[PATCH v4 03/11] drm/vkms: Add KUnit test for YCbCr to RGB conversion matrices

The existing yuv_u16_to_argb_u16 test performs a full RGB->YCbCr->RGB
round trip with a tolerance of 0x1ff. That tolerance is required because
fully saturated primaries clamp their chroma at encode time and cannot
round-trip exactly, but it also makes the test unable to detect small
scaling errors in the conversion matrices (such as using a 2^n instead
of a 2^n - 1 full-range maximum for limited range, an error of ~129 at
gray - below the tolerance).

Add a parameterised test that checks the exact S31.32 fixed-point
coefficients and luma offset returned by
get_conversion_matrix_to_argb_u16() for each encoding and range against
reference values. This directly pins the matrices and catches
sub-tolerance regressions that the round-trip test cannot.

Assisted-by: Copilot:claude-opus-4.8
Signed-off-by: Harry Wentland <[email protected]>
---
 drivers/gpu/drm/vkms/tests/vkms_format_test.c | 122 ++++++++++++++++++
 1 file changed, 122 insertions(+)

diff --git a/drivers/gpu/drm/vkms/tests/vkms_format_test.c b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
index 789c59d07ffb..e746b18bf37b 100644
--- a/drivers/gpu/drm/vkms/tests/vkms_format_test.c
+++ b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
@@ -263,8 +263,130 @@ KUNIT_ARRAY_PARAM(yuv_u16_to_argb_u16, yuv_u16_to_argb_u16_cases,
 		  vkms_format_test_yuv_u16_to_argb_u16_case_desc
 );
 
+/*
+ * struct conversion_matrix_case - Reference matrix to test the YUV to RGB
+ * conversion matrices returned by get_conversion_matrix_to_argb_u16()
+ *
+ * @encoding: Encoding of the conversion matrix under test
+ * @range: Range of the conversion matrix under test
+ * @expected: Expected S31.32 fixed-point matrix and luma offset
+ *
+ * The limited-range coefficients use the studio-range scaling mandated by the
+ * DRM UAPI and IGT's igt_ycbcr_to_rgb_matrix(): the narrow range is expanded
+ * relative to a full-range maximum of 2^n - 1 (255 for 8-bit), i.e. luma by
+ * 255/(235 - 16) and chroma by 255/(240 - 128). Generating them with the
+ * common 2^n normalisation (as colour.matrix_YCbCr(is_legal=True) does) is off
+ * by a factor of 256/255. See the DRM_COLOROP_FM_YCBCR*_LIMITED_RGB
+ * documentation in <drm/drm_colorop.h>.
+ */
+struct conversion_matrix_case {
+	enum drm_color_encoding encoding;
+	enum drm_color_range range;
+	struct conversion_matrix expected;
+};
+
+static struct conversion_matrix_case conversion_matrix_cases[] = {
+	{
+		.encoding = DRM_COLOR_YCBCR_BT601,
+		.range = DRM_COLOR_YCBCR_FULL_RANGE,
+		.expected = { .matrix = {
+			{ 4294967296, 0,           6021544149 },
+			{ 4294967296, -1478054095, -3067191994 },
+			{ 4294967296, 7610682049,  0 },
+		}, .y_offset = 0 },
+	},
+	{
+		.encoding = DRM_COLOR_YCBCR_BT601,
+		.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
+		.expected = { .matrix = {
+			{ 5000989317, 0,           6854882848 },
+			{ 5000989317, -1682606224, -3491669458 },
+			{ 5000989317, 8663946082,  0 },
+		}, .y_offset = 16 },
+	},
+	{
+		.encoding = DRM_COLOR_YCBCR_BT709,
+		.range = DRM_COLOR_YCBCR_FULL_RANGE,
+		.expected = { .matrix = {
+			{ 4294967296, 0,          6763714498 },
+			{ 4294967296, -804551626, -2010578443 },
+			{ 4294967296, 7969741314, 0 },
+		}, .y_offset = 0 },
+	},
+	{
+		.encoding = DRM_COLOR_YCBCR_BT709,
+		.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
+		.expected = { .matrix = {
+			{ 5000989317, 0,          7699764272 },
+			{ 5000989317, -915895824, -2288828138 },
+			{ 5000989317, 9072696586, 0 },
+		}, .y_offset = 16 },
+	},
+	{
+		.encoding = DRM_COLOR_YCBCR_BT2020,
+		.range = DRM_COLOR_YCBCR_FULL_RANGE,
+		.expected = { .matrix = {
+			{ 4294967296, 0,          6333358775 },
+			{ 4294967296, -706750298, -2453942994 },
+			{ 4294967296, 8080551471, 0 },
+		}, .y_offset = 0 },
+	},
+	{
+		.encoding = DRM_COLOR_YCBCR_BT2020,
+		.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
+		.expected = { .matrix = {
+			{ 5000989317, 0,          7209850391 },
+			{ 5000989317, -804559491, -2793551177 },
+			{ 5000989317, 9198842076, 0 },
+		}, .y_offset = 16 },
+	},
+};
+
+/*
+ * vkms_format_test_conversion_matrix - Verify the YUV to RGB conversion matrices
+ *
+ * This test checks that get_conversion_matrix_to_argb_u16() returns the exact
+ * fixed-point coefficients expected for each encoding and range. Unlike the
+ * round-trip test above, it is sensitive to small (sub-tolerance) scaling
+ * errors such as using a 2^n instead of a 2^n - 1 full-range maximum for the
+ * limited-range matrices.
+ */
+static void vkms_format_test_conversion_matrix(struct kunit *test)
+{
+	const struct conversion_matrix_case *param = test->param_value;
+	struct conversion_matrix matrix;
+
+	get_conversion_matrix_to_argb_u16(DRM_FORMAT_NV12, param->encoding,
+					  param->range, false, &matrix);
+
+	for (size_t i = 0; i < 3; i++) {
+		for (size_t j = 0; j < 3; j++) {
+			KUNIT_EXPECT_EQ_MSG(test, matrix.matrix[i][j],
+					    param->expected.matrix[i][j],
+					    "matrix[%zu][%zu] mismatch for %s - %s",
+					    i, j,
+					    drm_get_color_encoding_name(param->encoding),
+					    drm_get_color_range_name(param->range));
+		}
+	}
+
+	KUNIT_EXPECT_EQ(test, matrix.y_offset, param->expected.y_offset);
+}
+
+static void vkms_format_test_conversion_matrix_case_desc(struct conversion_matrix_case *t,
+							 char *desc)
+{
+	snprintf(desc, KUNIT_PARAM_DESC_SIZE, "%s - %s",
+		 drm_get_color_encoding_name(t->encoding), drm_get_color_range_name(t->range));
+}
+
+KUNIT_ARRAY_PARAM(conversion_matrix, conversion_matrix_cases,
+		  vkms_format_test_conversion_matrix_case_desc
+);
+
 static struct kunit_case vkms_format_test_cases[] = {
 	KUNIT_CASE_PARAM(vkms_format_test_yuv_u16_to_argb_u16, yuv_u16_to_argb_u16_gen_params),
+	KUNIT_CASE_PARAM(vkms_format_test_conversion_matrix, conversion_matrix_gen_params),
 	{}
 };
 
-- 
2.55.0