drm/i915: Fix readout degamma_lut mismatch on ilk/snb

On ilk/snb the pipe may be configured to place the LUT before or
after the CSC depending on various factors, but as there is only
one LUT (no split mode like on IVB+) we only advertise a gamma_lut
and no degamma_lut in the uapi to avoid confusing userspace.

This can cause a problem during readout if the VBIOS/GOP enabled
the LUT in the pre CSC configuration. The current code blindly
assigns the results of the readout to the degamma_lut, which will
cause a failure during the next atomic_check() as we aren't expecting
anything to be in degamma_lut since it's not visible to userspace.

Fix the problem by assigning whatever LUT we read out from the
hardware into gamma_lut.

Cc: stable@vger.kernel.org
Fixes: d2559299 ("drm/i915: Make ilk_read_luts() capable of degamma readout")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11608Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240710124137.16773-1-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>

drivers/gpu/drm/i915/display/intel_modeset_setup.c

@@ -326,6 +326,8 @@ static void intel_modeset_update_connector_atomic_state(struct drm_i915_private
 
 static void intel_crtc_copy_hw_to_uapi_state(struct intel_crtc_state *crtc_state)
 {
+	struct drm_i915_private *i915 = to_i915(crtc_state->uapi.crtc->dev);
+
 	if (intel_crtc_is_joiner_secondary(crtc_state))
 		return;
 
@@ -337,11 +339,30 @@ static void intel_crtc_copy_hw_to_uapi_state(struct intel_crtc_state *crtc_state
 	crtc_state->uapi.adjusted_mode = crtc_state->hw.adjusted_mode;
 	crtc_state->uapi.scaling_filter = crtc_state->hw.scaling_filter;
 
+	if (DISPLAY_INFO(i915)->color.degamma_lut_size) {
 		/* assume 1:1 mapping */
 		drm_property_replace_blob(&crtc_state->hw.degamma_lut,
 					  crtc_state->pre_csc_lut);
 		drm_property_replace_blob(&crtc_state->hw.gamma_lut,
 					  crtc_state->post_csc_lut);
+	} else {
+		/*
+		 * ilk/snb hw may be configured for either pre_csc_lut
+		 * or post_csc_lut, but we don't advertise degamma_lut as
+		 * being available in the uapi since there is only one
+		 * hardware LUT. Always assign the result of the readout
+		 * to gamma_lut as that is the only valid source of LUTs
+		 * in the uapi.
+		 */
+		drm_WARN_ON(&i915->drm, crtc_state->post_csc_lut &&
+			    crtc_state->pre_csc_lut);
+
+		drm_property_replace_blob(&crtc_state->hw.degamma_lut,
+					  NULL);
+		drm_property_replace_blob(&crtc_state->hw.gamma_lut,
+					  crtc_state->post_csc_lut ?:
+					  crtc_state->pre_csc_lut);
+	}
 
 	drm_property_replace_blob(&crtc_state->uapi.degamma_lut,
 				  crtc_state->hw.degamma_lut);