[PATCH 13/68] drm/i915/selftests: Force a rewind if at first we don't succeed

[Chris Wilson]

live_timeslice_rewind assumes a particular traversal and reordering
after the first timeslice yield. However, the outcome can be either
(A1, A2, B1) or (A1, B2, A2) depending on the path taken through the
dependency graph. So if we do not get the outcome we need at first, give
it a priority kick to force a rewind.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/selftest_execlists.c | 21 +++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/selftest_execlists.c b/drivers/gpu/drm/i915/gt/selftest_execlists.c
index 951e2bf867e1..68e1398704a4 100644
--- a/drivers/gpu/drm/i915/gt/selftest_execlists.c
+++ b/drivers/gpu/drm/i915/gt/selftest_execlists.c
@@ -1107,6 +1107,7 @@ static int live_timeslice_rewind(void *arg)
 		struct i915_request *rq[3] = {};
 		struct intel_context *ce;
 		unsigned long timeslice;
+		unsigned long timeout;
 		int i, err = 0;
 		u32 *slot;
 
@@ -1173,11 +1174,29 @@ static int live_timeslice_rewind(void *arg)
 
 		/* ELSP[] = { { A:rq1, A:rq2 }, { B:rq1 } } */
 		ENGINE_TRACE(engine, "forcing tasklet for rewind\n");
-		while (i915_request_is_active(rq[A2])) { /* semaphore yield! */
+		i = 0;
+		timeout = jiffies + HZ;
+		while (i915_request_is_active(rq[A2]) &&
+		       time_before(jiffies, timeout)) { /* semaphore yield! */
 			/* Wait for the timeslice to kick in */
 			del_timer(&engine->execlists.timer);
 			tasklet_hi_schedule(&engine->execlists.tasklet);
 			intel_engine_flush_submission(engine);
+
+			/*
+			 * Unfortunately this assumes that during the
+			 * search of the wait tree it sees the requests
+			 * in a particular order. That order is not
+			 * strictly determined and it may pick either
+			 * A2 or B1 to immediately follow A1.
+			 *
+			 * Break the tie with a set-priority. This defeats
+			 * the goal of trying to cause a rewind with a
+			 * timeslice, but alas, a rewind is better than
+			 * none.
+			 */
+			if (i++)
+				i915_request_set_priority(rq[B1], 1);
 		}
 		/* -> ELSP[] = { { A:rq1 }, { B:rq1 } } */
 		GEM_BUG_ON(!i915_request_is_active(rq[A1]));
-- 
2.20.1