drm/exynos: mixer: document YCbCr magic numbers

The output stage of the mixer uses YCbCr for the internal
computations, which is the reason that some registers take
YCbCr related data as input. In particular this applies
to MXR_BG_COLOR{0,1,2} and MXR_CM_COEFF_{Y,CB,CR}.

Document the formatting of the data which we write to
these registers.

While at it, unify wording of comments in the register header.
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -45,6 +45,22 @@
 #define MIXER_WIN_NR		3
 #define VP_DEFAULT_WIN		2
 
+/*
+ * Mixer color space conversion coefficient triplet.
+ * Used for CSC from RGB to YCbCr.
+ * Each coefficient is a 10-bit fixed point number with
+ * sign and no integer part, i.e.
+ * [0:8] = fractional part (representing a value y = x / 2^9)
+ * [9] = sign
+ * Negative values are encoded with two's complement.
+ */
+#define MXR_CSC_C(x) ((int)((x) * 512.0) & 0x3ff)
+#define MXR_CSC_CT(a0, a1, a2) \
+  ((MXR_CSC_C(a0) << 20) | (MXR_CSC_C(a1) << 10) | (MXR_CSC_C(a2) << 0))
+
+/* YCbCr value, used for mixer background color configuration. */
+#define MXR_YCBCR_VAL(y, cb, cr) (((y) << 16) | ((cb) << 8) | ((cr) << 0))
+
 /* The pixelformats that are natively supported by the mixer. */
 #define MXR_FORMAT_RGB565	4
 #define MXR_FORMAT_ARGB1555	5
@@ -390,13 +406,14 @@ static void mixer_cfg_rgb_fmt(struct mixer_context *ctx, unsigned int height)
 	case 1080:
 	default:
 		val = MXR_CFG_RGB709_16_235;
+		/* Configure the BT.709 CSC matrix for full range RGB. */
 		mixer_reg_write(res, MXR_CM_COEFF_Y,
-				(1 << 30) | (94 << 20) | (314 << 10) |
-				(32 << 0));
+			MXR_CSC_CT( 0.184,  0.614,  0.063) |
+			MXR_CM_COEFF_RGB_FULL);
 		mixer_reg_write(res, MXR_CM_COEFF_CB,
-				(972 << 20) | (851 << 10) | (225 << 0));
+			MXR_CSC_CT(-0.102, -0.338,  0.440));
 		mixer_reg_write(res, MXR_CM_COEFF_CR,
-				(225 << 20) | (820 << 10) | (1004 << 0));
+			MXR_CSC_CT( 0.440, -0.399, -0.040));
 		break;
 	}
 
@@ -714,10 +731,10 @@ static void mixer_win_reset(struct mixer_context *ctx)
 	/* reset default layer priority */
 	mixer_reg_write(res, MXR_LAYER_CFG, 0);
 
-	/* setting background color */
-	mixer_reg_write(res, MXR_BG_COLOR0, 0x008080);
-	mixer_reg_write(res, MXR_BG_COLOR1, 0x008080);
-	mixer_reg_write(res, MXR_BG_COLOR2, 0x008080);
+	/* set all background colors to RGB (0,0,0) */
+	mixer_reg_write(res, MXR_BG_COLOR0, MXR_YCBCR_VAL(0, 128, 128));
+	mixer_reg_write(res, MXR_BG_COLOR1, MXR_YCBCR_VAL(0, 128, 128));
+	mixer_reg_write(res, MXR_BG_COLOR2, MXR_YCBCR_VAL(0, 128, 128));
 
 	if (test_bit(MXR_BIT_VP_ENABLED, &ctx->flags)) {
 		/* configuration of Video Processor Registers */

drivers/gpu/drm/exynos/regs-mixer.h

@@ -140,11 +140,11 @@
 #define MXR_INT_EN_VSYNC		(1 << 11)
 #define MXR_INT_EN_ALL			(0x0f << 8)
 
-/* bit for MXR_INT_STATUS */
+/* bits for MXR_INT_STATUS */
 #define MXR_INT_CLEAR_VSYNC		(1 << 11)
 #define MXR_INT_STATUS_VSYNC		(1 << 0)
 
-/* bit for MXR_LAYER_CFG */
+/* bits for MXR_LAYER_CFG */
 #define MXR_LAYER_CFG_GRP1_VAL(x)	MXR_MASK_VAL(x, 11, 8)
 #define MXR_LAYER_CFG_GRP1_MASK		MXR_LAYER_CFG_GRP1_VAL(~0)
 #define MXR_LAYER_CFG_GRP0_VAL(x)	MXR_MASK_VAL(x, 7, 4)
@@ -152,5 +152,8 @@
 #define MXR_LAYER_CFG_VP_VAL(x)		MXR_MASK_VAL(x, 3, 0)
 #define MXR_LAYER_CFG_VP_MASK		MXR_LAYER_CFG_VP_VAL(~0)
 
+/* bits for MXR_CM_COEFF_Y */
+#define MXR_CM_COEFF_RGB_FULL		(1 << 30)
+
 #endif /* SAMSUNG_REGS_MIXER_H */