drm/i915: Skip the linear degamma LUT load on ICL+

Don't load the linear degamma LUT on ICL. The hardware no longer
has any silly linkages between the CSC enable and degamma LUT
enable so the degamma LUT is only needed when it's actually
enabled.

Also add comments to explain the situation on GLK.

v2: Drop useless parens around 1<<16
v3: Add missing const
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-11-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_color.c

@@ -273,6 +273,14 @@ static void ilk_load_csc_matrix(const struct intel_crtc_state *crtc_state)
 				    ilk_csc_coeff_limited_range,
 				    ilk_csc_postoff_limited_range);
 	} else if (crtc_state->csc_enable) {
+		/*
+		 * On GLK+ both pipe CSC and degamma LUT are controlled
+		 * by csc_enable. Hence for the cases where the degama
+		 * LUT is needed but CSC is not we need to load an
+		 * identity matrix.
+		 */
+		WARN_ON(!IS_CANNONLAKE(dev_priv) && !IS_GEMINILAKE(dev_priv));
+
 		ilk_update_pipe_csc(crtc, ilk_csc_off_zero,
 				    ilk_csc_coeff_identity,
 				    ilk_csc_off_zero);
@@ -559,6 +567,7 @@ static void glk_load_degamma_lut(const struct intel_crtc_state *crtc_state)
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 	enum pipe pipe = crtc->pipe;
 	const u32 lut_size = INTEL_INFO(dev_priv)->color.degamma_lut_size;
+	const struct drm_color_lut *lut = crtc_state->base.degamma_lut->data;
 	u32 i;
 
 	/*
@@ -569,9 +578,6 @@ static void glk_load_degamma_lut(const struct intel_crtc_state *crtc_state)
 	I915_WRITE(PRE_CSC_GAMC_INDEX(pipe), 0);
 	I915_WRITE(PRE_CSC_GAMC_INDEX(pipe), PRE_CSC_GAMC_AUTO_INCREMENT);
 
-	if (crtc_state->base.degamma_lut) {
-		struct drm_color_lut *lut = crtc_state->base.degamma_lut->data;
-
 	for (i = 0; i < lut_size; i++) {
 		/*
 		 * First 33 entries represent range from 0 to 1.0
@@ -588,23 +594,53 @@ static void glk_load_degamma_lut(const struct intel_crtc_state *crtc_state)
 		 */
 		I915_WRITE(PRE_CSC_GAMC_DATA(pipe), lut[i].green);
 	}
-	} else {
-		/* load a linear table. */
+
+	/* Clamp values > 1.0. */
+	while (i++ < 35)
+		I915_WRITE(PRE_CSC_GAMC_DATA(pipe), 1 << 16);
+}
+
+static void glk_load_degamma_lut_linear(const struct intel_crtc_state *crtc_state)
+{
+	struct intel_crtc *crtc = to_intel_crtc(crtc_state->base.crtc);
+	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
+	enum pipe pipe = crtc->pipe;
+	const u32 lut_size = INTEL_INFO(dev_priv)->color.degamma_lut_size;
+	u32 i;
+
+	/*
+	 * When setting the auto-increment bit, the hardware seems to
+	 * ignore the index bits, so we need to reset it to index 0
+	 * separately.
+	 */
+	I915_WRITE(PRE_CSC_GAMC_INDEX(pipe), 0);
+	I915_WRITE(PRE_CSC_GAMC_INDEX(pipe), PRE_CSC_GAMC_AUTO_INCREMENT);
+
 	for (i = 0; i < lut_size; i++) {
-			u32 v = (i * (1 << 16)) / (lut_size - 1);
+		u32 v = (i << 16) / (lut_size - 1);
 
 		I915_WRITE(PRE_CSC_GAMC_DATA(pipe), v);
 	}
-	}
 
 	/* Clamp values > 1.0. */
 	while (i++ < 35)
-		I915_WRITE(PRE_CSC_GAMC_DATA(pipe), (1 << 16));
+		I915_WRITE(PRE_CSC_GAMC_DATA(pipe), 1 << 16);
 }
 
 static void glk_load_luts(const struct intel_crtc_state *crtc_state)
 {
+	/*
+	 * On GLK+ both pipe CSC and degamma LUT are controlled
+	 * by csc_enable. Hence for the cases where the CSC is
+	 * needed but degamma LUT is not we need to load a
+	 * linear degamma LUT. In fact we'll just always load
+	 * the degama LUT so that we don't have to reload
+	 * it every time the pipe CSC is being enabled.
+	 */
+	if (crtc_state->base.degamma_lut)
 		glk_load_degamma_lut(crtc_state);
+	else
+		glk_load_degamma_lut_linear(crtc_state);
 
 	if (crtc_state_is_legacy_gamma(crtc_state))
 		i9xx_load_luts(crtc_state);
@@ -614,6 +650,7 @@ static void glk_load_luts(const struct intel_crtc_state *crtc_state)
 
 static void icl_load_luts(const struct intel_crtc_state *crtc_state)
 {
+	if (crtc_state->base.degamma_lut)
 		glk_load_degamma_lut(crtc_state);
 
 	if (crtc_state_is_legacy_gamma(crtc_state))