drm/i915: Mark up "sentinel" requests

Sometimes we want to emit a terminator request, a request that flushes
the pipeline and allows no request to come after it. This can be used
for a "preempt-to-idle" to ensure that upon processing the
context-switch to that request, all other active contexts have been
flushed.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191012070136.32058-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -1208,7 +1208,8 @@ static bool can_merge_rq(const struct i915_request *prev,
 	if (i915_request_completed(next))
 		return true;
 
-	if (unlikely(prev->flags ^ next->flags) & I915_REQUEST_NOPREEMPT)
+	if (unlikely((prev->flags ^ next->flags) &
+		     (I915_REQUEST_NOPREEMPT | I915_REQUEST_SENTINEL)))
 		return false;
 
 	if (!can_merge_ctx(prev->hw_context, next->hw_context))
@@ -1659,6 +1660,9 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 				if (last->hw_context == rq->hw_context)
 					goto done;
 
+				if (i915_request_has_sentinel(last))
+					goto done;
+
 				/*
 				 * If GVT overrides us we only ever submit
 				 * port[0], leaving port[1] empty. Note that we

drivers/gpu/drm/i915/i915_request.h

@@ -218,6 +218,7 @@ struct i915_request {
 	unsigned long flags;
 #define I915_REQUEST_WAITBOOST	BIT(0)
 #define I915_REQUEST_NOPREEMPT	BIT(1)
+#define I915_REQUEST_SENTINEL	BIT(2)
 
 	/** timeline->request entry for this request */
 	struct list_head link;
@@ -440,6 +441,11 @@ static inline bool i915_request_has_nopreempt(const struct i915_request *rq)
 	return unlikely(rq->flags & I915_REQUEST_NOPREEMPT);
 }
 
+static inline bool i915_request_has_sentinel(const struct i915_request *rq)
+{
+	return unlikely(rq->flags & I915_REQUEST_SENTINEL);
+}
+
 static inline struct intel_timeline *
 i915_request_timeline(struct i915_request *rq)
 {