V4L/DVB: media-spec: Fix documentation mistakes regarding I/O streaming
The media spec contains several errors in the description of the
I/O streaming ioctls, in particular with respect to the userptr
I/O method.
The most important change is that you really need to set count
and index in v4l2_requestbuffer and v4l2_buffer when dealing with
user pointer streaming.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/Documentation/DocBook/v4l/vidioc-reqbufs.xml b/Documentation/DocBook/v4l/vidioc-reqbufs.xml
index bab3808..1c08163 100644
--- a/Documentation/DocBook/v4l/vidioc-reqbufs.xml
+++ b/Documentation/DocBook/v4l/vidioc-reqbufs.xml
@@ -54,23 +54,23 @@
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.</para>
+driver into user pointer I/O mode and to setup some internal structures.</para>
- <para>To allocate device buffers applications initialize three
-fields of a <structname>v4l2_requestbuffers</structname> structure.
+ <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, and <structfield>memory</structfield>
-must be set to <constant>V4L2_MEMORY_MMAP</constant>. When the ioctl
-is called with a pointer to this structure the driver attempts to
-allocate the requested number of buffers and stores the actual number
+the desired number of buffers, <structfield>memory</structfield>
+must be set to the requested I/O method and the reserved 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 possible when the driver requires
-more buffers to function correctly.<footnote>
- <para>For example video output requires at least two buffers,
+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>
- </footnote> When memory mapping I/O is not supported the ioctl
+ <para>When the I/O method is not supported the ioctl
returns an &EINVAL;.</para>
<para>Applications can call <constant>VIDIOC_REQBUFS</constant>
@@ -81,14 +81,6 @@
reason why munmap()ping one or even all buffers must imply
streamoff.--></para>
- <para>To negotiate user pointer I/O, applications initialize only
-the <structfield>type</structfield> field and set
-<structfield>memory</structfield> to
-<constant>V4L2_MEMORY_USERPTR</constant>. When the ioctl is called
-with a pointer to this structure the driver prepares for user pointer
-I/O, when this I/O method is not supported the ioctl returns an
-&EINVAL;.</para>
-
<table pgwide="1" frame="none" id="v4l2-requestbuffers">
<title>struct <structname>v4l2_requestbuffers</structname></title>
<tgroup cols="3">
@@ -97,9 +89,7 @@
<row>
<entry>__u32</entry>
<entry><structfield>count</structfield></entry>
- <entry>The number of buffers requested or granted. This
-field is only used when <structfield>memory</structfield> is set to
-<constant>V4L2_MEMORY_MMAP</constant>.</entry>
+ <entry>The number of buffers requested or granted.</entry>
</row>
<row>
<entry>&v4l2-buf-type;</entry>
@@ -120,7 +110,7 @@
<entry><structfield>reserved</structfield>[2]</entry>
<entry>A place holder for future extensions and custom
(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
-higher.</entry>
+higher. This array should be zeroed by applications.</entry>
</row>
</tbody>
</tgroup>