ipa: rkisp1: awb: Expand comment

The RGB to YCbCr conversion matrix mentioned in a comment, coming from
the hardware documentation, does not match any of the canonical matrices
specified by any standard. While researching where the values came from,
it became apparent they are likely Bt.601 limited range coefficients
rounded to 6 bits of decimal precision. Record this in comments.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>

src/ipa/rkisp1/algorithms/awb.cpp

@@ -210,13 +210,18 @@ void Awb::process(IPAContext &context,
 		});
 
 		/*
-		 * Convert from YCbCr to RGB.
-		 * The hardware uses the following formulas:
-		 * Y = 16 + 0.2500 R + 0.5000 G + 0.1094 B
+		 * Convert from YCbCr to RGB. The hardware uses the following
+		 * formulas:
+		 *
+		 * Y  =  16 + 0.2500 R + 0.5000 G + 0.1094 B
 		 * Cb = 128 - 0.1406 R - 0.2969 G + 0.4375 B
 		 * Cr = 128 + 0.4375 R - 0.3750 G - 0.0625 B
 		 *
-		 * The inverse matrix is thus:
+		 * This seems to be based on limited range BT.601 with Q1.6
+		 * precision.
+		 *
+		 * The inverse matrix is:
+		 *
 		 * [[1,1636, -0,0623,  1,6008]
 		 *  [1,1636, -0,4045, -0,7949]
 		 *  [1,1636,  1,9912, -0,0250]]