[PATCH 08/11] drm/vkms: add support to non-uniform LUT for internal color curves
From: Leandro Ribeiro
Date: Tue Aug 04 2026 - 16:36:53 EST
This extends VKMS internal LUT implementation, allowing it to represent
non-uniform LUTs.
Such LUTs have the X axis. LUTs that come from userspace (gamma LUT) are
always uniform, so they don't have a X axis.
Note: vkms_color_srgb_inv_srgb() error tolerance reduced from 1/255 to
119/65535 ~= 0.46/255. Before this patch, the test compared the results
after quantizing them to 8-bit precision, while the updated test
compares directly in 16-bit. The LUT precision does not change with this
patch.
Signed-off-by: Leandro Ribeiro <leandro.ribeiro@xxxxxxxxxxxxx>
---
drivers/gpu/drm/vkms/tests/vkms_color_test.c | 20 +----
drivers/gpu/drm/vkms/vkms_composer.c | 85 ++++++++++++++++++--
drivers/gpu/drm/vkms/vkms_drv.h | 9 +++
drivers/gpu/drm/vkms/vkms_luts.c | 57 +++++++++++--
4 files changed, 139 insertions(+), 32 deletions(-)
diff --git a/drivers/gpu/drm/vkms/tests/vkms_color_test.c b/drivers/gpu/drm/vkms/tests/vkms_color_test.c
index bb9d84377b97..571b1b579310 100644
--- a/drivers/gpu/drm/vkms/tests/vkms_color_test.c
+++ b/drivers/gpu/drm/vkms/tests/vkms_color_test.c
@@ -98,19 +98,6 @@ static void vkms_color_test_get_uniform_lut_index(struct kunit *test)
lut_index = get_uniform_lut_index(&test_linear_lut, test_linear_array[i].red);
KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(lut_index), i);
}
-
- KUNIT_EXPECT_EQ(test, drm_fixp2int(get_uniform_lut_index(&srgb_eotf, 0x0)), 0x0);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_eotf, 0x0)), 0x0);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_eotf, 0x101)), 0x1);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_eotf, 0x202)), 0x2);
-
- KUNIT_EXPECT_EQ(test, drm_fixp2int(get_uniform_lut_index(&srgb_inv_eotf, 0x0)), 0x0);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_inv_eotf, 0x0)), 0x0);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_inv_eotf, 0x101)), 0x1);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_inv_eotf, 0x202)), 0x2);
-
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_eotf, 0xfefe)), 0xfe);
- KUNIT_EXPECT_EQ(test, drm_fixp2int_ceil(get_uniform_lut_index(&srgb_eotf, 0xffff)), 0xff);
}
static void vkms_color_test_lerp(struct kunit *test)
@@ -136,13 +123,14 @@ static void vkms_color_test_linear(struct kunit *test)
static void vkms_color_srgb_inv_srgb(struct kunit *test)
{
u16 srgb, final;
+ u16 tolerance = 119;
for (int i = 0; i < srgb_eotf.lut_length; i++) {
- srgb = apply_lut_to_channel_value(&srgb_eotf, i * 0x101, LUT_RED);
+ srgb = apply_lut_to_channel_value(&srgb_eotf, srgb_eotf.x[i], LUT_RED);
final = apply_lut_to_channel_value(&srgb_inv_eotf, srgb, LUT_RED);
- KUNIT_EXPECT_GE(test, final / 0x101, i - 1);
- KUNIT_EXPECT_LE(test, final / 0x101, i + 1);
+ KUNIT_EXPECT_GE(test, final, (int)srgb_eotf.x[i] - tolerance);
+ KUNIT_EXPECT_LE(test, final, (int)srgb_eotf.x[i] + tolerance);
}
}
diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 1a5e899e5b4a..4ae1ffd31406 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -93,27 +93,81 @@ static inline u16 lut_channel_value(const struct drm_color_lut *lut,
return 0;
}
+static u16 apply_non_uniform_lut(const struct vkms_color_lut *lut,
+ u16 channel_value,
+ enum lut_channel channel)
+{
+ const struct drm_color_lut *lut_y_lo, *lut_y_hi;
+ u16 y_lo, y_hi;
+ u16 x_lo, x_hi;
+ unsigned int lo, hi, mid;
+ s64 t;
+
+ /*
+ * Handle values out of LUT domain.
