drm/i915/bdw: Avoid non-lite-restore preemptions

In the current Execlists feeding mechanism, full preemption is not
supported yet: only lite-restores are allowed (this is: the GPU
simply samples a new tail pointer for the context currently in
execution).

But we have identified an scenario in which a full preemption occurs:
1) We submit two contexts for execution (A & B).
2) The GPU finishes with the first one (A), switches to the second one
(B) and informs us.
3) We submit B again (hoping to cause a lite restore) together with C,
but in the time we spend writing to the ELSP, the GPU finishes B.
4) The GPU start executing B again (since we told it so).
5) We receive a B finished interrupt and, mistakenly, we submit C (again)
and D, causing a full preemption of B.

The race is avoided by keeping track of how many times a context has been
submitted to the hardware and by better discriminating the received context
switch interrupts: in the example, when we have submitted B twice, we won´t
submit C and D as soon as we receive the notification that B is completed
because we were expecting to get a LITE_RESTORE and we didn´t, so we know a
second completion will be received shortly.

Without this explicit checking, somehow, the batch buffer execution order
gets messed with. This can be verified with the IGT test I sent together with
the series. I don´t know the exact mechanism by which the pre-emption messes
with the execution order but, since other people is working on the Scheduler
+ Preemption on Execlists, I didn´t try to fix it. In these series, only Lite
Restores are supported (other kind of preemptions WARN).

v2: elsp_submitted belongs in the new intel_ctx_submit_request. Several
rebase changes.

v3: Clarify how the race is avoided, as requested by Daniel.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Align function parameters ...]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -268,6 +268,7 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
 		} else if (req0->ctx == cursor->ctx) {
 			/* Same ctx: ignore first request, as second request
 			 * will update tail past first request's workload */
+			cursor->elsp_submitted = req0->elsp_submitted;
 			list_del(&req0->execlist_link);
 			queue_work(dev_priv->wq, &req0->work);
 			req0 = cursor;
@@ -277,9 +278,15 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
 		}
 	}
 
+	WARN_ON(req1 && req1->elsp_submitted);
+
 	WARN_ON(execlists_submit_context(ring, req0->ctx, req0->tail,
 					 req1 ? req1->ctx : NULL,
 					 req1 ? req1->tail : 0));
+
+	req0->elsp_submitted++;
+	if (req1)
+		req1->elsp_submitted++;
 }
 
 static bool execlists_check_remove_request(struct intel_engine_cs *ring,
@@ -298,9 +305,14 @@ static bool execlists_check_remove_request(struct intel_engine_cs *ring,
 		struct drm_i915_gem_object *ctx_obj =
 				head_req->ctx->engine[ring->id].state;
 		if (intel_execlists_ctx_id(ctx_obj) == request_id) {
-			list_del(&head_req->execlist_link);
-			queue_work(dev_priv->wq, &head_req->work);
-			return true;
+			WARN(head_req->elsp_submitted == 0,
+			     "Never submitted head request\n");
+
+			if (--head_req->elsp_submitted <= 0) {
+				list_del(&head_req->execlist_link);
+				queue_work(dev_priv->wq, &head_req->work);
+				return true;
+			}
 		}
 	}
 
@@ -333,7 +345,16 @@ void intel_execlists_handle_ctx_events(struct intel_engine_cs *ring)
 		status_id = I915_READ(RING_CONTEXT_STATUS_BUF(ring) +
 				(read_pointer % 6) * 8 + 4);
 
-		if (status & GEN8_CTX_STATUS_COMPLETE) {
+		if (status & GEN8_CTX_STATUS_PREEMPTED) {
+			if (status & GEN8_CTX_STATUS_LITE_RESTORE) {
+				if (execlists_check_remove_request(ring, status_id))
+					WARN(1, "Lite Restored request removed from queue\n");
+			} else
+				WARN(1, "Preemption without Lite Restore\n");
+		}
+
+		 if ((status & GEN8_CTX_STATUS_ACTIVE_IDLE) ||
+		     (status & GEN8_CTX_STATUS_ELEMENT_SWITCH)) {
 			if (execlists_check_remove_request(ring, status_id))
 				submit_contexts++;
 		}

drivers/gpu/drm/i915/intel_lrc.h

@@ -66,6 +66,8 @@ struct intel_ctx_submit_request {
 
 	struct list_head execlist_link;
 	struct work_struct work;
+
+	int elsp_submitted;
 };
 
 void intel_execlists_handle_ctx_events(struct intel_engine_cs *ring);