[Intel-gfx] [PATCH] drm/i915: Fix Limited Range Color Handling

[Uma Shankar]

From: Johnson Lin <johnson.lin at intel.com>

Some panels support limited range output (16-235) compared
to full range RGB values (0-255). Also userspace can control
the RGB range using "Broadcast RGB" property. Currently the
code to handle full range to limited range is broken. This
patch fixes the same by properly scaling down all the full
range co-efficients with limited range scaling factor.

v2: Fixed Ville's review comments.

Signed-off-by: Johnson Lin <johnson.lin at intel.com>
Signed-off-by: Uma Shankar <uma.shankar at intel.com>
---
 drivers/gpu/drm/i915/intel_color.c |   29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_color.c b/drivers/gpu/drm/i915/intel_color.c
index aa66e95..55408e2 100644
--- a/drivers/gpu/drm/i915/intel_color.c
+++ b/drivers/gpu/drm/i915/intel_color.c
@@ -84,26 +84,27 @@ static bool crtc_state_is_legacy_gamma(struct drm_crtc_state *state)
 
 /*
  * When using limited range, multiply the matrix given by userspace by
- * the matrix that we would use for the limited range. We do the
- * multiplication in U2.30 format.
+ * the matrix that we would use for the limited range.
  */
 static void ctm_mult_by_limited(uint64_t *result, int64_t *input)
 {
 	int i;
 
-	for (i = 0; i < 9; i++)
-		result[i] = 0;
+	for (i = 0; i < 9; i++) {
+		uint64_t user_coeff = input[i];
+		uint32_t limited_coeff = CTM_COEFF_LIMITED_RANGE;
+		uint32_t abs_coeff = (uint32_t)(clamp_val(
+						CTM_COEFF_ABS(user_coeff),
+						0,
+						CTM_COEFF_4_0 - 1) >> 2);
 
-	for (i = 0; i < 3; i++) {
-		int64_t user_coeff = input[i * 3 + i];
-		uint64_t limited_coeff = CTM_COEFF_LIMITED_RANGE >> 2;
-		uint64_t abs_coeff = clamp_val(CTM_COEFF_ABS(user_coeff),
-					       0,
-					       CTM_COEFF_4_0 - 1) >> 2;
-
-		result[i * 3 + i] = (limited_coeff * abs_coeff) >> 27;
-		if (CTM_COEFF_NEGATIVE(user_coeff))
-			result[i * 3 + i] |= CTM_COEFF_SIGN;
+		/*
+		 * By scaling every co-efficient with limited range (16-235)
+		 * vs full range (0-255) the final o/p will be scaled down to
+		 * fit in the limited range supported by the panel.
+		 */
+		result[i] = mul_u32_u32(limited_coeff, abs_coeff) >> 30;
+		result[i] |= user_coeff & CTM_COEFF_SIGN;
 	}
 }
 
-- 
1.7.9.5