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-rw-r--r--Documentation/DocBook/media/v4l/io.xml228
1 files changed, 196 insertions, 32 deletions
diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
index fd6aca2922b..388a3403265 100644
--- a/Documentation/DocBook/media/v4l/io.xml
+++ b/Documentation/DocBook/media/v4l/io.xml
@@ -331,7 +331,7 @@ application until one or more buffers can be dequeued. By default
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. The
-&func-select; or &func-poll; function are always available.</para>
+&func-select; or &func-poll; functions are always available.</para>
<para>To start and stop capturing or output applications call the
&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctl. Note
@@ -472,6 +472,165 @@ rest should be evident.</para>
</footnote></para>
</section>
+ <section id="dmabuf">
+ <title>Streaming I/O (DMA buffer importing)</title>
+
+ <note>
+ <title>Experimental</title>
+ <para>This is an <link linkend="experimental"> experimental </link>
+ interface and may change in the future.</para>
+ </note>
+
+<para>The DMABUF framework provides a generic method for sharing buffers
+between multiple devices. Device drivers that support DMABUF can export a DMA
+buffer to userspace as a file descriptor (known as the exporter role), import a
+DMA buffer from userspace using a file descriptor previously exported for a
+different or the same device (known as the importer role), or both. This
+section describes the DMABUF importer role API in V4L2.</para>
+
+ <para>Refer to <link linked="vidioc-expbuf"> DMABUF exporting </link> for
+details about exporting V4L2 buffers as DMABUF file descriptors.</para>
+
+<para>Input and output devices support the streaming I/O method when the
+<constant>V4L2_CAP_STREAMING</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability; returned by
+the &VIDIOC-QUERYCAP; ioctl is set. Whether importing DMA buffers through
+DMABUF file descriptors is supported is determined by calling the
+&VIDIOC-REQBUFS; ioctl with the memory type set to
+<constant>V4L2_MEMORY_DMABUF</constant>.</para>
+
+ <para>This I/O method is dedicated to sharing DMA buffers between different
+devices, which may be V4L devices or other video-related devices (e.g. DRM).
+Buffers (planes) are allocated by a driver on behalf of an application. Next,
+these buffers are exported to the application as file descriptors using an API
+which is specific for an allocator driver. Only such file descriptor are
+exchanged. The descriptors and meta-information are passed in &v4l2-buffer; (or
+in &v4l2-plane; in the multi-planar API case). The driver must be switched
+into DMABUF I/O mode by calling the &VIDIOC-REQBUFS; with the desired buffer
+type.</para>
+
+ <example>
+ <title>Initiating streaming I/O with DMABUF file descriptors</title>
+
+ <programlisting>
+&v4l2-requestbuffers; reqbuf;
+
+memset(&amp;reqbuf, 0, sizeof (reqbuf));
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_DMABUF;
+reqbuf.count = 1;
+
+if (ioctl(fd, &VIDIOC-REQBUFS;, &amp;reqbuf) == -1) {
+ if (errno == EINVAL)
+ printf("Video capturing or DMABUF streaming is not supported\n");
+ else
+ perror("VIDIOC_REQBUFS");
+
+ exit(EXIT_FAILURE);
+}
+ </programlisting>
+ </example>
+
+ <para>The buffer (plane) file descriptor is passed on the fly with the
+&VIDIOC-QBUF; ioctl. In case of multiplanar buffers, every plane can be
+associated with a different DMABUF descriptor. Although buffers are commonly
+cycled, applications can pass a different DMABUF descriptor at each
+<constant>VIDIOC_QBUF</constant> call.</para>
+
+ <example>
+ <title>Queueing DMABUF using single plane API</title>
+
+ <programlisting>
+int buffer_queue(int v4lfd, int index, int dmafd)
+{
+ &v4l2-buffer; buf;
+
+ memset(&amp;buf, 0, sizeof buf);
+ buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+ buf.memory = V4L2_MEMORY_DMABUF;
+ buf.index = index;
+ buf.m.fd = dmafd;
+
+ if (ioctl(v4lfd, &VIDIOC-QBUF;, &amp;buf) == -1) {
+ perror("VIDIOC_QBUF");
+ return -1;
+ }
+
+ return 0;
+}
+ </programlisting>
+ </example>
+
+ <example>