+ */
+ if (channel_value <= lut->x[0])
+ return lut_channel_value(&lut->y[0], channel);
+ if (channel_value >= lut->x[lut->lut_length - 1])
+ return lut_channel_value(&lut->y[lut->lut_length - 1], channel);
+
+ /*
+ * Binary search to find the largest index lo such that
+ * x[lo] <= channel_value.
+ */
+ lo = 0;
+ hi = lut->lut_length - 1;
+ while (lo < hi) {
+ mid = lo + (hi - lo + 1) / 2;
+ if (lut->x[mid] <= channel_value)
+ lo = mid;
+ else
+ hi = mid - 1;
+ }
+ lut_y_lo = &lut->y[lo];
+
+ /*
+ * As x[0] < channel_value < x[lut_length - 1] and
+ * x[lo] <= channel_value, lo + 1 is a valid index.
+ */
+ lut_y_hi = &lut->y[lo + 1];
+
+ x_lo = lut->x[lo];
+ x_hi = lut->x[lo + 1];
+ y_lo = lut_channel_value(lut_y_lo, channel);
+ y_hi = lut_channel_value(lut_y_hi, channel);
+
+ /* Avoid division by zero when two consecutive x values are equal. */
+ if (x_hi == x_lo)
+ return y_lo;
+
+ t = drm_fixp_div(drm_int2fixp(channel_value - x_lo),
+ drm_int2fixp(x_hi - x_lo));
+
+ return lerp_u16(y_lo, y_hi, t);
+}
+
VISIBLE_IF_KUNIT s64 get_uniform_lut_index(const struct vkms_color_lut *lut, u16 channel_value)
{
s64 color_channel_fp = drm_int2fixp(channel_value);
+ if (lut->x) {
+ DRM_DEBUG_DRIVER("Non-uniform LUT should not use get_uniform_lut_index()");
+ return 0;
+ }
+
return drm_fixp_mul(color_channel_fp, lut->channel_value2index_ratio);
}
EXPORT_SYMBOL_IF_KUNIT(get_uniform_lut_index);
-VISIBLE_IF_KUNIT u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 channel_value,
- enum lut_channel channel)
+static u16 apply_uniform_lut(const struct vkms_color_lut *lut,
+ u16 channel_value,
+ enum lut_channel channel)
{
const struct drm_color_lut *lut_y_floor, *lut_y_ceil;
s64 lut_index = get_uniform_lut_index(lut, channel_value);
u16 floor_channel_value, ceil_channel_value;
- /*
- * This checks if `struct drm_color_lut` has any gap added by the compiler
- * between the struct fields.
- */
- static_assert(sizeof(struct drm_color_lut) == sizeof(__u16) * 4);
-
lut_y_floor = &lut->y[drm_fixp2int(lut_index)];
if (drm_fixp2int(lut_index) == (lut->lut_length - 1))
/* We're at the end of the LUT array, use same value for ceil and floor */
@@ -127,6 +181,21 @@ VISIBLE_IF_KUNIT u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut
return lerp_u16(floor_channel_value, ceil_channel_value,
lut_index & DRM_FIXED_DECIMAL_MASK);
}
+
+VISIBLE_IF_KUNIT u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 channel_value,
+ enum lut_channel channel)
+{
+ /*
+ * This checks if `struct drm_color_lut` has any gap added by the compiler
+ * between the struct fields.
+ */
+ static_assert(sizeof(struct drm_color_lut) == sizeof(__u16) * 4);
+
+ if (lut->x)
+ return apply_non_uniform_lut(lut, channel_value, channel);
+
+ return apply_uniform_lut(lut, channel_value, channel);
+}
EXPORT_SYMBOL_IF_KUNIT(apply_lut_to_channel_value);
diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
index 55a3ea184e44..be81844dfd22 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.h
+++ b/drivers/gpu/drm/vkms/vkms_drv.h
@@ -158,7 +158,16 @@ struct vkms_plane {
struct drm_plane base;
};
+/**
+ * struct vkms_color_lut - Driver specific color LUT representation
+ * @x: LUT x-values, must be non-decreasing and may be non-uniformly spaced.
