[media] DocBook media: make explicit that standard/timings never change automatically

A driver might detect a new standard or DV timings, but it will never change to
those new timings automatically. Instead it will send an event and let the application
take care of it.

Make this explicit in the documentation.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

2 files changed, 22 insertions, 2 deletions

diff --git a/Documentation/DocBook/media/v4l/vidioc-query-dv-timings.xml b/Documentation/DocBook/media/v4l/vidioc-query-dv-timings.xml
index e9c70a8f3476..0c93677d16b4 100644
--- a/Documentation/DocBook/media/v4l/vidioc-query-dv-timings.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-query-dv-timings.xml
@@ -60,9 +60,19 @@ input</refpurpose>
 automatically, similar to sensing the video standard. To do so, applications
 call <constant>VIDIOC_QUERY_DV_TIMINGS</constant> with a pointer to a
 &v4l2-dv-timings;. Once the hardware detects the timings, it will fill in the
-timings structure.
+timings structure.</para>
 
-If the timings could not be detected because there was no signal, then
+<para>Please note that drivers shall <emphasis>not</emphasis> switch timings automatically
+if new timings are detected. Instead, drivers should send the
+<constant>V4L2_EVENT_SOURCE_CHANGE</constant> event (if they support this) and expect
+that userspace will take action by calling <constant>VIDIOC_QUERY_DV_TIMINGS</constant>.
+The reason is that new timings usually mean different buffer sizes as well, and you
+cannot change buffer sizes on the fly. In general, applications that receive the
+Source Change event will have to call <constant>VIDIOC_QUERY_DV_TIMINGS</constant>,
+and if the detected timings are valid they will have to stop streaming, set the new
+timings, allocate new buffers and start streaming again.</para>
+
+<para>If the timings could not be detected because there was no signal, then
 <errorcode>ENOLINK</errorcode> is returned. If a signal was detected, but
 it was unstable and the receiver could not lock to the signal, then
 <errorcode>ENOLCK</errorcode> is returned. If the receiver could lock to the signal,
diff --git a/Documentation/DocBook/media/v4l/vidioc-querystd.xml b/Documentation/DocBook/media/v4l/vidioc-querystd.xml
index 222348542182..3ceae35fab03 100644
--- a/Documentation/DocBook/media/v4l/vidioc-querystd.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-querystd.xml
@@ -59,6 +59,16 @@ then the driver will return V4L2_STD_UNKNOWN. When detection is not
 possible or fails, the set must contain all standards supported by the
 current video input or output.</para>
 
+<para>Please note that drivers shall <emphasis>not</emphasis> switch the video standard
+automatically if a new video standard is detected. Instead, drivers should send the
+<constant>V4L2_EVENT_SOURCE_CHANGE</constant> event (if they support this) and expect
+that userspace will take action by calling <constant>VIDIOC_QUERYSTD</constant>.
+The reason is that a new video standard can mean different buffer sizes as well, and you
+cannot change buffer sizes on the fly. In general, applications that receive the
+Source Change event will have to call <constant>VIDIOC_QUERYSTD</constant>,
+and if the detected video standard is valid they will have to stop streaming, set the new
+standard, allocate new buffers and start streaming again.</para>
+
   </refsect1>
 
   <refsect1>