drm/amd/display: add comments to describe DM crtc color mgmt behavior

Describe some expected behavior of the AMD DM color mgmt programming.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c

@@ -661,13 +661,25 @@ int amdgpu_dm_update_crtc_color_mgmt(struct dm_crtc_state *crtc)
 		crtc->cm_is_degamma_srgb = true;
 		stream->out_transfer_func->type = TF_TYPE_DISTRIBUTED_POINTS;
 		stream->out_transfer_func->tf = TRANSFER_FUNCTION_SRGB;
-
+		/*
+		 * Note: although 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: there is no implicit sRGB regamma here. 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;