[Intel-gfx] [PATCH] drm/i915/gt: Skip request resubmission if lite-restore w/a already applied

[Chris Wilson]

Be careful that we never submit the same request again if we have
already applied the LiteRestore w/a upon it -- as the HW may complete
the request as we submit the ELSP and so become confused by the request
to execute an empty ring.

To submit the same request in ELSP[0] three times (so triggering the
LiteRestore w/a and resubmitting with it already applied) would require
preemption, and on preemption we should be unwinding the request tail as
we know the HW hasn't advanced beyond the normal time. So in practice,
this should never occur...

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index c7ea8a055005..2fb761063606 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1776,16 +1776,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 
 				return;
 			}
-
-			/*
-			 * WaIdleLiteRestore:bdw,skl
-			 * Apply the wa NOOPs to prevent
-			 * ring:HEAD == rq:TAIL as we resubmit the
-			 * request. See gen8_emit_fini_breadcrumb() for
-			 * where we prepare the padding after the
-			 * end of the request.
-			 */
-			last->tail = last->wa_tail;
 		}
 	}
 
@@ -1943,6 +1933,24 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 				    ctx_single_port_submission(rq->hw_context))
 					goto done;
 
+				/*
+				 * WaIdleLiteRestore:bdw,skl
+				 *
+				 * If we have already submitted this request
+				 * using the wa_tail, we race with the HW and
+				 * HEAD may reach wa_tail before it processes
+				 * the ELSP[]. If it sees a context with an
+				 * empty ring, the HW gets confused.
+				 *
+				 * To prevent subsequent resubmission (lite
+				 * restores) with an empty ring, we emitted a
+				 * couple of NOOPs in gen8_emit_wa_tail()
+				 * beyond the normal end of the request.
+				 */
+				if (unlikely(last->tail == last->wa_tail))
+					goto done;
+
+				last->tail = last->wa_tail;
 				merge = false;
 			}
 
-- 
2.24.0