diff --git a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
index 0a4b90f..c07c657 100644
--- a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
@@ -199,13 +199,50 @@ also be zero.</entry>
 	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>error_idx</structfield></entry>
-	    <entry>Set by the driver in case of an error. If it is equal
-to <structfield>count</structfield>, then no actual changes were made to
-controls. In other words, the error was not associated with setting a particular
-control. If it is another value, then only the controls up to <structfield>error_idx-1</structfield>
-were modified and control <structfield>error_idx</structfield> is the one that
-caused the error. The <structfield>error_idx</structfield> value is undefined
-if the ioctl returned 0 (success).</entry>
+	    <entry><para>Set by the driver in case of an error. If the error is
+associated with a particular control, then <structfield>error_idx</structfield>
+is set to the index of that control. If the error is not related to a specific
+control, then <structfield>error_idx</structfield> is set to <structfield>count</structfield>.</para>
+
+<para>The behavior is different for <constant>VIDIOC_G_EXT_CTRLS</constant> and
+<constant>VIDIOC_S_EXT_CTRLS</constant>: if
+<structfield>error_idx</structfield> is equal to <structfield>count</structfield>,
+then no actual changes were made to the controls. For example, if you try to
+write to a read-only control, then <constant>VIDIOC_TRY_EXT_CTRLS</constant>
+will set <structfield>error_idx</structfield> to the index of that read-only
+control, but <constant>VIDIOC_S_EXT_CTRLS</constant> will set
+<structfield>error_idx</structfield> to <structfield>count</structfield> instead
+and none of the controls in the list will be set.</para>
+
+<para>The same is true when trying to e.g. read a write-only control:
+<constant>VIDIOC_G_EXT_CTRLS</constant> will set <structfield>error_idx</structfield>
+to <structfield>count</structfield> and none of the controls in the list will
+have been retrieved.</para>
+
+<para>The reason for this behavior is that it is important when setting and getting
+controls to do this as atomically as possible, so simple sanity checks like testing
+for read-only controls are done first before an attempt is made to apply/retrieve the new
+control values to/from the hardware. It is important for an application to know whether
+<constant>VIDIOC_S_EXT_CTRLS</constant> or <constant>VIDIOC_G_EXT_CTRLS</constant> actually
+made changes to controls or not. So if <structfield>error_idx</structfield> is equal
+to <structfield>count</structfield>, then you know that no actual controls were set or
+retrieved. In the case of <constant>VIDIOC_S_EXT_CTRLS</constant> you can call
+<constant>VIDIOC_TRY_EXT_CTRLS</constant> with the same control list to see if the
+problem was with a specific control or not (<constant>VIDIOC_TRY_EXT_CTRLS</constant>
+never modifies controls, so that ioctl will just set <structfield>error_idx</structfield>
+to the control that caused the problem). No such option exists for <constant>VIDIOC_G_EXT_CTRLS</constant>
+though, unfortunately.</para>
+
+<para>If <structfield>error_idx</structfield> as returned by <constant>VIDIOC_S_EXT_CTRLS</constant>
+or <constant>VIDIOC_G_EXT_CTRLS</constant> is less than <structfield>count</structfield>,
+then only the controls up to <structfield>error_idx-1</structfield> were modified and control
+<structfield>error_idx</structfield> is the one that caused the error. In the case of
+<constant>VIDIOC_S_EXT_CTRLS</constant> this might have left the hardware in an
+inconsistent state. These types of errors should not normally happen, and such
+errors are typically caused by problems in communicating with the hardware.</para>
+
+<para>Finally, note that the <structfield>error_idx</structfield> value is undefined
+if the ioctl returned 0 (success).</para></entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
