drm/i915: Document ILK+ pipe csc matrix better

Add comments to explain the ilk pipe csc operation a bit better.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-11-ville.syrjala@linux.intel.comReviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>

drivers/gpu/drm/i915/display/intel_color.c

@@ -42,6 +42,21 @@
 
 #define LEGACY_LUT_LENGTH		256
 
+/*
+ * ILK+ csc matrix:
+ *
+ * |R/Cr|   | c0 c1 c2 |   ( |R/Cr|   |preoff0| )   |postoff0|
+ * |G/Y | = | c3 c4 c5 | x ( |G/Y | + |preoff1| ) + |postoff1|
+ * |B/Cb|   | c6 c7 c8 |   ( |B/Cb|   |preoff2| )   |postoff2|
+ *
+ * ILK/SNB don't have explicit post offsets, and instead
+ * CSC_MODE_YUV_TO_RGB and CSC_BLACK_SCREEN_OFFSET are used:
+ *  CSC_MODE_YUV_TO_RGB=0 + CSC_BLACK_SCREEN_OFFSET=0 -> 1/2, 0, 1/2
+ *  CSC_MODE_YUV_TO_RGB=0 + CSC_BLACK_SCREEN_OFFSET=1 -> 1/2, 1/16, 1/2
+ *  CSC_MODE_YUV_TO_RGB=1 + CSC_BLACK_SCREEN_OFFSET=0 -> 0, 0, 0
+ *  CSC_MODE_YUV_TO_RGB=1 + CSC_BLACK_SCREEN_OFFSET=1 -> 1/16, 1/16, 1/16
+ */
+
 /*
  * Extract the CSC coefficient from a CTM coefficient (in U32.32 fixed point
  * format). This macro takes the coefficient we want transformed and the
@@ -59,37 +74,38 @@
 
 #define ILK_CSC_POSTOFF_LIMITED_RANGE (16 * (1 << 12) / 255)
 
+/* Nop pre/post offsets */
 static const u16 ilk_csc_off_zero[3] = {};
 
+/* Identity matrix */
 static const u16 ilk_csc_coeff_identity[9] = {
 	ILK_CSC_COEFF_1_0, 0, 0,
 	0, ILK_CSC_COEFF_1_0, 0,
 	0, 0, ILK_CSC_COEFF_1_0,
 };
 
+/* Limited range RGB post offsets */
 static const u16 ilk_csc_postoff_limited_range[3] = {
 	ILK_CSC_POSTOFF_LIMITED_RANGE,
 	ILK_CSC_POSTOFF_LIMITED_RANGE,
 	ILK_CSC_POSTOFF_LIMITED_RANGE,
 };
 
+/* Full range RGB -> limited range RGB matrix */
 static const u16 ilk_csc_coeff_limited_range[9] = {
 	ILK_CSC_COEFF_LIMITED_RANGE, 0, 0,
 	0, ILK_CSC_COEFF_LIMITED_RANGE, 0,
 	0, 0, ILK_CSC_COEFF_LIMITED_RANGE,
 };
 
-/*
- * These values are direct register values specified in the Bspec,
- * for RGB->YUV conversion matrix (colorspace BT709)
- */
+/* BT.709 full range RGB -> limited range YCbCr matrix */
 static const u16 ilk_csc_coeff_rgb_to_ycbcr[9] = {
 	0x1e08, 0x9cc0, 0xb528,
 	0x2ba8, 0x09d8, 0x37e8,
 	0xbce8, 0x9ad8, 0x1e08,
 };
 
-/* Post offset values for RGB->YCBCR conversion */
+/* Limited range YCbCr post offsets */
 static const u16 ilk_csc_postoff_rgb_to_ycbcr[3] = {
 	0x0800, 0x0100, 0x0800,
 };