drm/i915: Split breadcrumbs spinlock into two

As we now take the breadcrumbs spinlock within the interrupt handler, we
wish to minimise its hold time. During the interrupt we do not care
about the state of the full rbtree, only that of the first element, so
we can guard that with a separate lock.

v2: Rename first_wait to irq_wait to make it clearer that it is guarded
by irq_lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303190824.1330-1-chris@chris-wilson.co.uk

1 files changed, 5 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 55a6a3f8274c..9d6b83a16cc8 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -235,10 +235,12 @@ struct intel_engine_cs {
 	 * the overhead of waking that client is much preferred.
 	 */
 	struct intel_breadcrumbs {
-		spinlock_t lock; /* protects the lists of requests; irqsafe */
+		spinlock_t irq_lock; /* protects irq_*; irqsafe */
+		struct intel_wait *irq_wait; /* oldest waiter by retirement */
+
+		spinlock_t rb_lock; /* protects the rb and wraps irq_lock */
 		struct rb_root waiters; /* sorted by retirement, priority */
 		struct rb_root signals; /* sorted by retirement */
-		struct intel_wait *first_wait; /* oldest waiter by retirement */
 		struct task_struct *signaler; /* used for fence signalling */
 		struct drm_i915_gem_request __rcu *first_signal;
 		struct timer_list fake_irq; /* used after a missed interrupt */
@@ -639,7 +641,7 @@ void intel_engine_cancel_signaling(struct drm_i915_gem_request *request);
 
 static inline bool intel_engine_has_waiter(const struct intel_engine_cs *engine)
 {
-	return READ_ONCE(engine->breadcrumbs.first_wait);
+	return READ_ONCE(engine->breadcrumbs.irq_wait);
 }
 
 unsigned int intel_engine_wakeup(struct intel_engine_cs *engine);