[PATCH 3/7] drm/i915/execlists: Make submission tasklet hardirq safe

[Chris Wilson]

Prepare to allow the execlists submission to be run from underneath a
hardirq timer context (and not just the current softirq context) as is
required for fast preemption resets and context switches.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_lrc.c | 35 ++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index f9f4064dec0e..ff7c6a741721 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -554,7 +554,7 @@ static void inject_preempt_context(struct intel_engine_cs *engine)
 	execlists_set_active(&engine->execlists, EXECLISTS_ACTIVE_PREEMPT);
 }
 
-static void execlists_dequeue(struct intel_engine_cs *engine)
+static bool __execlists_dequeue(struct intel_engine_cs *engine)
 {
 	struct intel_engine_execlists * const execlists = &engine->execlists;
 	struct execlist_port *port = execlists->port;
@@ -564,6 +564,8 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	struct rb_node *rb;
 	bool submit = false;
 
+	lockdep_assert_held(&engine->timeline.lock);
+
 	/* Hardware submission is through 2 ports. Conceptually each port
 	 * has a (RING_START, RING_HEAD, RING_TAIL) tuple. RING_START is
 	 * static for a context, and unique to each, so we only execute
@@ -585,7 +587,6 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	 * and context switches) submission.
 	 */
 
-	spin_lock_irq(&engine->timeline.lock);
 	rb = execlists->first;
 	GEM_BUG_ON(rb_first(&execlists->queue) != rb);
 
@@ -600,7 +601,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 						EXECLISTS_ACTIVE_USER));
 		GEM_BUG_ON(!port_count(&port[0]));
 		if (port_count(&port[0]) > 1)
-			goto unlock;
+			return false;
 
 		/*
 		 * If we write to ELSP a second time before the HW has had
@@ -610,11 +611,11 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		 * the HW to indicate that it has had a chance to respond.
 		 */
 		if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
-			goto unlock;
+			return false;
 
 		if (need_preempt(engine, last, execlists->queue_priority)) {
 			inject_preempt_context(engine);
-			goto unlock;
+			return false;
 		}
 
 		/*
@@ -639,7 +640,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 		 * priorities of the ports haven't been switch.
 		 */
 		if (port_count(&port[1]))
-			goto unlock;
+			return false;
 
 		/*
 		 * WaIdleLiteRestore:bdw,skl
@@ -744,13 +745,25 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
 	/* We must always keep the beast fed if we have work piled up */
 	GEM_BUG_ON(execlists->first && !port_isset(execlists->port));
 
-unlock:
-	spin_unlock_irq(&engine->timeline.lock);
-
-	if (submit) {
+	/* Re-evaluate the executing context setup after each preemptive kick */
+	if (last)
 		execlists_user_begin(execlists, execlists->port);
+
+	return submit;
+}
+
+static void execlists_dequeue(struct intel_engine_cs *engine)
+{
+	struct intel_engine_execlists * const execlists = &engine->execlists;
+	unsigned long flags;
+	bool submit;
+
+	spin_lock_irqsave(&engine->timeline.lock, flags);
+	submit = __execlists_dequeue(engine);
+	spin_unlock_irqrestore(&engine->timeline.lock, flags);
+
+	if (submit)
 		execlists_submit_ports(engine);
-	}
 
 	GEM_BUG_ON(port_isset(execlists->port) &&
 		   !execlists_is_active(execlists, EXECLISTS_ACTIVE_USER));
-- 
2.17.0