drm/i915/dp_mst: Force modeset CRTC if DSC toggling requires it

Enabling / disabling DSC decompression in the branch device downstream
of the source may reset the whole branch device. To avoid this while the
streams are still active, force a modeset on all CRTC/ports connected to
this branch device.

v2:
- Check the CRTC state for each connector in the topology, instead of
  the CRTC being checked for a modeset requirement. (Ville)
- Add DocBook for the new function.
v3:
- Rebased on a change not to use
  intel_modeset_pipes_in_mask_early().

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-8-imre.deak@intel.com

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

@@ -6481,6 +6481,9 @@ int intel_atomic_check(struct drm_device *dev,
 		if (!new_crtc_state->hw.enable || intel_crtc_needs_modeset(new_crtc_state))
 			continue;
 
+		if (intel_dp_mst_crtc_needs_modeset(state, crtc))
+			intel_crtc_flag_modeset(new_crtc_state);
+
 		if (intel_dp_mst_is_slave_trans(new_crtc_state)) {
 			enum transcoder master = new_crtc_state->mst_master_transcoder;
 

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

@@ -1628,3 +1628,91 @@ int intel_dp_mst_add_topology_state_for_crtc(struct intel_atomic_state *state,
 
 	return 0;
 }
+
+static struct intel_connector *
+get_connector_in_state_for_crtc(struct intel_atomic_state *state,
+				const struct intel_crtc *crtc)
+{
+	struct drm_connector_state *old_conn_state;
+	struct drm_connector_state *new_conn_state;
+	struct drm_connector *_connector;
+	int i;
+
+	for_each_oldnew_connector_in_state(&state->base, _connector,
+					   old_conn_state, new_conn_state, i) {
+		struct intel_connector *connector =
+			to_intel_connector(_connector);
+
+		if (old_conn_state->crtc == &crtc->base ||
+		    new_conn_state->crtc == &crtc->base)
+			return connector;
+	}
+
+	return NULL;
+}
+
+/**
+ * intel_dp_mst_crtc_needs_modeset - check if changes in topology need to modeset the given CRTC
+ * @state: atomic state
+ * @crtc: CRTC for which to check the modeset requirement
+ *
+ * Check if any change in a MST topology requires a forced modeset on @crtc in
+ * this topology. One such change is enabling/disabling the DSC decompression
+ * state in the first branch device's UFP DPCD as required by one CRTC, while
+ * the other @crtc in the same topology is still active, requiring a full modeset
+ * on @crtc.
+ */
+bool intel_dp_mst_crtc_needs_modeset(struct intel_atomic_state *state,
+				     struct intel_crtc *crtc)
+{
+	const struct intel_connector *crtc_connector;
+	const struct drm_connector_state *conn_state;
+	const struct drm_connector *_connector;
+	int i;
+
+	if (!intel_crtc_has_type(intel_atomic_get_new_crtc_state(state, crtc),
+				 INTEL_OUTPUT_DP_MST))
+		return false;
+
+	crtc_connector = get_connector_in_state_for_crtc(state, crtc);
+
+	if (!crtc_connector)
+		/* None of the connectors in the topology needs modeset */
+		return false;
+
+	for_each_new_connector_in_state(&state->base, _connector, conn_state, i) {
+		const struct intel_connector *connector =
+			to_intel_connector(_connector);
+		const struct intel_crtc_state *new_crtc_state;
+		const struct intel_crtc_state *old_crtc_state;
+		struct intel_crtc *crtc_iter;
+
+		if (connector->mst_port != crtc_connector->mst_port ||
+		    !conn_state->crtc)
+			continue;
+
+		crtc_iter = to_intel_crtc(conn_state->crtc);
+
+		new_crtc_state = intel_atomic_get_new_crtc_state(state, crtc_iter);
+		old_crtc_state = intel_atomic_get_old_crtc_state(state, crtc_iter);
+
+		if (!intel_crtc_needs_modeset(new_crtc_state))
+			continue;
+
+		if (old_crtc_state->dsc.compression_enable ==
+		    new_crtc_state->dsc.compression_enable)
+			continue;
+		/*
+		 * Toggling the decompression flag because of this stream in
+		 * the first downstream branch device's UFP DPCD may reset the
+		 * whole branch device. To avoid the reset while other streams
+		 * are also active modeset the whole MST topology in this
+		 * case.
+		 */
+		if (connector->dp.dsc_decompression_aux ==
+		    &connector->mst_port->aux)
+			return true;
+	}
+
+	return false;
+}

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

@@ -22,5 +22,7 @@ bool intel_dp_mst_is_slave_trans(const struct intel_crtc_state *crtc_state);
 bool intel_dp_mst_source_support(struct intel_dp *intel_dp);
 int intel_dp_mst_add_topology_state_for_crtc(struct intel_atomic_state *state,
 					     struct intel_crtc *crtc);
+bool intel_dp_mst_crtc_needs_modeset(struct intel_atomic_state *state,
+				     struct intel_crtc *crtc);
 
 #endif /* __INTEL_DP_MST_H__ */