[igt-dev] [PATCH i-g-t] lib/i915/gem_engine_topology.c - intel_get_current_engine invalid result

[Dale B Stimson]

Function intel_get_current_engine() should return NULL (instead of
engine 0) if there are no engines.

Function intel_init_engine_list() should not store potential engine
data in the output structure unless the engine is present.

Function intel_init_engine_list() should arguably not filter the static
engine list with gem_has_ring if fd == -1, so that subtests can still
be individually invoked to show subtest FAIL instead of test notrun.

Symptom: A device open failure in gem_ctx_isolation resulted in
an endless __for_each_physical_engine "per-engine" loop with the
purported last potential engine being processed every time.

Diagnosis: device open (or debugfs open) failed, leaving fd == -1.
Control skipped the rest of the initial igt_fixture block, after
which an attempt was made to iterate through engines using macro
__for_each_physical_engine.

Macro __for_each_physical_engine called intel_init_engine_list()
to initialize the loop control data.  Because fd == -1,
intel_init_engine_list() fell back to using __for_each_static_engine().
All of the engines in the static engine list are rejected due to
gem_has_ring returning false (because of fd == -1), leaving 0 engines.
That resulted in loop control data with engine_data.nengines == 0
and the data for the last engine considered stored at index 0.

Still in macro __for_each_physical_engine, intel_get_current_engine()
was called to get the engine to process.  It should have returned NULL,
but instead returned the engine entry at index 0, which
had received information describing the last potential engine.
This happened without end.

Signed-off-by: Dale B Stimson <dale.b.stimson at intel.com>
---
 lib/i915/gem_engine_topology.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/lib/i915/gem_engine_topology.c b/lib/i915/gem_engine_topology.c
index 9daa03df4..b8ed49bc9 100644
--- a/lib/i915/gem_engine_topology.c
+++ b/lib/i915/gem_engine_topology.c
@@ -156,10 +156,10 @@ static void query_engine_list(int fd, struct intel_engine_data *ed)
 struct intel_execution_engine2 *
 intel_get_current_engine(struct intel_engine_data *ed)
 {
-	if (!ed->n)
-		ed->current_engine = &ed->engines[0];
-	else if (ed->n >= ed->nengines)
+	if (ed->n >= ed->nengines)
 		ed->current_engine = NULL;
+	else if (!ed->n)
+		ed->current_engine = &ed->engines[0];
 
 	return ed->current_engine;
 }
@@ -222,18 +222,21 @@ struct intel_engine_data intel_init_engine_list(int fd, uint32_t ctx_id)
 		igt_debug("using pre-allocated engine list\n");
 
 		__for_each_static_engine(e2) {
-			struct intel_execution_engine2 *__e2 =
-				&engine_data.engines[engine_data.nengines];
-
-			strcpy(__e2->name, e2->name);
-			__e2->instance   = e2->instance;
-			__e2->class      = e2->class;
-			__e2->flags      = e2->flags;
-			__e2->is_virtual = false;
-
 			if (igt_only_list_subtests() ||
-			    gem_has_ring(fd, e2->flags))
+			    (fd < 0) ||
+			    gem_has_ring(fd, e2->flags)) {
+				struct intel_execution_engine2 *__e2 =
+					&engine_data.engines[
+					engine_data.nengines];
+
+				strcpy(__e2->name, e2->name);
+				__e2->instance   = e2->instance;
+				__e2->class      = e2->class;
+				__e2->flags      = e2->flags;
+				__e2->is_virtual = false;
+
 				engine_data.nengines++;
+                        }
 		}
 		return engine_data;
 	}
-- 
2.25.0