[PATCH v6 2/7] gpu: ipu-v3: ipu-ic: Fix BT.601 coefficients

[Steve Longerbeam]

The ycbcr2rgb and inverse rgb2ycbcr tables define the BT.601 Y'CbCr
encoding coefficients.

The rgb2ycbcr table specifically describes the BT.601 encoding from
full range RGB to full range YUV. Add table comments to make this more
clear.

The ycbcr2rgb inverse table describes encoding YUV limited range to RGB
full range. To be consistent with the rgb2ycbcr table, convert this to
YUV full range to RGB full range, and adjust/expand on the comments.

The ic_csc_rgb2rgb table is just an identity matrix, so rename to
ic_encode_identity.

Fixes: 1aa8ea0d2bd5d ("gpu: ipu-v3: Add Image Converter unit")

Suggested-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
Signed-off-by: Steve Longerbeam <slongerbeam@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 drivers/gpu/ipu-v3/ipu-ic.c | 61 ++++++++++++++++++++++---------------
 1 file changed, 37 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/ipu-v3/ipu-ic.c b/drivers/gpu/ipu-v3/ipu-ic.c
index 18816ccf600e..b63a2826b629 100644
--- a/drivers/gpu/ipu-v3/ipu-ic.c
+++ b/drivers/gpu/ipu-v3/ipu-ic.c
@@ -175,7 +175,7 @@ static inline void ipu_ic_write(struct ipu_ic *ic, u32 value, unsigned offset)
 	writel(value, ic->priv->base + offset);
 }
 
-struct ic_csc_params {
+struct ic_encode_coeff {
 	s16 coeff[3][3];	/* signed 9-bit integer coefficients */
 	s16 offset[3];		/* signed 11+2-bit fixed point offset */
 	u8 scale:2;		/* scale coefficients * 2^(scale-1) */
@@ -183,13 +183,15 @@ struct ic_csc_params {
 };
 
 /*
- * Y = R *  .299 + G *  .587 + B *  .114;
- * U = R * -.169 + G * -.332 + B *  .500 + 128.;
- * V = R *  .500 + G * -.419 + B * -.0813 + 128.;
+ * BT.601 encoding from RGB full range to YUV full range:
+ *
+ * Y =  .2990 * R + .5870 * G + .1140 * B
+ * U = -.1687 * R - .3313 * G + .5000 * B + 128
+ * V =  .5000 * R - .4187 * G - .0813 * B + 128
  */
-static const struct ic_csc_params ic_csc_rgb2ycbcr = {
+static const struct ic_encode_coeff ic_encode_rgb2ycbcr_601 = {
 	.coeff = {
-		{ 77, 150, 29 },
+		{  77, 150,  29 },
 		{ 469, 427, 128 },
 		{ 128, 405, 491 },
 	},
@@ -197,8 +199,11 @@ static const struct ic_csc_params ic_csc_rgb2ycbcr = {
 	.scale = 1,
 };
 
-/* transparent RGB->RGB matrix for graphics combining */
-static const struct ic_csc_params ic_csc_rgb2rgb = {
+/*
+ * identity matrix, used for transparent RGB->RGB graphics
+ * combining.
+ */
+static const struct ic_encode_coeff ic_encode_identity = {
 	.coeff = {
 		{ 128, 0, 0 },
 		{ 0, 128, 0 },
@@ -208,17 +213,25 @@ static const struct ic_csc_params ic_csc_rgb2rgb = {
 };
 
 /*
- * R = (1.164 * (Y - 16)) + (1.596 * (Cr - 128));
- * G = (1.164 * (Y - 16)) - (0.392 * (Cb - 128)) - (0.813 * (Cr - 128));
- * B = (1.164 * (Y - 16)) + (2.017 * (Cb - 128);
+ * Inverse BT.601 encoding from YUV full range to RGB full range:
+ *
+ * R = 1. * Y +      0 * (Cb - 128) + 1.4020 * (Cr - 128)
+ * G = 1. * Y -  .3442 * (Cb - 128) - 0.7142 * (Cr - 128)
+ * B = 1. * Y + 1.7720 * (Cb - 128) +      0 * (Cr - 128)
+ *
+ * equivalently (factoring out the offsets):
+ *
+ * R = 1. * Y  +      0 * Cb + 1.4020 * Cr - 179.456
+ * G = 1. * Y  -  .3442 * Cb - 0.7142 * Cr + 135.475
+ * B = 1. * Y  + 1.7720 * Cb +      0 * Cr - 226.816
  */
-static const struct ic_csc_params ic_csc_ycbcr2rgb = {
+static const struct ic_encode_coeff ic_encode_ycbcr2rgb_601 = {
 	.coeff = {
-		{ 149, 0, 204 },
-		{ 149, 462, 408 },
-		{ 149, 255, 0 },
+		{ 128,   0, 179 },
+		{ 128, 468, 421 },
+		{ 128, 227,   0 },
 	},
-	.offset = { -446, 266, -554 },
+	.offset = { -359, 271, -454 },
 	.scale = 2,
 };
 
@@ -228,7 +241,7 @@ static int init_csc(struct ipu_ic *ic,
 		    int csc_index)
 {
 	struct ipu_ic_priv *priv = ic->priv;
-	const struct ic_csc_params *params;
+	const struct ic_encode_coeff *coeff;
 	u32 __iomem *base;
 	const u16 (*c)[3];
 	const u16 *a;
@@ -238,26 +251,26 @@ static int init_csc(struct ipu_ic *ic,
 		(priv->tpmem_base + ic->reg->tpmem_csc[csc_index]);
 
 	if (inf == IPUV3_COLORSPACE_YUV && outf == IPUV3_COLORSPACE_RGB)
-		params = &ic_csc_ycbcr2rgb;
+		coeff = &ic_encode_ycbcr2rgb_601;
 	else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_YUV)
-		params = &ic_csc_rgb2ycbcr;
+		coeff = &ic_encode_rgb2ycbcr_601;
 	else if (inf == IPUV3_COLORSPACE_RGB && outf == IPUV3_COLORSPACE_RGB)
-		params = &ic_csc_rgb2rgb;
+		coeff = &ic_encode_identity;
 	else {
 		dev_err(priv->ipu->dev, "Unsupported color space conversion\n");
 		return -EINVAL;
 	}
 
 	/* Cast to unsigned */
-	c = (const u16 (*)[3])params->coeff;
-	a = (const u16 *)params->offset;
+	c = (const u16 (*)[3])coeff->coeff;
+	a = (const u16 *)coeff->offset;
 
 	param = ((a[0] & 0x1f) << 27) | ((c[0][0] & 0x1ff) << 18) |
 		((c[1][1] & 0x1ff) << 9) | (c[2][2] & 0x1ff);
 	writel(param, base++);
 
-	param = ((a[0] & 0x1fe0) >> 5) | (params->scale << 8) |
-		(params->sat << 10);
+	param = ((a[0] & 0x1fe0) >> 5) | (coeff->scale << 8) |
+		(coeff->sat << 10);
 	writel(param, base++);
 
 	param = ((a[1] & 0x1f) << 27) | ((c[0][1] & 0x1ff) << 18) |
-- 
2.17.1