+ * Only required for non-uniform LUTs.
+ * @y: LUT y-values.
+ * @lut_length: The LUT length.
+ * @channel_value2index_ratio: helper for uniform LUTs (no x-values).
+ */
struct vkms_color_lut {
+ u16 *x;
struct drm_color_lut *y;
size_t lut_length;
s64 channel_value2index_ratio;
diff --git a/drivers/gpu/drm/vkms/vkms_luts.c b/drivers/gpu/drm/vkms/vkms_luts.c
index 7b0c8eaf83b8..c2d5f9e502ae 100644
--- a/drivers/gpu/drm/vkms/vkms_luts.c
+++ b/drivers/gpu/drm/vkms/vkms_luts.c
@@ -12,7 +12,42 @@
* https://gitlab.freedesktop.org/hwentland/lutgen
*/
-static struct drm_color_lut srgb_array[] = {
+static u16 linear_x[] = {
+ 0x0000, 0x0101, 0x0202, 0x0303, 0x0404, 0x0505, 0x0606, 0x0707,
+ 0x0808, 0x0909, 0x0a0a, 0x0b0b, 0x0c0c, 0x0d0d, 0x0e0e, 0x0f0f,
+ 0x1010, 0x1111, 0x1212, 0x1313, 0x1414, 0x1515, 0x1616, 0x1717,
+ 0x1818, 0x1919, 0x1a1a, 0x1b1b, 0x1c1c, 0x1d1d, 0x1e1e, 0x1f1f,
+ 0x2020, 0x2121, 0x2222, 0x2323, 0x2424, 0x2525, 0x2626, 0x2727,
+ 0x2828, 0x2929, 0x2a2a, 0x2b2b, 0x2c2c, 0x2d2d, 0x2e2e, 0x2f2f,
+ 0x3030, 0x3131, 0x3232, 0x3333, 0x3434, 0x3535, 0x3636, 0x3737,
+ 0x3838, 0x3939, 0x3a3a, 0x3b3b, 0x3c3c, 0x3d3d, 0x3e3e, 0x3f3f,
+ 0x4040, 0x4141, 0x4242, 0x4343, 0x4444, 0x4545, 0x4646, 0x4747,
+ 0x4848, 0x4949, 0x4a4a, 0x4b4b, 0x4c4c, 0x4d4d, 0x4e4e, 0x4f4f,
+ 0x5050, 0x5151, 0x5252, 0x5353, 0x5454, 0x5555, 0x5656, 0x5757,
+ 0x5858, 0x5959, 0x5a5a, 0x5b5b, 0x5c5c, 0x5d5d, 0x5e5e, 0x5f5f,
+ 0x6060, 0x6161, 0x6262, 0x6363, 0x6464, 0x6565, 0x6666, 0x6767,
+ 0x6868, 0x6969, 0x6a6a, 0x6b6b, 0x6c6c, 0x6d6d, 0x6e6e, 0x6f6f,
+ 0x7070, 0x7171, 0x7272, 0x7373, 0x7474, 0x7575, 0x7676, 0x7777,
+ 0x7878, 0x7979, 0x7a7a, 0x7b7b, 0x7c7c, 0x7d7d, 0x7e7e, 0x7f7f,
+ 0x8080, 0x8181, 0x8282, 0x8383, 0x8484, 0x8585, 0x8686, 0x8787,
+ 0x8888, 0x8989, 0x8a8a, 0x8b8b, 0x8c8c, 0x8d8d, 0x8e8e, 0x8f8f,
+ 0x9090, 0x9191, 0x9292, 0x9393, 0x9494, 0x9595, 0x9696, 0x9797,
+ 0x9898, 0x9999, 0x9a9a, 0x9b9b, 0x9c9c, 0x9d9d, 0x9e9e, 0x9f9f,
+ 0xa0a0, 0xa1a1, 0xa2a2, 0xa3a3, 0xa4a4, 0xa5a5, 0xa6a6, 0xa7a7,
