drm/i915/guc: Ensure multi-lrc fini breadcrumb math is correct

Realized that the GuC multi-lrc fini breadcrumb emit code is very
delicate as the math this code does relies on functions it calls to emit
a certain number of DWs. Add a few GEM_BUG_ONs to assert the math is
correct.

v2:
  - Rebase + resend for CI
 (Checkpatch)
  - Fix blank line warning
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/20220119210639.33053-1-matthew.brost@intel.com

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

@@ -4441,27 +4441,31 @@ static inline bool skip_handshake(struct i915_request *rq)
 	return test_bit(I915_FENCE_FLAG_SKIP_PARALLEL, &rq->fence.flags);
 }
 
+#define NON_SKIP_LEN	6
 static u32 *
 emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
 						 u32 *cs)
 {
 	struct intel_context *ce = rq->context;
+	__maybe_unused u32 *before_fini_breadcrumb_user_interrupt_cs;
+	__maybe_unused u32 *start_fini_breadcrumb_cs = cs;
 
 	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.
+		 * the NON_SKIP_LEN 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;
+		       (ce->engine->emit_fini_breadcrumb_dw - NON_SKIP_LEN));
+		cs += ce->engine->emit_fini_breadcrumb_dw - NON_SKIP_LEN;
 	} else {
 		cs = __emit_fini_breadcrumb_parent_no_preempt_mid_batch(rq, cs);
 	}
 
 	/* Emit fini breadcrumb */
+	before_fini_breadcrumb_user_interrupt_cs = cs;
 	cs = gen8_emit_ggtt_write(cs,
 				  rq->fence.seqno,
 				  i915_request_active_timeline(rq)->hwsp_offset,
@@ -4471,6 +4475,12 @@ emit_fini_breadcrumb_parent_no_preempt_mid_batch(struct i915_request *rq,
 	*cs++ = MI_USER_INTERRUPT;
 	*cs++ = MI_NOOP;
 
+	/* Ensure our math for skip + emit is correct */
+	GEM_BUG_ON(before_fini_breadcrumb_user_interrupt_cs + NON_SKIP_LEN !=
+		   cs);
+	GEM_BUG_ON(start_fini_breadcrumb_cs +
+		   ce->engine->emit_fini_breadcrumb_dw != cs);
+
 	rq->tail = intel_ring_offset(rq, cs);
 
 	return cs;
@@ -4513,22 +4523,25 @@ emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
 						u32 *cs)
 {
 	struct intel_context *ce = rq->context;
+	__maybe_unused u32 *before_fini_breadcrumb_user_interrupt_cs;
+	__maybe_unused u32 *start_fini_breadcrumb_cs = cs;
 
 	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.
+		 * the NON_SKIP_LEN 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;
+		       (ce->engine->emit_fini_breadcrumb_dw - NON_SKIP_LEN));
+		cs += ce->engine->emit_fini_breadcrumb_dw - NON_SKIP_LEN;
 	} else {
 		cs = __emit_fini_breadcrumb_child_no_preempt_mid_batch(rq, cs);
 	}
 
 	/* Emit fini breadcrumb */
+	before_fini_breadcrumb_user_interrupt_cs = cs;
 	cs = gen8_emit_ggtt_write(cs,
 				  rq->fence.seqno,
 				  i915_request_active_timeline(rq)->hwsp_offset,
@@ -4538,11 +4551,19 @@ emit_fini_breadcrumb_child_no_preempt_mid_batch(struct i915_request *rq,
 	*cs++ = MI_USER_INTERRUPT;
 	*cs++ = MI_NOOP;
 
+	/* Ensure our math for skip + emit is correct */
+	GEM_BUG_ON(before_fini_breadcrumb_user_interrupt_cs + NON_SKIP_LEN !=
+		   cs);
+	GEM_BUG_ON(start_fini_breadcrumb_cs +
+		   ce->engine->emit_fini_breadcrumb_dw != cs);
+
 	rq->tail = intel_ring_offset(rq, cs);
 
 	return cs;
 }
 
+#undef NON_SKIP_LEN
+
 static struct intel_context *
 guc_create_virtual(struct intel_engine_cs **siblings, unsigned int count,
 		   unsigned long flags)