[media] DocBook: media: Document ALPHA_COMPONENT control usage on output devices

Extend the V4L2_CID_ALPHA_COMPONENT control for use on output devices,
to set the alpha component value when the output format doesn't have an
alpha channel.

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

1 files changed, 10 insertions, 7 deletions

diff --git a/Documentation/DocBook/media/v4l/controls.xml b/Documentation/DocBook/media/v4l/controls.xml
index cc0087efc8d2..74f575560f1b 100644
--- a/Documentation/DocBook/media/v4l/controls.xml
+++ b/Documentation/DocBook/media/v4l/controls.xml
@@ -408,14 +408,17 @@ to work.</entry>
           <row id="v4l2-alpha-component">
             <entry><constant>V4L2_CID_ALPHA_COMPONENT</constant></entry>
             <entry>integer</entry>
-            <entry> Sets the alpha color component on the capture device or on
-            the capture buffer queue of a mem-to-mem device. When a mem-to-mem
-            device produces frame format that includes an alpha component
+            <entry>Sets the alpha color component. When a capture device (or
+            capture queue of a mem-to-mem device) produces a frame format that
+            includes an alpha component
             (e.g. <link linkend="rgb-formats">packed RGB image formats</link>)
-            and the alpha value is not defined by the mem-to-mem input data
-            this control lets you select the alpha component value of all
-            pixels. It is applicable to any pixel format that contains an alpha
-            component.
+            and the alpha value is not defined by the device or the mem-to-mem
+            input data this control lets you select the alpha component value of
+            all pixels. When an output device (or output queue of a mem-to-mem
+            device) consumes a frame format that doesn't include an alpha
+            component and the device supports alpha channel processing this
+            control lets you set the alpha component value of all pixels for
+            further processing in the device.
             </entry>
           </row>
           <row>