+ 0xa8a8, 0xa9a9, 0xaaaa, 0xabab, 0xacac, 0xadad, 0xaeae, 0xafaf,
+ 0xb0b0, 0xb1b1, 0xb2b2, 0xb3b3, 0xb4b4, 0xb5b5, 0xb6b6, 0xb7b7,
+ 0xb8b8, 0xb9b9, 0xbaba, 0xbbbb, 0xbcbc, 0xbdbd, 0xbebe, 0xbfbf,
+ 0xc0c0, 0xc1c1, 0xc2c2, 0xc3c3, 0xc4c4, 0xc5c5, 0xc6c6, 0xc7c7,
+ 0xc8c8, 0xc9c9, 0xcaca, 0xcbcb, 0xcccc, 0xcdcd, 0xcece, 0xcfcf,
+ 0xd0d0, 0xd1d1, 0xd2d2, 0xd3d3, 0xd4d4, 0xd5d5, 0xd6d6, 0xd7d7,
+ 0xd8d8, 0xd9d9, 0xdada, 0xdbdb, 0xdcdc, 0xdddd, 0xdede, 0xdfdf,
+ 0xe0e0, 0xe1e1, 0xe2e2, 0xe3e3, 0xe4e4, 0xe5e5, 0xe6e6, 0xe7e7,
+ 0xe8e8, 0xe9e9, 0xeaea, 0xebeb, 0xecec, 0xeded, 0xeeee, 0xefef,
+ 0xf0f0, 0xf1f1, 0xf2f2, 0xf3f3, 0xf4f4, 0xf5f5, 0xf6f6, 0xf7f7,
+ 0xf8f8, 0xf9f9, 0xfafa, 0xfbfb, 0xfcfc, 0xfdfd, 0xfefe, 0xffff
+};
+
+static struct drm_color_lut srgb_y[] = {
{ 0x0, 0x0, 0x0, 0 },
{ 0x13, 0x13, 0x13, 0 },
{ 0x27, 0x27, 0x27, 0 },
@@ -271,14 +306,17 @@ static struct drm_color_lut srgb_array[] = {
{ 0xffff, 0xffff, 0xffff, 0 },
};
+static_assert(ARRAY_SIZE(linear_x) == ARRAY_SIZE(srgb_y),
+ "srgb x and y must have the same number of entries");
+
const struct vkms_color_lut srgb_eotf = {
- .y = srgb_array,
- .lut_length = ARRAY_SIZE(srgb_array),
- .channel_value2index_ratio = 0xff00ffll
+ .x = linear_x,
+ .y = srgb_y,
+ .lut_length = ARRAY_SIZE(srgb_y)
};
EXPORT_SYMBOL(srgb_eotf);
-static struct drm_color_lut srgb_inv_array[] = {
+static struct drm_color_lut srgb_inv_y[] = {
{ 0x0, 0x0, 0x0, 0 },
{ 0xcc2, 0xcc2, 0xcc2, 0 },
{ 0x15be, 0x15be, 0x15be, 0 },
@@ -537,9 +575,12 @@ static struct drm_color_lut srgb_inv_array[] = {
{ 0xffff, 0xffff, 0xffff, 0 },
};
+static_assert(ARRAY_SIZE(linear_x) == ARRAY_SIZE(srgb_inv_y),
+ "srgb_inv x and y must have the same number of entries");
+
const struct vkms_color_lut srgb_inv_eotf = {
- .y = srgb_inv_array,
- .lut_length = ARRAY_SIZE(srgb_inv_array),
- .channel_value2index_ratio = 0xff00ffll
+ .x = linear_x,
+ .y = srgb_inv_y,
+ .lut_length = ARRAY_SIZE(srgb_inv_y)
};
EXPORT_SYMBOL(srgb_inv_eotf);
--
2.55.0