media: v4l2-ctrls: Document V4L2_CID_NOTIFY_GAINS control

Add documentation for the V4L2_CID_NOTIFY_GAINS control.

This control is required by sensors that need to know what colour
gains will be applied to pixels by downstream processing (such as by
an ISP), though the sensor does not apply these gains itself.
Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/ext-ctrls-image-source.rst

@@ -72,3 +72,23 @@ Image Source Control IDs
     * - __u32
       - ``height``
       - Height of the area.
+
+``V4L2_CID_NOTIFY_GAINS (integer array)``
+    The sensor is notified what gains will be applied to the different
+    colour channels by subsequent processing (such as by an ISP). The
+    sensor is merely informed of these values in case it performs
+    processing that requires them, but it does not apply them itself to
+    the output pixels.
+
+    Currently it is defined only for Bayer sensors, and is an array
+    control taking 4 gain values, being the gains for each of the
+    Bayer channels. The gains are always in the order B, Gb, Gr and R,
+    irrespective of the exact Bayer order of the sensor itself.
+
+    The use of an array allows this control to be extended to sensors
+    with, for example, non-Bayer CFAs (colour filter arrays).
+
+    The units for the gain values are linear, with the default value
+    representing a gain of exactly 1.0. For example, if this default value
+    is reported as being (say) 128, then a value of 192 would represent
+    a gain of exactly 1.5.