drm/i915: Ignore long hpds on eDP ports

Turning vdd on/off can generate a long hpd pulse on eDP ports. In order
to handle hpd we would need to turn on vdd to perform aux transfers.
This would lead to an endless cycle of
"vdd off -> long hpd -> vdd on -> detect -> vdd off -> ..."

So ignore long hpd pulses on eDP ports. eDP panels should be physically
tied to the machine anyway so they should not actually disappear and
thus don't need long hpd handling. Short hpds are still needed for link
re-train and whatnot so we can't just turn off the hpd interrupt
entirely for eDP ports. Perhaps we could turn it off whenever the panel
is disabled, but just ignoring the long hpd seems sufficient.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: NDave Airlie <airlied@redhat.com>
Reviewed-by: NTodd Previte <tprevite@gmail.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -4498,6 +4498,18 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
 	if (intel_dig_port->base.type != INTEL_OUTPUT_EDP)
 		intel_dig_port->base.type = INTEL_OUTPUT_DISPLAYPORT;
 
+	if (long_hpd && intel_dig_port->base.type == INTEL_OUTPUT_EDP) {
+		/*
+		 * vdd off can generate a long pulse on eDP which
+		 * would require vdd on to handle it, and thus we
+		 * would end up in an endless cycle of
+		 * "vdd off -> long hpd -> vdd on -> detect -> vdd off -> ..."
+		 */
+		DRM_DEBUG_KMS("ignoring long hpd on eDP port %c\n",
+			      port_name(intel_dig_port->port));
+		return false;
+	}
+
 	DRM_DEBUG_KMS("got hpd irq on port %c - %s\n",
 		      port_name(intel_dig_port->port),
 		      long_hpd ? "long" : "short");