linux-kernel - [RFC PATCH v2 08/18] drm/amd/display: add comments to describe DM crtc color mgmt behavior

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Date:   Mon,  9 Jan 2023 13:38:36 -0100
From:   Melissa Wen <mwen@...lia.com>
To:     harry.wentland@....com, sunpeng.li@....com,
        Rodrigo.Siqueira@....com, alexander.deucher@....com,
        christian.koenig@....com, Xinhui.Pan@....com, airlied@...il.com,
        daniel@...ll.ch
Cc:     Joshua Ashton <joshua@...ggi.es>, alex.hung@....com,
        nicholas.kazlauskas@....com, sungjoon.kim@....com,
        seanpaul@...omium.org, bhawanpreet.lakha@....com,
        Shashank Sharma <shashank.sharma@....com>,
        ville.syrjala@...ux.intel.com, maarten.lankhorst@...ux.intel.com,
        mripard@...nel.org, tzimmermann@...e.de, kernel-dev@...lia.com,
        laurent.pinchart+renesas@...asonboard.com,
        Melissa Wen <mwen@...lia.com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH v2 08/18] drm/amd/display: add comments to describe DM crtc color mgmt behavior

Describe some expected behavior of the AMD DM color mgmt programming.

Signed-off-by: Melissa Wen <mwen@...lia.com>
---
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
index 10a29d131424..b54ef1392895 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
@@ -428,12 +428,23 @@ int amdgpu_dm_update_crtc_color_mgmt(struct dm_crtc_state *crtc)
 		stream->out_transfer_func->type = TF_TYPE_DISTRIBUTED_POINTS;
 		stream->out_transfer_func->tf = TRANSFER_FUNCTION_SRGB;
 
+		/* Note: even if we pass has_rom as parameter here, we never
+		 * actually use ROM because the color module only takes the ROM
+		 * path if transfer_func->type == PREDEFINED.
+		 *
+		 * See more in mod_color_calculate_regamma_params()
+		 */
 		r = __set_legacy_tf(stream->out_transfer_func, regamma_lut,
 				    regamma_size, has_rom);
 		if (r)
 			return r;
 	} else if (has_regamma) {
-		/* If atomic regamma, CRTC RGM goes into RGM LUT. */
+		/* CRTC RGM goes into RGM LUT.
+		 *
+		 * Note: here there is no implicit sRGB regamma. We are using
+		 * degamma calculation from color module to calculate the curve
+		 * from a linear base.
+		 */
 		stream->out_transfer_func->type = TF_TYPE_DISTRIBUTED_POINTS;
 		stream->out_transfer_func->tf = TRANSFER_FUNCTION_LINEAR;
 
-- 
2.35.1