[RFC 19/22] DocBook: media: Document the subdev selection API

[Sakari Ailus <sakari.ailus@linux.intel.com>]

From: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

Now that the subdev crop API is considered obsolete, the selection API
that replaces it deserves a full documentation instead of referring to
the crop API documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
---
 .../media/v4l/vidioc-subdev-g-selection.xml        | 37 ++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Documentation/DocBook/media/v4l/vidioc-subdev-g-selection.xml b/Documentation/DocBook/media/v4l/vidioc-subdev-g-selection.xml
index 8346b2e..faac955 100644
--- a/Documentation/DocBook/media/v4l/vidioc-subdev-g-selection.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-subdev-g-selection.xml
@@ -63,6 +63,43 @@
     more information on how each selection target affects the image
     processing pipeline inside the subdevice.</para>
 
+    <para>To retrieve a current selection rectangle applications set the
+    <structfield>pad</structfield> field of a &v4l2-subdev-selection; to the
+    desired pad number as reported by the media API, the
+    <structfield>which</structfield> field to
+    <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant> and the
+    <structfield>target</structfield> to the target selection rectangle. They
+    then call the <constant>VIDIOC_SUBDEV_G_SELECTION</constant> ioctl with a
+    pointer to this structure. The driver fills the members of the
+    <structfield>r</structfield> field or returns &EINVAL; if the input
+    arguments are invalid, or if selection is not supported on the given pad.
+    </para>
+
+    <para>To change a current selection rectangle applications set the
+    <structfield>pad</structfield>, <structfield>which</structfield> and
+    <structfield>target</structfield> fields and all members of the
+    <structfield>r</structfield> field. They then call the
+    <constant>VIDIOC_SUBDEV_S_SELECTION</constant> ioctl with a pointer to this
+    structure. The driver verifies the requested selection rectangle, adjusts it
+    based on the hardware capabilities and configures the device. Upon return
+    the &v4l2-subdev-selection; contains the current selection rectangle as
+    would be returned by a <constant>VIDIOC_SUBDEV_G_SELECTION</constant> call.
+    </para>
+
+    <para>Applications can query the device capabilities by setting the
+    <structfield>which</structfield> to
+    <constant>V4L2_SUBDEV_FORMAT_TRY</constant>. When set, 'try' selection
+    rectangles are not applied to the device by the driver, but are mangled
+    exactly as active selection rectangles and stored in the sub-device file
+    handle. Two applications querying the same sub-device would thus not
+    interfere with each other.</para>
+
+    <para>Drivers must not return an error solely because the requested
+    selection rectangle doesn't match the device capabilities. They must instead
+    modify the rectangle to match what the hardware can provide. The modified
+    selection rectangle should be as close as possible to the original request.
+    </para>
+
     <refsect2>
       <title>Types of selection targets</title>
 
-- 
1.9.1