Re: [PATCHv9 02/25] Documentation: media: description of DMABUF importing in V4L2

5 Oct 2012

Hi Tomasz,
Just a few more stylistic issues...
On Tue October 2 2012 16:27:13 Tomasz Stanislawski wrote:
...
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
CC: linux-doc@vger.kernel.org

Documentation/DocBook/media/v4l/compat.xml         |    4 +
 Documentation/DocBook/media/v4l/io.xml             |  180 +++++++++++++++++++-
 .../DocBook/media/v4l/vidioc-create-bufs.xml       |   16 +-
 Documentation/DocBook/media/v4l/vidioc-qbuf.xml    |   17 ++
 Documentation/DocBook/media/v4l/vidioc-reqbufs.xml |   47 ++---
 5 files changed, 234 insertions(+), 30 deletions(-)

diff --git a/Documentation/DocBook/media/v4l/compat.xml b/Documentation/DocBook/media/v4l/compat.xml
index faa0fd1..a46f95b 100644
--- a/Documentation/DocBook/media/v4l/compat.xml
+++ b/Documentation/DocBook/media/v4l/compat.xml
@@ -2621,6 +2621,10 @@ ioctls.</para>
         <listitem>
     <para>Support for frequency band enumeration: &VIDIOC-ENUM-FREQ-BANDS; ioctl.</para>
         </listitem>

   <listitem>


 <para>Importing DMABUF file descriptors as a new IO method described


 in <xref linkend="dmabuf" />.</para>


   </listitem>
 </itemizedlist>

   </section>


diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
index 1885cc0..5b58657 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 +472,161 @@ 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>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 for sharing DMA buffers between V4L and

+other APIs.
It's more accurate to say something along the lines of: "...is dedicated to sharing
DMA buffers between different devices, which may be V4L devices or other
video-related devices (e.g. DRM)."
One of the uses of this API is to share buffers between different V4L devices, so
that's a bit more generic than the original sentence suggests.
...
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));
no space after memset
...
+reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
+reqbuf.memory = V4L2_MEMORY_DMABUF;
+reqbuf.count = 1;



+if (ioctl (fd, &VIDIOC-REQBUFS;, &amp;reqbuf) == -1) {
no space after ioctl
...

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>Filled or displayed buffers are dequeued with the

Filled -> Captured
...
+&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; or &func-poll; function are always available.</para>
or -> and
function -> functions
...


<para>To start and stop capturing or output applications call the

output -> displaying
...
+&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>


@@ -673,6 +828,14 @@ 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>



@@ -746,6 +909,15 @@ 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>



@@ -973,6 +1145,12 @@ 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>


diff --git a/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml b/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
index a8cda1a..cd99436 100644
--- a/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
@@ -6,7 +6,8 @@
   <refnamediv>
     <refname>VIDIOC_CREATE_BUFS</refname>
-    <refpurpose>Create buffers for Memory Mapped or User Pointer I/O</refpurpose>
+    <refpurpose>Create buffers for Memory Mapped or User Pointer or DMA Buffer
+    I/O</refpurpose>
   </refnamediv>
   <refsynopsisdiv>
@@ -55,11 +56,11 @@
     </note>
 <para>This ioctl is used to create buffers for <link linkend="mmap">memory

-mapped</link> or <link linkend="userp">user pointer</link>
-I/O. It can be used as an alternative or in addition to the
-<constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter control over buffers
-is required. This ioctl can be called multiple times to create buffers of
-different sizes.</para>
+mapped</link> or <link linkend="userp">user pointer</link> or <link
+linkend="dmabuf">DMA buffer</link> I/O. It can be used as an alternative or in
+addition to the <constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter
+control over buffers is required. This ioctl can be called multiple times to
+create buffers of different sizes.</para>
 <para>To allocate device buffers applications initialize relevant fields of

the <structname>v4l2_create_buffers</structname> structure. They set the
@@ -109,7 +110,8 @@ information.</para>
       <entry>__u32</entry>
       <entry><structfield>memory</structfield></entry>
       <entry>Applications set this field to
-<constant>V4L2_MEMORY_MMAP</constant> or
+<constant>V4L2_MEMORY_MMAP</constant>,
+<constant>V4L2_MEMORY_DMABUF</constant> or
 <constant>V4L2_MEMORY_USERPTR</constant>. See <xref linkend="v4l2-memory"
 /></entry>
     </row>
diff --git a/Documentation/DocBook/media/v4l/vidioc-qbuf.xml b/Documentation/DocBook/media/v4l/vidioc-qbuf.xml
index 77ff5be..f40dfa0 100644
--- a/Documentation/DocBook/media/v4l/vidioc-qbuf.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-qbuf.xml
@@ -109,6 +109,23 @@ 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>To enqueue a <link linkend="dmabuf">DMABUF</link> buffer applications

