[PATCH 2/3] drm/i915/dp_mst: Fix side-band message timeouts due to long PPS delays

[Imre Deak]

The Panel Power Sequencer lock held on an eDP port (a) blocks a DP AUX
transfer on another port (b), since the PPS lock is device global, thus
shared by all ports. The PPS lock can be held on port (a) for a longer
period due to the various PPS delays (panel/backlight on/off,
power-cycle delays). This in turn can cause an MST down-message request
on port (b) time out, if the above PPS delay defers the handling of the
reply to the request by more than 100ms: the MST branch device sending
the reply (signaling this via the DP_DOWN_REP_MSG_RDY flag in the
DP_DEVICE_SERVICE_IRQ_VECTOR DPCD register) may cancel the reply
(clearing DP_DOWN_REP_MSG_RDY and the reply message buffer) after 110
ms, if the reply is not processed by that time.

Avoid MST down-message timeouts described above, by locking the PPS
state for AUX transfers only if this is actually required: on eDP ports,
where the VDD power depends on the PPS state and on all DP and eDP ports
on VLV/CHV, where the PPS is a pipe instance and hence a modeset on any
port possibly affecting the PPS state.

Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/i915/display/intel_pps.c | 34 ++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_pps.c b/drivers/gpu/drm/i915/display/intel_pps.c
index 3c078fd53fbfa..7d7157983f25e 100644
--- a/drivers/gpu/drm/i915/display/intel_pps.c
+++ b/drivers/gpu/drm/i915/display/intel_pps.c
@@ -26,6 +26,11 @@ static void vlv_steal_power_sequencer(struct intel_display *display,
 static void pps_init_delays(struct intel_dp *intel_dp);
 static void pps_init_registers(struct intel_dp *intel_dp, bool force_disable_vdd);
 
+static bool intel_pps_is_pipe_instance(struct intel_display *display)
+{
+	return display->platform.valleyview || display->platform.cherryview;
+}
+
 static const char *pps_name(struct intel_dp *intel_dp)
 {
 	struct intel_display *display = to_intel_display(intel_dp);
@@ -955,10 +960,32 @@ void intel_pps_vdd_off(struct intel_dp *intel_dp)
 		intel_pps_vdd_off_unlocked(intel_dp, false);
 }
 
+static bool aux_needs_pps_lock(struct intel_dp *intel_dp)
+{
+	struct intel_display *display = to_intel_display(intel_dp);
+
+	/*
+	 * The PPS state needs to be locked for:
+	 * - eDP on all platforms, since AUX transfers on eDP need VDD power
+	 *   (either forced or via panel power) which depends on the PPS
+	 *   state.
+	 * - non-eDP on platforms where the PPS is a pipe instance (VLV/CHV),
+	 *   since changing the PPS state (via a parallel modeset for
+	 *   instance) may interfere with the AUX transfers on a non-eDP
+	 *   output as well.
+	 */
+	return intel_dp_is_edp(intel_dp) || intel_pps_is_pipe_instance(display);
+}
+
 intel_wakeref_t intel_pps_lock_for_aux(struct intel_dp *intel_dp, bool *vdd_ref)
 {
 	intel_wakeref_t wakeref;
 
+	if (!aux_needs_pps_lock(intel_dp)) {
+		*vdd_ref = false;
+		return NULL;
+	}
+
 	wakeref = intel_pps_lock(intel_dp);
 
 	/*
@@ -976,6 +1003,13 @@ intel_wakeref_t intel_pps_lock_for_aux(struct intel_dp *intel_dp, bool *vdd_ref)
 
 void intel_pps_unlock_for_aux(struct intel_dp *intel_dp, intel_wakeref_t wakeref, bool vdd_ref)
 {
+	struct intel_display *display = to_intel_display(intel_dp);
+
+	if (!wakeref) {
+		drm_WARN_ON(display->drm, vdd_ref || aux_needs_pps_lock(intel_dp));
+		return;
+	}
+
 	if (vdd_ref)
 		intel_pps_vdd_off_unlocked(intel_dp, false);
 
-- 
2.44.2