[Intel-gfx] [PATCH] drm/i915/scheduler: Assert that we do not have a dep cycle back to request

[Chris Wilson]

When reprioritising a request, we build a list of its dependencies in
topological order. This should leave our origin request as the first
element in our list, if it moves we have a dependency cycle and severe
breakage. Assert that it doesn't move.

Complete, but expensive checking is done by swfence, this assert is more
about documenting the topologically ordered list.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 4e150b095a11..42705e3875cd 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1050,6 +1050,13 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 		}
 	}
 
+	/*
+	 * We should never have a dep cycle back to ourselves, leaving
+	 * the original request as the origin of our topologically sorted
+	 * list.
+	 */
+	GEM_BUG_ON(list_first_entry(&dfs, typeof(stack), dfs_link) != &stack);
+
 	/*
 	 * If we didn't need to bump any existing priorities, and we haven't
 	 * yet submitted this request (i.e. there is no potential race with
-- 
2.15.1