drm/i915/execlists: Simply walk back along request timeline on reset

The request's timeline will only contain requests from this context, in
order of execution. Therefore, we can simply look back along this
timeline to find the currently executing request.

If we do find that the current context has completed its last request,
that does not imply that all requests are completed in the context, so
only advance the ring->head up to the end of the known completions!
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028124125.25176-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -250,24 +250,16 @@ static void mark_eio(struct i915_request *rq)
 	i915_request_mark_complete(rq);
 }
 
-static struct i915_request *active_request(struct i915_request *rq)
+static struct i915_request *
+active_request(const struct intel_timeline * const tl, struct i915_request *rq)
 {
-	const struct intel_context * const ce = rq->hw_context;
-	struct i915_request *active = NULL;
-	struct list_head *list;
-
-	if (!i915_request_is_active(rq)) /* unwound, but incomplete! */
-		return rq;
+	struct i915_request *active = rq;
 
 	rcu_read_lock();
-	list = &rcu_dereference(rq->timeline)->requests;
-	list_for_each_entry_from_reverse(rq, list, link) {
+	list_for_each_entry_continue_reverse(rq, &tl->requests, link) {
 		if (i915_request_completed(rq))
 			break;
 
-		if (rq->hw_context != ce)
-			break;
-
 		active = rq;
 	}
 	rcu_read_unlock();
@@ -1073,6 +1065,7 @@ static void reset_active(struct i915_request *rq,
 			 struct intel_engine_cs *engine)
 {
 	struct intel_context * const ce = rq->hw_context;
+	u32 head;
 
 	/*
 	 * The executing context has been cancelled. We want to prevent
@@ -1093,11 +1086,11 @@ static void reset_active(struct i915_request *rq,
 		  __func__, engine->name, rq->fence.context, rq->fence.seqno);
 
 	/* On resubmission of the active request, payload will be scrubbed */
-	rq = active_request(rq);
-	if (rq)
-		ce->ring->head = intel_ring_wrap(ce->ring, rq->head);
+	if (i915_request_completed(rq))
+		head = rq->tail;
 	else
-		ce->ring->head = ce->ring->tail;
+		head = active_request(ce->timeline, rq)->head;
+	ce->ring->head = intel_ring_wrap(ce->ring, head);
 	intel_ring_update_space(ce->ring);
 
 	/* Scrub the context image to prevent replaying the previous batch */
@@ -2990,16 +2983,17 @@ static void __execlists_reset(struct intel_engine_cs *engine, bool stalled)
 	ce = rq->hw_context;
 	GEM_BUG_ON(!i915_vma_is_pinned(ce->state));
 
-	rq = active_request(rq);
-	if (!rq) {
+	if (i915_request_completed(rq)) {
 		/* Idle context; tidy up the ring so we can restart afresh */
-		ce->ring->head = ce->ring->tail;
+		ce->ring->head = intel_ring_wrap(ce->ring, rq->tail);
 		goto out_replay;
 	}
 
 	/* Context has requests still in-flight; it should not be idle! */
 	GEM_BUG_ON(i915_active_is_idle(&ce->active));
+	rq = active_request(ce->timeline, rq);
 	ce->ring->head = intel_ring_wrap(ce->ring, rq->head);
+	GEM_BUG_ON(ce->ring->head == ce->ring->tail);
 
 	/*
 	 * If this request hasn't started yet, e.g. it is waiting on a
@@ -3043,7 +3037,7 @@ static void __execlists_reset(struct intel_engine_cs *engine, bool stalled)
 	restore_default_state(ce, engine);
 
 out_replay:
-	GEM_TRACE("%s replay {head:%04x, tail:%04x\n",
+	GEM_TRACE("%s replay {head:%04x, tail:%04x}\n",
 		  engine->name, ce->ring->head, ce->ring->tail);
 	intel_ring_update_space(ce->ring);
 	__execlists_reset_reg_state(ce, engine);