[media] V4L2 Spec: fix extended control documentation

Update the spec to the behavior implemented by the control framework.
This should have been documented long ago but for some reason it was
never done.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
index 6f1f9a629dc3..b17a7aac6997 100644
--- a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
@@ -183,7 +183,12 @@ applications must set the array to zero.</entry>
             <entry>__u32</entry>
             <entry><structfield>ctrl_class</structfield></entry>
             <entry>The control class to which all controls belong, see
-<xref linkend="ctrl-class" />.</entry>
+<xref linkend="ctrl-class" />. Drivers that use a kernel framework for handling
+controls will also accept a value of 0 here, meaning that the controls can
+belong to any control class. Whether drivers support this can be tested by setting
+<structfield>ctrl_class</structfield> to 0 and calling <constant>VIDIOC_TRY_EXT_CTRLS</constant>
+with a <structfield>count</structfield> of 0. If that succeeds, then the driver
+supports this feature.</entry>
           </row>
           <row>
             <entry>__u32</entry>
@@ -194,10 +199,13 @@ also be zero.</entry>
           <row>
             <entry>__u32</entry>
             <entry><structfield>error_idx</structfield></entry>
-            <entry>Set by the driver in case of an error. It is the
-index of the control causing the error or equal to 'count' when the
-error is not associated with a particular control. Undefined when the
-ioctl returns 0 (success).</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>
           </row>
           <row>
             <entry>__u32</entry>