+ <title>Queueing DMABUF using multi plane API</title>
+
+ <programlisting>
+int buffer_queue_mp(int v4lfd, int index, int dmafd[], int n_planes)
+{
+ &v4l2-buffer; buf;
+ &v4l2-plane; planes[VIDEO_MAX_PLANES];
+ int i;
+
+ memset(&amp;buf, 0, sizeof buf);
+ buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
+ buf.memory = V4L2_MEMORY_DMABUF;
+ buf.index = index;
+ buf.m.planes = planes;
+ buf.length = n_planes;
+
+ memset(&amp;planes, 0, sizeof planes);
+
+ for (i = 0; i &lt; n_planes; ++i)
+ buf.m.planes[i].m.fd = dmafd[i];
+
+ if (ioctl(v4lfd, &VIDIOC-QBUF;, &amp;buf) == -1) {
+ perror("VIDIOC_QBUF");
+ return -1;
+ }
+
+ return 0;
+}
+ </programlisting>
+ </example>
+
+ <para>Captured or displayed buffers are dequeued with the
+&VIDIOC-DQBUF; ioctl. The driver can unlock the buffer at any
+time between the completion of the DMA and this ioctl. The memory is
+also unlocked when &VIDIOC-STREAMOFF; is called, &VIDIOC-REQBUFS;, or
+when the device is closed.</para>
+
+ <para>For capturing applications it is customary to enqueue a
+number of empty buffers, to start capturing and enter the read loop.
+Here the application waits until a filled buffer can be dequeued, and
+re-enqueues the buffer when the data is no longer needed. Output
+applications fill and enqueue buffers, when enough buffers are stacked
+up output is started. In the write loop, when the application
+runs out of free buffers it must wait until an empty buffer can be
+dequeued and reused. Two methods exist to suspend execution of the
+application until one or more buffers can be dequeued. 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. The
+&func-select; and &func-poll; functions are always available.</para>
+
+ <para>To start and stop capturing or displaying applications call the
+&VIDIOC-STREAMON; and &VIDIOC-STREAMOFF; ioctls. Note that
+<constant>VIDIOC_STREAMOFF</constant> removes all buffers from both queues and
+unlocks all buffers as a side effect. Since there is no notion of doing
+anything "now" on a multitasking system, if an application needs to synchronize
+with another event it should examine the &v4l2-buffer;
+<structfield>timestamp</structfield> of captured buffers, or set the field
+before enqueuing buffers for output.</para>
+
+ <para>Drivers implementing DMABUF importing I/O must support the
+<constant>VIDIOC_REQBUFS</constant>, <constant>VIDIOC_QBUF</constant>,
+<constant>VIDIOC_DQBUF</constant>, <constant>VIDIOC_STREAMON</constant> and
+<constant>VIDIOC_STREAMOFF</constant> ioctls, and the
+<function>select()</function> and <function>poll()</function> functions.</para>
+
+ </section>
+
<section id="async">
<title>Asynchronous I/O</title>
@@ -613,8 +772,8 @@ field is independent of the <structfield>timestamp</structfield> and
<entry>__u32</entry>
<entry><structfield>sequence</structfield></entry>
<entry></entry>
- <entry>Set by the driver, counting the frames in the
-sequence.</entry>
+ <entry>Set by the driver, counting the frames (not fields!) in
+sequence. This field is set for both input and output devices.</entry>
</row>
<row>
<entry spanname="hspan"><para>In <link
@@ -673,32 +832,36 @@ memory, set by the application. See <xref linkend="userp" /> for details.
<structname>v4l2_buffer</structname> structure.</entry>
</row>
<row>
+ <entry></entry>
+ <entry>int</entry>
+ <entry><structfield>fd</structfield></entry>
+ <entry>For the single-plane API and when
+<structfield>memory</structfield> is <constant>V4L2_MEMORY_DMABUF</constant> this
+is the file descriptor associated with a DMABUF buffer.</entry>
+ </row>
+ <row>
<entry>__u32</entry>
<entry><structfield>length</structfield></entry>
<entry></entry>
<entry>Size of the buffer (not the payload) in bytes for the
- single-planar API. For the multi-planar API should contain the
- number of elements in the <structfield>planes</structfield> array.
+ single-planar API. For the multi-planar API the application sets
+ this to the number of elements in the <structfield>planes</structfield>
+ array. The driver will fill in the actual number of valid elements in
+ that array.
