This over-scales every limited-range conversion by a factor of 256/255,
producing an error of up to ~3/255 (8-bit) and causing IGT tests that
compare software-computed surfaces against VKMS-processed ones to fail.
Recompute the three limited-range matrices from first principles rather
than relying on colour's is_legal scaling:
1. Start from the standard ITU-R YCbCr -> RGB relations for the luma
weights Kr, Kb (Kg = 1 - Kr - Kb) of each encoding (BT.601, BT.709
and BT.2020 non-constant luminance). These are the same relations
that produce the existing full-range matrices.
2. Expand the studio input range to full range relative to a full-range
maximum of 2^n - 1: the luma coefficient by 255/(235 - 16) and the
chroma coefficients by 255/(240 - 16). This matches the DRM UAPI
definition and IGT's igt_ycbcr_to_rgb_matrix().
3. Convert each coefficient to S31.32 fixed point (round(coeff * 2^32)).
The full-range matrices (is_legal=False) are unaffected as they use exact
1.0/2.0 scaling.
Update the limited-range reference values in the vkms-format KUnit test
accordingly: the expected YCbCr inputs now use the studio-range encoding
consistent with the corrected matrices (8-bit value v placed at v * 257,
i.e. relative to a full-range maximum of 255).
Fixes: fe22d21e9342 ("drm/vkms: Add YUV support")
Assisted-by: Copilot:claude-opus-4.8
Signed-off-by: Harry Wentland <[email protected]>
---
drivers/gpu/drm/vkms/tests/vkms_format_test.c | 36 ++++++------
drivers/gpu/drm/vkms/vkms_formats.c | 55 +++++++++++++------
2 files changed, 55 insertions(+), 36 deletions(-)
diff --git a/drivers/gpu/drm/vkms/tests/vkms_format_test.c
b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
index a7788fbc45dc..789c59d07ffb 100644
--- a/drivers/gpu/drm/vkms/tests/vkms_format_test.c
+++ b/drivers/gpu/drm/vkms/tests/vkms_format_test.c
@@ -101,12 +101,12 @@ static struct yuv_u16_to_argb_u16_case
yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
- { "red", { 0x517b, 0x5a34, 0xf000 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
- { "green", { 0x908e, 0x35cc, 0x2237 }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x28f7, 0xf000, 0x6dc9 }, { 0xffff,
0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x51cd, 0x5a8e, 0xf0f0 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
+ { "green", { 0x911e, 0x3602, 0x2259 }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x2920, 0xf0f0, 0x6e37 }, { 0xffff,
0x0000, 0x0000, 0xffff }},
}
},
/*
@@ -151,12 +151,12 @@ static struct yuv_u16_to_argb_u16_case
yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
- { "red", { 0x3e8f, 0x6656, 0xf000 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
- { "green", { 0xaca1, 0x29aa, 0x1a45 }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x1fd0, 0xf000, 0x75bb }, { 0xffff,
0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x3ece, 0x66bc, 0xf0f0 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
+ { "green", { 0xad4e, 0x29d4, 0x1a5f }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x1ff0, 0xf0f0, 0x7631 }, { 0xffff,
0x0000, 0x0000, 0xffff }},
}
},
/*
@@ -201,12 +201,12 @@ static struct yuv_u16_to_argb_u16_case
yuv_u16_to_argb_u16_cases[] = {
.range = DRM_COLOR_YCBCR_LIMITED_RANGE,
.n_colors = 6,
.colors = {
- { "white", { 0xeb00, 0x8000, 0x8000 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
- { "gray", { 0x7dee, 0x8000, 0x8000 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
- { "black", { 0x1000, 0x8000, 0x8000 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
- { "red", { 0x4988, 0x60b9, 0xf000 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
- { "green", { 0xa47b, 0x2f47, 0x1902 }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
- { "blue", { 0x1cfd, 0xf000, 0x76fe }, { 0xffff,
0x0000, 0x0000, 0xffff }},
+ { "white", { 0xebeb, 0x8080, 0x8080 }, { 0xffff,
0xffff, 0xffff, 0xffff }},
+ { "gray", { 0x7e6c, 0x8080, 0x8080 }, { 0xffff,
0x8080, 0x8080, 0x8080 }},
+ { "black", { 0x1010, 0x8080, 0x8080 }, { 0xffff,
0x0000, 0x0000, 0x0000 }},
+ { "red", { 0x49d2, 0x611a, 0xf0f0 }, { 0xffff,
0xffff, 0x0000, 0x0000 }},
+ { "green", { 0xa520, 0x2f76, 0x191b }, { 0xffff,
0x0000, 0xffff, 0x0000 }},
+ { "blue", { 0x1d1a, 0xf0f0, 0x7775 }, { 0xffff,
0x0000, 0x0000, 0xffff }},
}
},
};
diff --git a/drivers/gpu/drm/vkms/vkms_formats.c
b/drivers/gpu/drm/vkms/vkms_formats.c
index dfb8e13cba87..4d5fcaeb82c5 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -788,15 +788,36 @@ static const struct conversion_matrix yuv_bt601_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.601"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.601 limited-/studio-range YCbCr to full-range RGB.
