[Intel-gfx] [PATCH 2/4] drm/i915: Report back whether the irq was armed when adding the waiter

[Chris Wilson]

The important condition that we need to check after enabling the
interrupt for signaling is whether the request completed in the process
(and so we missed that interrupt). A large cost in enabling the
signaling (rather than waiters) is in waking up the auxiliary signaling
thread, but we only need to do so to catch that missed interrupt. If we
know we didn't miss any interrupts (because we didn't arm the interrupt)
then we can skip waking the auxiliary thread.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_breadcrumbs.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
index 183afcb036aa..dbcad9f6b2d5 100644
--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
@@ -329,7 +329,7 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
 {
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
 	struct rb_node **p, *parent, *completed;
-	bool first;
+	bool first, irq_armed;
 	u32 seqno;
 
 	/* Insert the request into the retirement ordered list
@@ -344,6 +344,7 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
 	 * removing stale elements in the tree, we may be able to reduce the
 	 * ping-pong between the old bottom-half and ourselves as first-waiter.
 	 */
+	irq_armed = false;
 	first = true;
 	parent = NULL;
 	completed = NULL;
@@ -390,6 +391,7 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
 
 	if (first) {
 		spin_lock(&b->irq_lock);
+		irq_armed = !b->irq_armed;
 		b->irq_wait = wait;
 		/* After assigning ourselves as the new bottom-half, we must
 		 * perform a cursory check to prevent a missed interrupt.
@@ -426,20 +428,24 @@ static bool __intel_engine_add_wait(struct intel_engine_cs *engine,
 	GEM_BUG_ON(!b->irq_armed);
 	GEM_BUG_ON(rb_first(&b->waiters) != &b->irq_wait->node);
 
-	return first;
+	return irq_armed;
 }
 
 bool intel_engine_add_wait(struct intel_engine_cs *engine,
 			   struct intel_wait *wait)
 {
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
-	bool first;
+	bool irq_armed;
 
 	spin_lock_irq(&b->rb_lock);
-	first = __intel_engine_add_wait(engine, wait);
+	irq_armed = __intel_engine_add_wait(engine, wait);
 	spin_unlock_irq(&b->rb_lock);
+	if (irq_armed)
+		return irq_armed;
 
-	return first;
+	/* Make the caller recheck if its request has already started. */
+	return i915_seqno_passed(intel_engine_get_seqno(engine),
+				 wait->seqno - 1);
 }
 
 static inline bool chain_wakeup(struct rb_node *rb, int priority)
-- 
2.11.0