[Intel-gfx] [PATCH] drm/i915/gt: Be defensive in the face of false CS events

[Chris Wilson]

If the HW throws a curve ball and reports either en event before it is
possible, or just a completely impossible event, we have to grin and
bear it. The first few events, we will likely not notice as we would be
expecting some event, but as soon as we stop expecting an event and yet
they still keep coming, then we enter into undefined state territory.
In which case, bail out, stop processing the events, and reset the
engine and our set of queued requests to recover.

The sporadic hangs and warnings will continue to plague CI, but at least
system stability should not be compromised.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2045
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index fbcfeaed6441..c86324d2d2bb 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2567,6 +2567,7 @@ static void process_csb(struct intel_engine_cs *engine)
 	tail = READ_ONCE(*execlists->csb_write);
 	if (unlikely(head == tail))
 		return;
+	execlists->csb_head = tail;
 
 	/*
 	 * Hopefully paired with a wmb() in HW!
@@ -2613,6 +2614,9 @@ static void process_csb(struct intel_engine_cs *engine)
 		if (promote) {
 			struct i915_request * const *old = execlists->active;
 
+			if (GEM_WARN_ON(!*execlists->pending))
+				break;
+
 			ring_set_paused(engine, 0);
 
 			/* Point active to the new ELSP; prevent overwriting */
@@ -2635,7 +2639,8 @@ static void process_csb(struct intel_engine_cs *engine)
 
 			WRITE_ONCE(execlists->pending[0], NULL);
 		} else {
-			GEM_BUG_ON(!*execlists->active);
+			if (GEM_WARN_ON(!*execlists->active))
+				break;
 
 			/* port0 completed, advanced to port1 */
 			trace_ports(execlists, "completed", execlists->active);
@@ -2686,7 +2691,6 @@ static void process_csb(struct intel_engine_cs *engine)
 		}
 	} while (head != tail);
 
-	execlists->csb_head = head;
 	set_timeslice(engine);
 
 	/*
-- 
2.20.1