+set the <structfield>memory</structfield> field to
+<constant>V4L2_MEMORY_DMABUF</constant> and the <structfield>m.fd</structfield>
+field to a file descriptor associated with a DMABUF buffer. When the
+multi-planar API is used the <structfield>m.fd</structfield> fields 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 buffer. Locking a buffer means passing it to a driver for an
+access by hardware.
"an access by hardware" -> "hardware access (usually DMA)"
...
If an application accesses (reads/writes) a locked buffer
+then the result is undefined.  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
diff --git a/Documentation/DocBook/media/v4l/vidioc-reqbufs.xml b/Documentation/DocBook/media/v4l/vidioc-reqbufs.xml
index d7c9505..20f4323 100644
--- a/Documentation/DocBook/media/v4l/vidioc-reqbufs.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-reqbufs.xml
@@ -48,28 +48,30 @@
   <refsect1>
     <title>Description</title>

<para>This ioctl is used to initiate <link linkend="mmap">memory

-mapped</link> or <link linkend="userp">user pointer</link>
-I/O. Memory mapped buffers are located in device memory and must be
-allocated with this ioctl before they can be mapped into the
-application's address space. User buffers are allocated by
-applications themselves, and this ioctl is merely used to switch the
-driver into user pointer I/O mode and to setup some internal structures.</para>
+<para>This ioctl is used to initiate <link linkend="mmap">memory mapped</link>,
+<link linkend="userp">user pointer</link> or <link
+linkend="dmabuf">DMABUF</link> based I/O.  Memory mapped buffers are located in
+device memory and must be allocated with this ioctl before they can be mapped
+into the application's address space. User buffers are allocated by
+applications themselves, and this ioctl is merely used to switch the driver
+into user pointer I/O mode and to setup some internal structures.
+Similarly, DMABUF buffers are allocated by applications through a device
+driver, and this ioctl only configures the driver into DMABUF I/O mode without
+performing any direct allocation.</para>

<para>To allocate device buffers applications initialize all

-fields of the <structname>v4l2_requestbuffers</structname> structure.
-They set the <structfield>type</structfield> field to the respective
-stream or buffer type, the <structfield>count</structfield> field to
-the desired number of buffers, <structfield>memory</structfield>
-must be set to the requested I/O method and the <structfield>reserved</structfield> array
-must be zeroed. When the ioctl
-is called with a pointer to this structure the driver will attempt to allocate
-the requested number of buffers and it stores the actual number
-allocated in the <structfield>count</structfield> field. It can be
-smaller than the number requested, even zero, when the driver runs out
-of free memory. A larger number is also possible when the driver requires
-more buffers to function correctly. For example video output requires at least two buffers,
-one displayed and one filled by the application.</para>

<para>To allocate device buffers applications initialize all fields of the

+<structname>v4l2_requestbuffers</structname> structure.  They set the
+<structfield>type</structfield> field to the respective stream or buffer type,
+the <structfield>count</structfield> field to the desired number of buffers,
+<structfield>memory</structfield> must be set to the requested I/O method and
+the <structfield>reserved</structfield> array must be zeroed. When the ioctl is
+called with a pointer to this structure the driver will attempt to allocate the
+requested number of buffers and it stores the actual number allocated in the
+<structfield>count</structfield> field. It can be smaller than the number
+requested, even zero, when the driver runs out of free memory. A larger number
+is also possible when the driver requires more buffers to function correctly.
+For example video output requires at least two buffers, one displayed and one
+filled by the application.</para>
     <para>When the I/O method is not supported the ioctl
 returns an &EINVAL;.</para>
@@ -102,7 +104,8 @@ as the &v4l2-format; <structfield>type</structfield> field. See <xref
       <entry>__u32</entry>
       <entry><structfield>memory</structfield></entry>
       <entry>Applications set this field to
-<constant>V4L2_MEMORY_MMAP</constant> or
+<constant>V4L2_MEMORY_MMAP</constant>,
+<constant>V4L2_MEMORY_DMABUF</constant> or
 <constant>V4L2_MEMORY_USERPTR</constant>. See <xref linkend="v4l2-memory"
 />.</entry>
     </row>
Regards,
Hans

    

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Re: [PATCHv9 02/25] Documentation: media: description of DMABUF importing in V4L2