drm/i915: Make sure engines are idle during GPU idling in LR mode

We assume that the GPU is idle once receiving the seqno via the last
request's user interrupt. In execlist mode the corresponding context
completed interrupt can be delayed though and until this latter
interrupt arrives we consider the request to be pending on the ELSP
submit port. This can cause a problem during system suspend where this
last request will be seen by the resume code as still pending. Such
pending requests are normally replayed after a GPU reset, but during
resume we reset both SW and HW tracking of the ring head/tail pointers,
so replaying the pending request with its stale tail pointer will leave
the ring in an inconsistent state. A subsequent request submission can
lead then to the GPU executing from uninitialized area in the ring
behind the above stale tail pointer.

Fix this by making sure any pending request on the ELSP port is
completed before suspending. I used a polling wait since the completion
time I measured was <1ms and since normally we only need to wait during
system suspend. GPU idling during runtime suspend is scheduled with a
delay (currently 50-100ms) after the retirement of the last request at
which point the context completed interrupt must have arrived already.

The chance of this bug was increased by

commit 1c777c5d1dcdf8fa0223fcff35fb387b5bb9517a
Author: Imre Deak <imre.deak@intel.com>
Date:   Wed Oct 12 17:46:37 2016 +0300

    drm/i915/hsw: Fix GPU hang during resume from S3-devices state

but it could happen even without the explicit GPU reset, since we
disable interrupts afterwards during the suspend sequence.

v2:
- Do an unlocked poll-wait first. (Chris)
v3-4:
- s/intel_lr_engines_idle/intel_execlists_idle/ and move
  i915.enable_execlists check to the new helper. (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98470
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478510405-11799-3-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/i915_gem.c

@@ -2752,6 +2752,13 @@ i915_gem_idle_work_handler(struct work_struct *work)
 	if (!READ_ONCE(dev_priv->gt.awake))
 		return;
 
+	/*
+	 * Wait for last execlists context complete, but bail out in case a
+	 * new request is submitted.
+	 */
+	wait_for(READ_ONCE(dev_priv->gt.active_requests) ||
+		 intel_execlists_idle(dev_priv), 10);
+
 	if (READ_ONCE(dev_priv->gt.active_requests))
 		return;
 
@@ -2776,6 +2783,9 @@ i915_gem_idle_work_handler(struct work_struct *work)
 	if (dev_priv->gt.active_requests)
 		goto out_unlock;
 
+	if (wait_for(intel_execlists_idle(dev_priv), 10))
+		DRM_ERROR("Timeout waiting for engines to idle\n");
+
 	for_each_engine(engine, dev_priv, id)
 		i915_gem_batch_pool_fini(&engine->batch_pool);
 

drivers/gpu/drm/i915/intel_lrc.c

@@ -522,6 +522,28 @@ static bool execlists_elsp_idle(struct intel_engine_cs *engine)
 	return !engine->execlist_port[0].request;
 }
 
+/**
+ * intel_execlists_idle() - Determine if all engine submission ports are idle
+ * @dev_priv: i915 device private
+ *
+ * Return true if there are no requests pending on any of the submission ports
+ * of any engines.
+ */
+bool intel_execlists_idle(struct drm_i915_private *dev_priv)
+{
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+
+	if (!i915.enable_execlists)
+		return true;
+
+	for_each_engine(engine, dev_priv, id)
+		if (!execlists_elsp_idle(engine))
+			return false;
+
+	return true;
+}
+
 static bool execlists_elsp_ready(struct intel_engine_cs *engine)
 {
 	int port;

drivers/gpu/drm/i915/intel_lrc.h

@@ -95,5 +95,6 @@ uint64_t intel_lr_context_descriptor(struct i915_gem_context *ctx,
 int intel_sanitize_enable_execlists(struct drm_i915_private *dev_priv,
 				    int enable_execlists);
 void intel_execlists_enable_submission(struct drm_i915_private *dev_priv);
+bool intel_execlists_idle(struct drm_i915_private *dev_priv);
 
 #endif /* _INTEL_LRC_H_ */