1 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/DocBook/v4l/dev-subdev.xml b/Documentation/DocBook/v4l/dev-subdev.xml
index fc62e65f45ef..e9eb8af0f303 100644
--- a/Documentation/DocBook/v4l/dev-subdev.xml
+++ b/Documentation/DocBook/v4l/dev-subdev.xml
@@ -275,6 +275,39 @@
       </para>
     </section>
 
+    <section>
+      <title>Cropping and scaling</title>
+
+      <para>Many sub-devices support cropping frames on their input or output
+      pads (or possible even on both). Cropping is used to select the area of
+      interest in an image, typically on a video sensor or video decoder. It can
+      also be used as part of digital zoom implementations to select the area of
+      the image that will be scaled up.</para>
+
+      <para>Crop settings are defined by a crop rectangle and represented in a
+      &v4l2-rect; by the coordinates of the top left corner and the rectangle
+      size. Both the coordinates and sizes are expressed in pixels.</para>
+
+      <para>The crop rectangle is retrieved and set using the
+      &VIDIOC-SUBDEV-G-CROP; and &VIDIOC-SUBDEV-S-CROP; ioctls. Like for pad
+      formats, drivers store try and active crop rectangles. The format
+      negotiation mechanism applies to crop settings as well.</para>
+
+      <para>On input pads, cropping is applied relatively to the current pad
+      format. The pad format represents the image size as received by the
+      sub-device from the previous block in the pipeline, and the crop rectangle
+      represents the sub-image that will be transmitted further inside the
+      sub-device for processing. The crop rectangle be entirely containted
+      inside the input image size.</para>
+
+      <para>Input crop rectangle are reset to their default value when the input
+      image format is modified. Drivers should use the input image size as the
+      crop rectangle default value, but hardware requirements may prevent this.
+      </para>
+
+      <para>Cropping behaviour on output pads is not defined.</para>
+
+    </section>
   </section>
 
   &sub-subdev-formats;