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

[Shankar, Uma]

> -----Original Message-----
> From: andrescj via sendgmr <andrescj at andrescj-cros-specialist.c.googlers.com>
> On Behalf Of Andres Calderon Jaramillo
> Sent: Tuesday, December 15, 2020 3:50 AM
> To: intel-gfx at lists.freedesktop.org
> Cc: Shankar, Uma <uma.shankar at intel.com>; Venkatesh Reddy, Sushma
> <sushma.venkatesh.reddy at intel.com>; seanpaul at chromium.org; Andres
> Calderon Jaramillo <andrescj at chromium.org>
> Subject: [PATCH] drm/i915/display: Prevent double YUV range correction on HDR
> planes
> 
> 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

Change looks good to me, double conversion should not be done.
Plane input csc coefficients take care of the limited range.

Reviewed-by: Uma Shankar <uma.shankar at intel.com>

> 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