drm/i915/tc: Abort DP AUX transfer on a disconnected TC port

On TC ports the 4ms AUX timeout combined with the 5 * 32 retry
attempts during DPCD accesses adds a 640ms delay to each access if the
sink is disconnected. This in turn slows down a modeset during which the
sink is disconnected (for instance a disabling modeset).

Prevent the above delay by aborting AUX transfers on a TC port with a
disconnected sink.

The DP 1.4a link CTS (4.2.1.5 Source Device Inactive HPD / Inactive AUX
Test") also requires not to initiate AUX transfers on disconnected DP
ports in general, however this patch doesn't change the behavior on
non-TC ports, leaving that for a follow-up.
Reported-and-tested-by: Chris Chiu <chris.chiu@canonical.com>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8279Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316131724.359612-2-imre.deak@intel.com

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

@@ -205,8 +205,19 @@ intel_dp_aux_xfer(struct intel_dp *intel_dp,
 	for (i = 0; i < ARRAY_SIZE(ch_data); i++)
 		ch_data[i] = intel_dp->aux_ch_data_reg(intel_dp, i);
 
-	if (is_tc_port)
+	if (is_tc_port) {
 		intel_tc_port_lock(dig_port);
+		/*
+		 * Abort transfers on a disconnected port as required by
+		 * DP 1.4a link CTS 4.2.1.5, also avoiding the long AUX
+		 * timeouts that would otherwise happen.
+		 * TODO: abort the transfer on non-TC ports as well.
+		 */
+		if (!intel_tc_port_connected_locked(&dig_port->base)) {
+			ret = -ENXIO;
+			goto out_unlock;
+		}
+	}
 
 	aux_domain = intel_aux_power_domain(dig_port);
 
@@ -367,7 +378,7 @@ intel_dp_aux_xfer(struct intel_dp *intel_dp,
 
 	intel_pps_unlock(intel_dp, pps_wakeref);
 	intel_display_power_put_async(i915, aux_domain, aux_wakeref);
-
+out_unlock:
 	if (is_tc_port)
 		intel_tc_port_unlock(dig_port);
 

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

@@ -768,16 +768,23 @@ void intel_tc_port_sanitize_mode(struct intel_digital_port *dig_port)
  * connected ports are usable, and avoids exposing to the users objects they
  * can't really use.
  */
+bool intel_tc_port_connected_locked(struct intel_encoder *encoder)
+{
+	struct intel_digital_port *dig_port = enc_to_dig_port(encoder);
+	struct drm_i915_private *i915 = to_i915(dig_port->base.base.dev);
+
+	drm_WARN_ON(&i915->drm, !intel_tc_port_ref_held(dig_port));
+
+	return tc_port_live_status_mask(dig_port) & BIT(dig_port->tc_mode);
+}
+
 bool intel_tc_port_connected(struct intel_encoder *encoder)
 {
 	struct intel_digital_port *dig_port = enc_to_dig_port(encoder);
 	bool is_connected;
 
 	intel_tc_port_lock(dig_port);
-
-	is_connected = tc_port_live_status_mask(dig_port) &
-		       BIT(dig_port->tc_mode);
-
+	is_connected = intel_tc_port_connected_locked(encoder);
 	intel_tc_port_unlock(dig_port);
 
 	return is_connected;

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

@@ -17,6 +17,7 @@ bool intel_tc_port_in_dp_alt_mode(struct intel_digital_port *dig_port);
 bool intel_tc_port_in_legacy_mode(struct intel_digital_port *dig_port);
 
 bool intel_tc_port_connected(struct intel_encoder *encoder);
+bool intel_tc_port_connected_locked(struct intel_encoder *encoder);
 
 u32 intel_tc_port_get_lane_mask(struct intel_digital_port *dig_port);
 u32 intel_tc_port_get_pin_assignment_mask(struct intel_digital_port *dig_port);