drm/i915: Rollback seqno when request creation fails

gem_ctx_create.basic-files can slam on kernel contexts to the extent
where request creation fails because the ring is full. When this happens
seqno numbers are skipped which can result the below GEM_BUG_ON blowing
in gt/intel_engine_pm.c:__engine_unpark:

GEM_BUG_ON(ce->timeline->seqno !=
           READ_ONCE(*ce->timeline->hwsp_seqno));

Fixup request creation code to roll back seqno when request creation
fails.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>

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

@@ -301,6 +301,11 @@ static u32 timeline_advance(struct intel_timeline *tl)
 	return tl->seqno += 1 + tl->has_initial_breadcrumb;
 }
 
+void intel_timeline_rollback_seqno(struct intel_timeline *tl)
+{
+	timeline_rollback(tl);
+}
+
 static noinline int
 __intel_timeline_get_seqno(struct intel_timeline *tl,
 			   u32 *seqno)

drivers/gpu/drm/i915/gt/intel_timeline.h

@@ -72,6 +72,7 @@ void intel_timeline_enter(struct intel_timeline *tl);
 int intel_timeline_get_seqno(struct intel_timeline *tl,
 			     struct i915_request *rq,
 			     u32 *seqno);
+void intel_timeline_rollback_seqno(struct intel_timeline *tl);
 void intel_timeline_exit(struct intel_timeline *tl);
 void intel_timeline_unpin(struct intel_timeline *tl);
 

drivers/gpu/drm/i915/i915_request.c

@@ -966,6 +966,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
 
 err_unwind:
 	ce->ring->emit = rq->head;
+	intel_timeline_rollback_seqno(tl);
 
 	/* Make sure we didn't add ourselves to external state before freeing */
 	GEM_BUG_ON(!list_empty(&rq->sched.signalers_list));