[PATCH 18/24] drm/i915: Only apply one barrier after a breadcrumb interrupt is posted

[Chris Wilson]

If we flag the seqno as potentially stale upon receiving an interrupt,
we can use that information to reduce the frequency that we apply the
heavyweight coherent seqno read (i.e. if we wake up a chain of waiters).

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_drv.h          | 15 ++++++++++++++-
 drivers/gpu/drm/i915/i915_irq.c          |  1 +
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 11 ++++++++++-
 drivers/gpu/drm/i915/intel_ringbuffer.h  |  1 +
 4 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 1bf94fd25e8a..3293b80999e5 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3819,7 +3819,20 @@ static inline bool __i915_request_irq_complete(struct drm_i915_gem_request *req)
 	 * but it is easier and safer to do it every time the waiter
 	 * is woken.
 	 */
-	if (engine->irq_seqno_barrier) {
+	if (engine->irq_seqno_barrier && READ_ONCE(engine->irq_posted)) {
+		/* The ordering of irq_posted versus applying the barrier
+		 * is crucial. The clearing of the current irq_posted must
+		 * be visible before we perform the barrier operation,
+		 * such that if a subsequent interrupt arrives, irq_posted
+		 * is reasserted and our task rewoken (which causes us to
+		 * do another __i915_request_irq_complete() immediately
+		 * and reapply the barrier). Conversely, if the clear
+		 * occurs after the barrier, then an interrupt that arrived
+		 * whilst we waited on the barrier would not trigger a
+		 * barrier on the next pass, and the read may not see the
+		 * seqno update.
+		 */
+		WRITE_ONCE(engine->irq_posted, false);
 		engine->irq_seqno_barrier(engine);
 		if (i915_gem_request_completed(req))
 			return true;
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 795e60b47eab..f62fcf3f6ea8 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -988,6 +988,7 @@ static void ironlake_rps_change_irq_handler(struct drm_i915_private *dev_priv)
 
 static void notify_ring(struct intel_engine_cs *engine)
 {
+	smp_store_mb(engine->irq_posted, true);
 	if (intel_engine_wakeup(engine)) {
 		trace_i915_gem_request_notify(engine);
 		engine->user_interrupts++;
diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 37f8fe19f122..578de43cb07e 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -43,12 +43,20 @@ static void intel_breadcrumbs_fake_irq(unsigned long data)
 
 static void irq_enable(struct intel_engine_cs *engine)
 {
+	/* Enabling the IRQ may miss the generation of the interrupt, but
+	 * we still need to force the barrier before reading the seqno,
+	 * just in case.
+	 */
+	engine->irq_posted = true;
+
 	WARN_ON(!engine->irq_get(engine));
 }
 
 static void irq_disable(struct intel_engine_cs *engine)
 {
 	engine->irq_put(engine);
+
+	engine->irq_posted = false;
 }
 
 static bool __intel_breadcrumbs_enable_irq(struct intel_breadcrumbs *b)
@@ -194,7 +202,8 @@ bool intel_engine_add_wait(struct intel_engine_cs *engine,
 			 * in case the seqno passed.
 			 */
 			__intel_breadcrumbs_enable_irq(b);
-			wake_up_process(to_wait(next)->task);
+			if (READ_ONCE(engine->irq_posted))
+				wake_up_process(to_wait(next)->task);
 		}
 
 		do {
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 8c45435d7865..a2e988068cb4 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -196,6 +196,7 @@ struct intel_engine_cs {
 	struct i915_ctx_workarounds wa_ctx;
 
 	unsigned irq_refcount; /* protected by dev_priv->irq_lock */
+	bool		irq_posted;
 	u32		irq_enable_mask;	/* bitmask to enable ring interrupt */
 	struct drm_i915_gem_request *trace_irq_req;
 	bool __must_check (*irq_get)(struct intel_engine_cs *ring);
-- 
2.8.1