[PATCH 04/21] drm/amd/display: Fix regamma not affecting full-intensity color values

[Harry Wentland]

From: "Leo (Sunpeng) Li" <sunpeng.li at amd.com>

Hardware understands the regamma LUT as a piecewise linear function,
with points spaced exponentially along the range. We previously
programmed the LUT for range [2^-10, 2^0). This causes (normalized)
color values of 1 (=2^0) to miss the programmed LUT, and fall onto the
end region.

For DCE, the end region is extrapolated using a single (base, slope)
pair, using the max y-value from the last point in the curve as base.
This presents a problem, since this value affects all three color
channels. Scaling down the intensity of say - the blue regamma curve -
will not affect it's end region. This is especially noticiable when
using RedShift. It scales down the blue and green channels, but leaves
full-intensity colors unshifted.

Therefore, extend the range to cover [2^-10, 2^1) by programming another
hardware segment, containing only one point. That way, we won't be
hitting the end region.

Note that things are a bit different for DCN, since the end region can
be set per-channel.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li at amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac at amd.com>
Acked-by: Harry Wentland <harry.wentland at amd.com>
---
 drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c b/drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c
index 71e4812217bb..0ff2a8092782 100644
--- a/drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c
+++ b/drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c
@@ -456,10 +456,13 @@ dce110_translate_regamma_to_hw_format(const struct dc_transfer_func *output_tf,
 
 	} else {
 		/* 10 segments
-		 * segment is from 2^-10 to 2^0
+		 * segment is from 2^-10 to 2^1
+		 * We include an extra segment for range [2^0, 2^1). This is to
+		 * ensure that colors with normalized values of 1 don't miss the
+		 * LUT.
 		 */
 		region_start = -10;
-		region_end = 0;
+		region_end = 1;
 
 		seg_distr[0] = 4;
 		seg_distr[1] = 4;
@@ -471,7 +474,7 @@ dce110_translate_regamma_to_hw_format(const struct dc_transfer_func *output_tf,
 		seg_distr[7] = 4;
 		seg_distr[8] = 4;
 		seg_distr[9] = 4;
-		seg_distr[10] = -1;
+		seg_distr[10] = 0;
 		seg_distr[11] = -1;
 		seg_distr[12] = -1;
 		seg_distr[13] = -1;
-- 
2.15.1