[PATCH 1/2] drm/amd/display: use halving distribution for all encode-to-linear curves

[Melissa Wen <mwen@igalia.com>]

In encode-to-linear conversions, LUT entries should be uniformly
distributed across the input range: non-linear encodings are already
approximately perceptually uniform, so every input code carries the same
weight. A fixed count per region does the opposite, concentrating
entries on the darker values and leaving few for the bright end, whereas
halving distribution spaces all 256 entries uniformly. This holds for
any encoded input, so remove the PQ/sRGB condition from de17c6bb7072 and
apply halving to all encode-to-linear operations (pre-defined TF or user
LUTs).

It fixes the following IGT kms_colorop subtests:
- plane-XR30-XR30-srgb_inv_eotf_lut-srgb_eotf_lut
- plane-XR30-XR30-gamma_2_2-gamma_2_2_inv-gamma_2_2

Fixes: de17c6bb7072 ("drm/amd/display: use halving distribution for PQ/sRGB linearizing LUT")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
---
 .../amd/display/dc/dcn30/dcn30_cm_common.c    | 34 ++++++-------------
 1 file changed, 10 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c b/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
index 66fe7f313ea3..62ca235cd649 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn30/dcn30_cm_common.c
@@ -320,6 +320,8 @@ static struct fixed31_32 interp_tf_pts(const struct fixed31_32 *output_tf_channe
 	return value;
 }
 
+#define NUM_DEGAMMA_REGIONS    9
+
 bool cm3_helper_translate_curve_to_degamma_hw_format(
 				const struct dc_transfer_func *output_tf,
 				struct pwl_params *lut_params)
@@ -343,31 +345,15 @@ bool cm3_helper_translate_curve_to_degamma_hw_format(
 	memset(lut_params, 0, sizeof(struct pwl_params));
 	memset(seg_distr, 0, sizeof(seg_distr));
 
-	if (output_tf->tf == TRANSFER_FUNCTION_PQ ||
-	    output_tf->tf == TRANSFER_FUNCTION_SRGB) {
-		/* 9 segments
-		 * segments are from 2^-9 to 0
-		 */
-		const uint8_t SEG_COUNT = 9;
-		seg_distr[0] = 0; // Since we only have one point in darkest region
-		for (k = 1; k < SEG_COUNT; k++)
-			seg_distr[k] = k - 1; // 2^(k-1) points per region; halves as k decreases
+	/* 9 segments
+	 * segments are from 2^-9 to 2^0
+	 */
+	seg_distr[0] = 0; // Since we only have one point in darkest region
+	for (k = 1; k < NUM_DEGAMMA_REGIONS; k++)
+		seg_distr[k] = k - 1; // 2^(k-1) points per region; halves as k decreases
 
-		region_start = -SEG_COUNT;
-		region_end = 0;
-	} else {
-		/* 12 segments
-		 * segments are from 2^-12 to 2^0
-		 * There are less than 256 points, for optimization
-		 */
-		const uint8_t SEG_COUNT = 12;
-
-		for (i = 0; i < SEG_COUNT; i++)
-			seg_distr[i] = 4;
-
-		region_start = -SEG_COUNT;
-		region_end = 0;
-	}
+	region_start = -NUM_DEGAMMA_REGIONS;
+	region_end = 0;
 
 	for (i = region_end - region_start; i < MAX_REGIONS_NUMBER ; i++)
 		seg_distr[i] = -1;
-- 
2.53.0