+ *
+ * The coefficients are derived as follows:
+ *
+ * 1. Take the standard ITU-R YCbCr -> RGB relations for luma weights
+ * Kr, Kb (Kg = 1 - Kr - Kb), with Y in [0, 1] and Cb, Cr in
+ * [-0.5, 0.5]. For BT.601 Kr = 0.299 and Kb = 0.114:
+ *
+ * R = Y + 2 * (1 - Kr) * Cr
+ * G = Y - 2 * (1 - Kb) * Kb / Kg * Cb - 2 * (1 - Kr) * Kr / Kg * Cr
+ * B = Y + 2 * (1 - Kb) * Cb
+ *
+ * These are exactly the yuv_bt601_full coefficients above.
+ *
+ * 2. Expand the studio input range to full range, relative to a
+ * full-range maximum of 2^n - 1 (255 for 8-bit): the luma
+ * coefficient is scaled by 255/(235 - 16) and the chroma
+ * coefficients by 255/(240 - 16). This matches the DRM UAPI
+ * definition and IGT's igt_ycbcr_to_rgb_matrix(). Note this differs
+ * from colour.matrix_YCbCr(is_legal=True), which normalises by 2^n
+ * and is thus off by a factor of 256/255.
+ *
+ * 3. Convert each coefficient to S31.32 fixed point, i.e.
+ * round(coeff * 2^32).
*/
static const struct conversion_matrix yuv_bt601_limited = {
.matrix = {
- { 5020601039, 0, 6881764740 },
- { 5020601039, -1689204679, -3505362278 },
- { 5020601039, 8697922339, 0 },
+ { 5000989317, 0, 6854882848 },
+ { 5000989317, -1682606224, -3491669458 },
+ { 5000989317, 8663946082, 0 },
},
.y_offset = 16,
};
@@ -816,15 +837,14 @@ static const struct conversion_matrix yuv_bt709_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.709"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.709 limited-range YCbCr to full-range RGB (Kr = 0.2126, Kb = 0.0722).
+ * Derived as described for yuv_bt601_limited.
*/
static const struct conversion_matrix yuv_bt709_limited = {
.matrix = {
- { 5020601039, 0, 7729959424 },
- { 5020601039, -919487572, -2297803934 },
- { 5020601039, 9108275786, 0 },
+ { 5000989317, 0, 7699764272 },
+ { 5000989317, -915895824, -2288828138 },
+ { 5000989317, 9072696586, 0 },
},
.y_offset = 16,
};
@@ -844,15 +864,14 @@ static const struct conversion_matrix yuv_bt2020_full = {
};
/*
- * numpy.around(colour.matrix_YCbCr(K=colour.WEIGHTS_YCBCR["ITU-R BT.2020"],
- * is_legal = True,
- * bits = 8) * 2**32).astype(int)
+ * BT.2020 non-constant-luminance limited-range YCbCr to full-range RGB
+ * (Kr = 0.2627, Kb = 0.0593). Derived as described for yuv_bt601_limited.
*/
static const struct conversion_matrix yuv_bt2020_limited = {
.matrix = {
- { 5020601039, 0, 7238124312 },
- { 5020601039, -807714626, -2804506279 },
- { 5020601039, 9234915964, 0 },
+ { 5000989317, 0, 7209850391 },
+ { 5000989317, -804559491, -2793551177 },
+ { 5000989317, 9198842076, 0 },
},
.y_offset = 16,
};
