drm/i915/dp: Disable the AUX DPCD probe quirk if it's not required

Reading DPCD registers has side-effects and some of these can cause a
problem for instance during link training. Based on this it's better to
avoid the probing quirk done before each DPCD register read, limiting
this to the monitor which requires it. The only known problematic
monitor is an external SST sink, so keep the quirk disabled always for
eDP and MST sinks. Reenable the quirk after a hotplug event and after
resuming from a power state without hotplug support, until the
subsequent EDID based detection.

v2: Add a helper for determining the need/setting the probing. (Jani)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250609125556.109538-2-imre.deak@intel.com

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

@@ -5790,6 +5790,28 @@ intel_dp_detect_sdp_caps(struct intel_dp *intel_dp)
 		drm_dp_as_sdp_supported(&intel_dp->aux, intel_dp->dpcd);
 }
 
+static bool intel_dp_needs_dpcd_probe(struct intel_dp *intel_dp, bool force_on_external)
+{
+	struct intel_connector *connector = intel_dp->attached_connector;
+
+	if (intel_dp_is_edp(intel_dp))
+		return false;
+
+	if (force_on_external)
+		return true;
+
+	if (intel_dp->is_mst)
+		return false;
+
+	return drm_edid_has_quirk(&connector->base, DRM_EDID_QUIRK_DP_DPCD_PROBE);
+}
+
+void intel_dp_dpcd_set_probe(struct intel_dp *intel_dp, bool force_on_external)
+{
+	drm_dp_dpcd_set_probe(&intel_dp->aux,
+			      intel_dp_needs_dpcd_probe(intel_dp, force_on_external));
+}
+
 static int
 intel_dp_detect(struct drm_connector *_connector,
 		struct drm_modeset_acquire_ctx *ctx,
@@ -5918,6 +5940,8 @@ out_unset_edid:
 	if (status != connector_status_connected && !intel_dp->is_mst)
 		intel_dp_unset_edid(intel_dp);
 
+	intel_dp_dpcd_set_probe(intel_dp, false);
+
 	if (!intel_dp_is_edp(intel_dp))
 		drm_dp_set_subconnector_property(&connector->base,
 						 status,
@@ -5948,6 +5972,8 @@ intel_dp_force(struct drm_connector *_connector)
 		return;
 
 	intel_dp_set_edid(intel_dp);
+
+	intel_dp_dpcd_set_probe(intel_dp, false);
 }
 
 static int intel_dp_get_modes(struct drm_connector *_connector)
@@ -6320,10 +6346,11 @@ intel_dp_hpd_pulse(struct intel_digital_port *dig_port, bool long_hpd)
 	 * complete the DP tunnel BW request for the latter connector/encoder
 	 * waiting for this encoder's DPRX read, perform a dummy read here.
 	 */
-	if (long_hpd)
+	if (long_hpd) {
+		intel_dp_dpcd_set_probe(intel_dp, true);
+
 		intel_dp_read_dprx_caps(intel_dp, dpcd);
 
-	if (long_hpd) {
 		intel_dp->reset_link_params = true;
 		intel_dp_invalidate_source_oui(intel_dp);
 

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

@@ -213,5 +213,6 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
 				const struct drm_connector_state *conn_state);
 
 int intel_dp_dsc_bpp_step_x16(const struct intel_connector *connector);
+void intel_dp_dpcd_set_probe(struct intel_dp *intel_dp, bool force_on_external);
 
 #endif /* __INTEL_DP_H__ */

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

@@ -834,6 +834,8 @@ void intel_dp_aux_init(struct intel_dp *intel_dp)
 
 	intel_dp->aux.transfer = intel_dp_aux_transfer;
 	cpu_latency_qos_add_request(&intel_dp->pm_qos, PM_QOS_DEFAULT_VALUE);
+
+	intel_dp_dpcd_set_probe(intel_dp, true);
 }
 
 static enum aux_ch default_aux_ch(struct intel_encoder *encoder)

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

@@ -33,6 +33,7 @@
 #include "intel_display_core.h"
 #include "intel_display_rpm.h"
 #include "intel_display_types.h"
+#include "intel_dp.h"
 #include "intel_hdcp.h"
 #include "intel_hotplug.h"
 #include "intel_hotplug_irq.h"
@@ -906,9 +907,14 @@ void intel_hpd_poll_enable(struct intel_display *display)
  */
 void intel_hpd_poll_disable(struct intel_display *display)
 {
+	struct intel_encoder *encoder;
+
 	if (!HAS_DISPLAY(display))
 		return;
 
+	for_each_intel_dp(display->drm, encoder)
+		intel_dp_dpcd_set_probe(enc_to_intel_dp(encoder), true);
+
 	WRITE_ONCE(display->hotplug.poll_enabled, false);
 
 	spin_lock_irq(&display->irq.lock);