[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.

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 |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_color.c b/drivers/gpu/drm/i915/intel_color.c
index aa66e95..777ce26 100644
--- a/drivers/gpu/drm/i915/intel_color.c
+++ b/drivers/gpu/drm/i915/intel_color.c
@@ -94,16 +94,24 @@ static void ctm_mult_by_limited(uint64_t *result, int64_t *input)
 	for (i = 0; i < 9; i++)
 		result[i] = 0;
 
-	for (i = 0; i < 3; i++) {
-		int64_t user_coeff = input[i * 3 + i];
+	for (i = 0; i < 9; i++) {
+		int64_t user_coeff = input[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;
+		/*
+		 * By scaling every co-efficient with limited range (235-16)
+		 * vs full range (0-255) the final o/p will be scaled down to
+		 * fit in the limited range supported by the panel.
+		 * Need to shift the multiplication result by 28 as the floating
+		 * count expected is of 32bits, multiplication here is done in
+		 * U2.30 so result need to be right shifted by 60-32 = 28
+		 */
+		result[i] = (limited_coeff * abs_coeff) >> 28;
 		if (CTM_COEFF_NEGATIVE(user_coeff))
-			result[i * 3 + i] |= CTM_COEFF_SIGN;
+			result[i] |= CTM_COEFF_SIGN;
 	}
 }
 
-- 
1.7.9.5