8 @@
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<refnamediv>
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<refname>VIDIOC_CREATE_BUFS</refname>
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<refpurpose>Create buffers for Memory Mapped or User Pointer I/O</refpurpose>
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<refpurpose>Create buffers for Memory Mapped or User Pointer or DMA Buffer
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I/O</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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@ -55,11 +56,11 @@
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</note>
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<para>This ioctl is used to create buffers for <link linkend="mmap">memory
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mapped</link> or <link linkend="userp">user pointer</link>
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I/O. It can be used as an alternative or in addition to the
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<constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter control over buffers
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is required. This ioctl can be called multiple times to create buffers of
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different sizes.</para>
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mapped</link> or <link linkend="userp">user pointer</link> or <link
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linkend="dmabuf">DMA buffer</link> I/O. It can be used as an alternative or in
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addition to the <constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter
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control over buffers is required. This ioctl can be called multiple times to
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create buffers of different sizes.</para>
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<para>To allocate device buffers applications initialize relevant fields of
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the <structname>v4l2_create_buffers</structname> structure. They set the
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@ -109,7 +110,8 @@ information.</para>
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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_MMAP</constant>,
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<constant>V4L2_MEMORY_DMABUF</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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@ -109,6 +109,23 @@ 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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@ -48,28 +48,30 @@
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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>This ioctl is used to initiate <link linkend="mmap">memory mapped</link>,
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<link linkend="userp">user pointer</link> or <link
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linkend="dmabuf">DMABUF</link> based I/O. Memory mapped buffers are located in
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device memory and must be allocated with this ioctl before they can be mapped
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into the application's address space. User buffers are allocated by
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applications themselves, and this ioctl is merely used to switch the driver
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into user pointer I/O mode and to setup some internal structures.
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Similarly, DMABUF buffers are allocated by applications through a device
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driver, and this ioctl only configures the driver into DMABUF I/O mode without
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performing any direct allocation.</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>To allocate device buffers applications initialize all fields of the
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<structname>v4l2_requestbuffers</structname> structure. They set the
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<structfield>type</structfield> field to the respective stream or buffer type,
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the <structfield>count</structfield> field to the desired number of buffers,
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<structfield>memory</structfield> must be set to the requested I/O method and
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the <structfield>reserved</structfield> array must be zeroed. When the ioctl is
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called with a pointer to this structure the driver will attempt to allocate the
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requested number of buffers and it stores the actual number allocated in the
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<structfield>count</structfield> field. It can be smaller than the number
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requested, even zero, when the driver runs out of free memory. A larger number
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is also possible when the driver requires more buffers to function correctly.
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For example video output requires at least two buffers, one displayed and one
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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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@ -102,7 +104,8 @@ as the &v4l2-format; <structfield>type</structfield> field. See <xref
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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_MMAP</constant>,
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<constant>V4L2_MEMORY_DMABUF</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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