[media] Docbook/media: improve data_offset/bytesused documentation

This patch explicitly documents the relationship between bytesused and data_offset.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
index 8c4ee746a73159f1c1a26fe5f08e486f9ce6a64f..e5e8325aa3d781095c0851d8419b57ccdcb5608a 100644 (file)
--- a/Documentation/DocBook/media/v4l/io.xml
+++ b/Documentation/DocBook/media/v4l/io.xml
@@ -870,7 +870,8 @@ should set this to 0.</entry>
              If the application sets this to 0 for an output stream, then
              <structfield>bytesused</structfield> will be set to the size of the
              plane (see the <structfield>length</structfield> field of this struct)
-             by the driver.</entry>
+             by the driver. Note that the actual image data starts at
+             <structfield>data_offset</structfield> which may not be 0.</entry>
          </row>
          <row>
            <entry>__u32</entry>
@@ -919,6 +920,10 @@ should set this to 0.</entry>
            <entry>Offset in bytes to video data in the plane.
              Drivers must set this field when <structfield>type</structfield>
              refers to an input stream, applications when it refers to an output stream.
+             Note that data_offset is included in <structfield>bytesused</structfield>.
+             So the size of the image in the plane is
+             <structfield>bytesused</structfield>-<structfield>data_offset</structfield> at
+             offset <structfield>data_offset</structfield> from the start of the plane.
            </entry>
          </row>
          <row>