</entry>
</row>
<row>
<entry>__u32</entry>
- <entry><structfield>input</structfield></entry>
+ <entry><structfield>reserved2</structfield></entry>
<entry></entry>
- <entry>Some video capture drivers support rapid and
-synchronous video input changes, a function useful for example in
-video surveillance applications. For this purpose applications set the
-<constant>V4L2_BUF_FLAG_INPUT</constant> flag, and this field to the
-number of a video input as in &v4l2-input; field
-<structfield>index</structfield>.</entry>
+ <entry>A place holder for future extensions. Applications
+should set this to 0.</entry>
</row>
<row>
<entry>__u32</entry>
<entry><structfield>reserved</structfield></entry>
<entry></entry>
- <entry>A place holder for future extensions and custom
-(driver defined) buffer types
-<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher. Applications
+ <entry>A place holder for future extensions. Applications
should set this to 0.</entry>
</row>
</tbody>
@@ -748,6 +911,15 @@ should set this to 0.</entry>
</entry>
</row>
<row>
+ <entry></entry>
+ <entry>int</entry>
+ <entry><structfield>fd</structfield></entry>
+ <entry>When the memory type in the containing &v4l2-buffer; is
+ <constant>V4L2_MEMORY_DMABUF</constant>, this is a file
+ descriptor associated with a DMABUF buffer, similar to the
+ <structfield>fd</structfield> field in &v4l2-buffer;.</entry>
+ </row>
+ <row>
<entry>__u32</entry>
<entry><structfield>data_offset</structfield></entry>
<entry></entry>
@@ -829,14 +1001,7 @@ should set this to 0.</entry>
<entry><constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant></entry>
<entry>8</entry>
<entry>Buffer for video output overlay (OSD), see <xref
- linkend="osd" />. Status: <link
-linkend="experimental">Experimental</link>.</entry>
- </row>
- <row>
- <entry><constant>V4L2_BUF_TYPE_PRIVATE</constant></entry>
- <entry>0x80</entry>
- <entry>This and higher values are reserved for custom
-(driver defined) buffer types.</entry>
+ linkend="osd" />.</entry>
</row>
</tbody>
</tgroup>
@@ -923,13 +1088,6 @@ Drivers set or clear this flag when the <constant>VIDIOC_DQBUF</constant>
ioctl is called.</entry>
</row>
<row>
- <entry><constant>V4L2_BUF_FLAG_INPUT</constant></entry>
- <entry>0x0200</entry>
- <entry>The <structfield>input</structfield> field is valid.
-Applications set or clear this flag before calling the
-<constant>VIDIOC_QBUF</constant> ioctl.</entry>
- </row>
- <row>
<entry><constant>V4L2_BUF_FLAG_PREPARED</constant></entry>
<entry>0x0400</entry>
<entry>The buffer has been prepared for I/O and can be queued by the
@@ -941,7 +1099,7 @@ application. Drivers set or clear this flag when the
</row>
<row>
<entry><constant>V4L2_BUF_FLAG_NO_CACHE_INVALIDATE</constant></entry>
- <entry>0x0400</entry>
+ <entry>0x0800</entry>
<entry>Caches do not have to be invalidated for this buffer.
Typically applications shall use this flag if the data captured in the buffer
is not going to be touched by the CPU, instead the buffer will, probably, be
@@ -950,7 +1108,7 @@ passed on to a DMA-capable hardware unit for further processing or output.
</row>
<row>
<entry><constant>V4L2_BUF_FLAG_NO_CACHE_CLEAN</constant></entry>
- <entry>0x0800</entry>
+ <entry>0x1000</entry>
<entry>Caches do not have to be cleaned for this buffer.
Typically applications shall use this flag for output buffers if the data
in this buffer has not been created by the CPU but by some DMA-capable unit,
@@ -982,6 +1140,12 @@ pointer</link> I/O.</entry>
<entry>3</entry>
<entry>[to do]</entry>
</row>
+ <row>
+ <entry><constant>V4L2_MEMORY_DMABUF</constant></entry>
+ <entry>4</entry>
+ <entry>The buffer is used for <link linkend="dmabuf">DMA shared
+buffer</link> I/O.</entry>
+ </row>
</tbody>
</tgroup>
</table>