drm/i915/guc: Handle errors in multi-lrc requests

If an error occurs in the front end when multi-lrc requests are getting
generated we need to skip these in the backend but we still need to
emit the breadcrumbs seqno. An issues arises because with multi-lrc
breadcrumbs there is a handshake between the parent and children to make
forward progress. If all the requests are not present this handshake
doesn't work. To work around this, if multi-lrc request has an error we
skip the handshake but still emit the breadcrumbs seqno.

v2:
 (John Harrison)
  - Add comment explaining the skipping of the handshake logic
  - Fix typos in the commit message
v3:
 (John Harrison)
  - Fix up some comments about the math to NOP the ring
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014172005.27155-22-matthew.brost@intel.com

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

@@ -4070,7 +4070,7 @@ static int emit_bb_start_child_no_preempt_mid_batch(struct i915_request *rq,
 }
 
 static u32 *
-emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
+__emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
 						   u32 *cs)
 {
 	struct intel_context *ce = rq->context;
@@ -4099,6 +4099,45 @@ emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
 				  get_children_go_addr(ce),
 				  0);
 
+	return cs;
+}
+
+/*
+ * If this true, a submission of multi-lrc requests had an error and the
+ * requests need to be skipped. The front end (execuf IOCTL) should've called
+ * i915_request_skip which squashes the BB but we still need to emit the fini
+ * breadrcrumbs seqno write. At this point we don't know how many of the
+ * requests in the multi-lrc submission were generated so we can't do the
+ * handshake between the parent and children (e.g. if 4 requests should be
+ * generated but 2nd hit an error only 1 would be seen by the GuC backend).
+ * Simply skip the handshake, but still emit the breadcrumbd seqno, if an error
+ * has occurred on any of the requests in submission / relationship.
+ */
+static inline bool skip_handshake(struct i915_request *rq)
+{
+	return test_bit(I915_FENCE_FLAG_SKIP_PARALLEL, &rq->fence.flags);
+}
+
+static u32 *
+emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
+						 u32 *cs)
+{
+	struct intel_context *ce = rq->context;
+
+	GEM_BUG_ON(!intel_context_is_parent(ce));
+
+	if (unlikely(skip_handshake(rq))) {
+		/*
+		 * NOP everything in __emit_fini_breadcrumb_parent_no_preempt_mid_batch,
+		 * the -6 comes from the length of the emits below.
+		 */
+		memset(cs, 0, sizeof(u32) *
+		       (ce->engine->emit_fini_breadcrumb_dw - 6));
+		cs += ce->engine->emit_fini_breadcrumb_dw - 6;
+	} else {
+		cs = __emit_fini_breadcrumb_parent_no_preempt_mid_batch(rq, cs);
+	}
+
 	/* Emit fini breadcrumb */
 	cs = gen8_emit_ggtt_write(cs,
 				  rq->fence.seqno,
@@ -4115,7 +4154,8 @@ emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
 }
 
 static u32 *
-emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq, u32 *cs)
+__emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
+						  u32 *cs)
 {
 	struct intel_context *ce = rq->context;
 	struct intel_context *parent = intel_context_to_parent(ce);
@@ -4142,6 +4182,29 @@ emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq, u32 *cs
 	*cs++ = get_children_go_addr(parent);
 	*cs++ = 0;
 
+	return cs;
+}
+
+static u32 *
+emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
+						u32 *cs)
+{
+	struct intel_context *ce = rq->context;
+
+	GEM_BUG_ON(!intel_context_is_child(ce));
+
+	if (unlikely(skip_handshake(rq))) {
+		/*
+		 * NOP everything in __emit_fini_breadcrumb_child_no_preempt_mid_batch,
+		 * the -6 comes from the length of the emits below.
+		 */
+		memset(cs, 0, sizeof(u32) *
+		       (ce->engine->emit_fini_breadcrumb_dw - 6));
+		cs += ce->engine->emit_fini_breadcrumb_dw - 6;
+	} else {
+		cs = __emit_fini_breadcrumb_child_no_preempt_mid_batch(rq, cs);
+	}
+
 	/* Emit fini breadcrumb */
 	cs = gen8_emit_ggtt_write(cs,
 				  rq->fence.seqno,