[PATCH 14/16] drm/i915/gt: Perform an arbitration check before busywaiting

[Chris Wilson]

During igt_reset_nop_engine, it was observed that an unexpected failed
engine reset lead to us busywaiting on the stop-ring semaphore (set
during the reset preparations) on the first request afterwards. There was
no explicit MI_ARB_CHECK in this sequence as the presumption was that
the failed MI_SEMAPHORE_WAIT would itself act as an arbitration point.
It did not in this circumstance, so force it.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/gen8_engine_cs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/gen8_engine_cs.c b/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
index 9a182652a35e..e9ac281644cc 100644
--- a/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
@@ -567,6 +567,7 @@ u32 *gen11_emit_fini_breadcrumb_rcs(struct i915_request *rq, u32 *cs)
 
 static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
 {
+	*cs++ = MI_ARB_CHECK;
 	*cs++ = MI_SEMAPHORE_WAIT_TOKEN |
 		MI_SEMAPHORE_GLOBAL_GTT |
 		MI_SEMAPHORE_POLL |
@@ -575,7 +576,6 @@ static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
 	*cs++ = preempt_address(rq->engine);
 	*cs++ = 0;
 	*cs++ = 0;
-	*cs++ = MI_NOOP;
 
 	return cs;
 }
-- 
2.20.1