[Intel-gfx] [PATCH] drm/i915/display: Prevent double YUV range correction on HDR planes

[Andres Calderon Jaramillo]

From: Andres Calderon Jaramillo <andrescj at chromium.org>

Prevent the ICL HDR plane pipeline from performing YUV color range
correction twice when the input is in limited range.

Before this patch the following could happen: user space gives us a YUV
buffer in limited range; per the pipeline in [1], the plane would first
go through a "YUV Range correct" stage that expands the range; the plane
would then go through the "Input CSC" stage which would also expand the
range because icl_program_input_csc() would use a matrix and an offset
that assume limited-range input; this would ultimately cause dark and
light colors to appear darker and lighter than they should respectively.

This is an issue because if a buffer switches between being scanned out
and being composited with the GPU, the user will see a color difference.
If this switching happens quickly and frequently, the user will perceive
this as a flickering.

[1] https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-icllp-vol12-displayengine_0.pdf#page=281

Signed-off-by: Andres Calderon Jaramillo <andrescj at chromium.org>
---
 drivers/gpu/drm/i915/display/intel_display.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 761be8deaa9b..aeea344b06ad 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -4811,6 +4811,13 @@ u32 glk_plane_color_ctl(const struct intel_crtc_state *crtc_state,
 			plane_color_ctl |= PLANE_COLOR_YUV_RANGE_CORRECTION_DISABLE;
 	} else if (fb->format->is_yuv) {
 		plane_color_ctl |= PLANE_COLOR_INPUT_CSC_ENABLE;
+
+		/*
+		 * Disable the YUV range correction stage because the input CSC
+		 * stage already takes care of range conversion by using separate
+		 * matrices and offsets depending on the color range.
+		 */
+		plane_color_ctl |= PLANE_COLOR_YUV_RANGE_CORRECTION_DISABLE;
 	}
 
 	return plane_color_ctl;
-- 
2.29.2.684.gfbc64c5ab5-goog