[media] v4l: Document timestamp buffer flag behaviour

Timestamp buffer flags are constant at the moment. Document them so that 1)
they're always valid and 2) not changed by the drivers. This leaves room to
extend the functionality later on if needed.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/io.xml

@@ -653,6 +653,20 @@ plane, are stored in struct <structname>v4l2_plane</structname> instead.
 In that case, struct <structname>v4l2_buffer</structname> contains an array of
 plane structures.</para>
 
+    <para>Dequeued video buffers come with timestamps. The driver
+    decides at which part of the frame and with which clock the
+    timestamp is taken. Please see flags in the masks
+    <constant>V4L2_BUF_FLAG_TIMESTAMP_MASK</constant> and
+    <constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant> in <xref
+    linkend="buffer-flags" />. These flags are always valid and constant
+    across all buffers during the whole video stream. Changes in these
+    flags may take place as a side effect of &VIDIOC-S-INPUT; or
+    &VIDIOC-S-OUTPUT; however. The
+    <constant>V4L2_BUF_FLAG_TIMESTAMP_COPY</constant> timestamp type
+    which is used by e.g. on mem-to-mem devices is an exception to the
+    rule: the timestamp source flags are copied from the OUTPUT video
+    buffer to the CAPTURE video buffer.</para>
+
     <table frame="none" pgwide="1" id="v4l2-buffer">
       <title>struct <structname>v4l2_buffer</structname></title>
       <tgroup cols="4">