drm/i915: Skip vcpi allocation for MSTB ports that are gone

Since we need to be able to allow DPMS on->off prop changes after an MST
port has disappeared from the system, we need to be able to make sure we
can compute a config for the resulting atomic commit. Currently this is
impossible when the port has disappeared, since the VCPI slot searching
we try to do in intel_dp_mst_compute_config() will fail with -EINVAL.

Since the only commits we want to allow on no-longer-present MST ports
are ones that shut off display hardware, we already know that no VCPI
allocations are needed. So, hardcode the VCPI slot count to 0 when
intel_dp_mst_compute_config() is called on an MST port that's gone.

Changes since V4:
- Don't use mst_port_gone at all, just check whether or not the drm
  connector is registered - Daniel Vetter
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181008232437.5571-5-lyude@redhat.com
(cherry picked from commit f67207d7)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_dp_mst.c

@@ -38,11 +38,11 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	struct intel_dp_mst_encoder *intel_mst = enc_to_mst(&encoder->base);
 	struct intel_digital_port *intel_dig_port = intel_mst->primary;
 	struct intel_dp *intel_dp = &intel_dig_port->dp;
-	struct intel_connector *connector =
-		to_intel_connector(conn_state->connector);
+	struct drm_connector *connector = conn_state->connector;
+	void *port = to_intel_connector(connector)->port;
 	struct drm_atomic_state *state = pipe_config->base.state;
 	int bpp;
-	int lane_count, slots;
+	int lane_count, slots = 0;
 	const struct drm_display_mode *adjusted_mode = &pipe_config->base.adjusted_mode;
 	int mst_pbn;
 	bool constant_n = drm_dp_has_quirk(&intel_dp->desc,
@@ -70,18 +70,24 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 
 	pipe_config->port_clock = intel_dp_max_link_rate(intel_dp);
 
-	if (drm_dp_mst_port_has_audio(&intel_dp->mst_mgr, connector->port))
+	if (drm_dp_mst_port_has_audio(&intel_dp->mst_mgr, port))
 		pipe_config->has_audio = true;
 
 	mst_pbn = drm_dp_calc_pbn_mode(adjusted_mode->crtc_clock, bpp);
 	pipe_config->pbn = mst_pbn;
 
-	slots = drm_dp_atomic_find_vcpi_slots(state, &intel_dp->mst_mgr,
-					      connector->port, mst_pbn);
+	/* Zombie connectors can't have VCPI slots */
+	if (READ_ONCE(connector->registered)) {
+		slots = drm_dp_atomic_find_vcpi_slots(state,
+						      &intel_dp->mst_mgr,
+						      port,
+						      mst_pbn);
 		if (slots < 0) {
-		DRM_DEBUG_KMS("failed finding vcpi slots:%d\n", slots);
+			DRM_DEBUG_KMS("failed finding vcpi slots:%d\n",
+				      slots);
 			return false;
 		}
+	}
 
 	intel_link_compute_m_n(bpp, lane_count,
 			       adjusted_mode->crtc_clock,