[media] media/Documentation: New flag EXECUTE_ON_WRITE

Document new flag V4L2_CTRL_FLAG_EXECUTE_ON_WRITE, and the new behavior
of CH_VALUE event on VOLATILE controls.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/v4l/vidioc-dqevent.xml

@@ -425,9 +425,9 @@
 	    <entry><constant>V4L2_EVENT_CTRL_CH_VALUE</constant></entry>
 	    <entry>0x0001</entry>
 	    <entry>This control event was triggered because the value of the control
-		changed. Special case: if a button control is pressed, then this
-		event is sent as well, even though there is not explicit value
-		associated with a button control.</entry>
+		changed. Special cases: Volatile controls do no generate this event;
+		If a control has the <constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant>
+		flag set, then this event is sent as well, regardless its value.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_EVENT_CTRL_CH_FLAGS</constant></entry>

Documentation/DocBook/media/v4l/vidioc-queryctrl.xml

@@ -600,7 +600,9 @@ writing a value will cause the device to carry out a given action
 changes continuously. A typical example would be the current gain value if the device
 is in auto-gain mode. In such a case the hardware calculates the gain value based on
 the lighting conditions which can change over time. Note that setting a new value for
-a volatile control will have no effect. The new value will just be ignored.</entry>
+a volatile control will have no effect and no <constant>V4L2_EVENT_CTRL_CH_VALUE</constant>
+will be sent, unless the <constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant> flag
+(see below) is also set. Otherwise the new value will just be ignored.</entry>
 	  </row>
 	  <row>
 	    <entry><constant>V4L2_CTRL_FLAG_HAS_PAYLOAD</constant></entry>
@@ -610,6 +612,14 @@ using one of the pointer fields of &v4l2-ext-control;. This flag is set for cont
 that are an array, string, or have a compound type. In all cases you have to set a
 pointer to memory containing the payload of the control.</entry>
 	  </row>
+	  <row>
+	    <entry><constant>V4L2_CTRL_FLAG_EXECUTE_ON_WRITE</constant></entry>
+	    <entry>0x0200</entry>
+	    <entry>The value provided to the control will be propagated to the driver
+even if remains constant. This is required when the control represents an action
+on the hardware. For example: clearing an error flag or triggering the flash. All the
+controls of the type <constant>V4L2_CTRL_TYPE_BUTTON</constant> have this flag set.</entry>
+	  </row>
 	</tbody>
       </tgroup>
     </table>

Documentation/video4linux/v4l2-controls.txt

@@ -344,7 +344,9 @@ implement g_volatile_ctrl like this:
 	}
 
 Note that you use the 'new value' union as well in g_volatile_ctrl. In general
-controls that need to implement g_volatile_ctrl are read-only controls.
+controls that need to implement g_volatile_ctrl are read-only controls. If they
+are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
+changes.
 
 To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE: