drm/i915: Clear DP M2/N2 when not doing DRRS

Make life simpler by always programming DP M2/N2 with a consistent
value. This will lets use do state readout+chec unconditionally.

I was first going to just set M2/N2=M1/N1 but then it occurred
to me that it might interfere with fastboot on account of BIOS
likely leaving the registers zeroed. So let's zero out the values
instead (except TU where a zero register value actually means '1').
Still not sure that's the best approach but lets go with it for
now.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-14-ville.syrjala@linux.intel.comReviewed-by: NJani Nikula <jani.nikula@intel.com>

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

@@ -3137,6 +3137,13 @@ static void intel_panel_sanitize_ssc(struct drm_i915_private *dev_priv)
 	}
 }
 
+void intel_zero_m_n(struct intel_link_m_n *m_n)
+{
+	/* corresponds to 0 register value */
+	memset(m_n, 0, sizeof(*m_n));
+	m_n->tu = 1;
+}
+
 void intel_set_m_n(struct drm_i915_private *i915,
 		   const struct intel_link_m_n *m_n,
 		   i915_reg_t data_m_reg, i915_reg_t data_n_reg,
@@ -3148,8 +3155,8 @@ void intel_set_m_n(struct drm_i915_private *i915,
 	intel_de_write(i915, link_n_reg, m_n->link_n);
 }
 
-static bool transcoder_has_m2_n2(struct drm_i915_private *dev_priv,
-				 enum transcoder transcoder)
+bool intel_cpu_transcoder_has_m2_n2(struct drm_i915_private *dev_priv,
+				    enum transcoder transcoder)
 {
 	if (IS_HASWELL(dev_priv))
 		return transcoder == TRANSCODER_EDP;
@@ -3180,7 +3187,7 @@ void intel_cpu_transcoder_set_m2_n2(struct intel_crtc *crtc,
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 
-	if (!transcoder_has_m2_n2(dev_priv, transcoder))
+	if (!intel_cpu_transcoder_has_m2_n2(dev_priv, transcoder))
 		return;
 
 	intel_set_m_n(dev_priv, m_n,
@@ -3878,7 +3885,7 @@ void intel_cpu_transcoder_get_m2_n2(struct intel_crtc *crtc,
 {
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 
-	if (!transcoder_has_m2_n2(dev_priv, transcoder))
+	if (!intel_cpu_transcoder_has_m2_n2(dev_priv, transcoder))
 		return;
 
 	intel_get_m_n(dev_priv, m_n,

drivers/gpu/drm/i915/display/intel_display.h

@@ -606,6 +606,7 @@ bool intel_fuzzy_clock_check(int clock1, int clock2);
 
 void intel_display_prepare_reset(struct drm_i915_private *dev_priv);
 void intel_display_finish_reset(struct drm_i915_private *dev_priv);
+void intel_zero_m_n(struct intel_link_m_n *m_n);
 void intel_set_m_n(struct drm_i915_private *i915,
 		   const struct intel_link_m_n *m_n,
 		   i915_reg_t data_m_reg, i915_reg_t data_n_reg,
@@ -614,6 +615,8 @@ void intel_get_m_n(struct drm_i915_private *i915,
 		   struct intel_link_m_n *m_n,
 		   i915_reg_t data_m_reg, i915_reg_t data_n_reg,
 		   i915_reg_t link_m_reg, i915_reg_t link_n_reg);
+bool intel_cpu_transcoder_has_m2_n2(struct drm_i915_private *dev_priv,
+				    enum transcoder transcoder);
 void intel_cpu_transcoder_set_m1_n1(struct intel_crtc *crtc,
 				    enum transcoder cpu_transcoder,
 				    const struct intel_link_m_n *m_n);

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

@@ -74,10 +74,14 @@ intel_drrs_compute_config(struct intel_dp *intel_dp,
 			  int output_bpp, bool constant_n)
 {
 	struct intel_connector *connector = intel_dp->attached_connector;
+	struct drm_i915_private *i915 = to_i915(connector->base.dev);
 	int pixel_clock;
 
-	if (!can_enable_drrs(connector, pipe_config))
+	if (!can_enable_drrs(connector, pipe_config)) {
+		if (intel_cpu_transcoder_has_m2_n2(i915, pipe_config->cpu_transcoder))
+			intel_zero_m_n(&pipe_config->dp_m2_n2);
 		return;
+	}
 
 	pipe_config->has_drrs = true;