drm/i915/gt: Add arbitration check before semaphore wait

Similar to commit 49b20dbf74 ("drm/i915/gt: Perform an arbitration
check before busywaiting"), also add a check prior to the busywait
on gen8+, as we have now seen (because we added a selftest to add fault
injection into the engine resets) the same engine reset failure leading
to an indefinite wait on the ring-stop semaphore. So not a Tigerlake
specific bug after all, though it still seems odd behaviour for the
busywait as we do get the arbitration point elsewhere on a miss.

Testcase: igt_reset_fail_engine
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210117110418.3361-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/gen8_engine_cs.c

@@ -488,6 +488,7 @@ static u32 *gen8_emit_wa_tail(struct i915_request *rq, u32 *cs)
 
 static u32 *emit_preempt_busywait(struct i915_request *rq, u32 *cs)
 {
+	*cs++ = MI_ARB_CHECK; /* trigger IDLE->ACTIVE first */
 	*cs++ = MI_SEMAPHORE_WAIT |
 		MI_SEMAPHORE_GLOBAL_GTT |
 		MI_SEMAPHORE_POLL |
@@ -495,6 +496,7 @@ static u32 *emit_preempt_busywait(struct i915_request *rq, u32 *cs)
 	*cs++ = 0;
 	*cs++ = preempt_address(rq->engine);
 	*cs++ = 0;
+	*cs++ = MI_NOOP;
 
 	return cs;
 }