drm/i915: Convert trace-irq to the breadcrumb waiter

If we convert the tracing over from direct use of ring->irq_get() and
over to the breadcrumb infrastructure, we only have a single user of the
ring->irq_get and so we will be able to simplify the driver routines
(eliminating the redundant validation and irq refcounting).

Process context is preferred over softirq (or even hardirq) for a couple
of reasons:

 - we already utilize process context to have fast wakeup of a single
   client (i.e. the client waiting for the GPU inspects the seqno for
   itself following an interrupt to avoid the overhead of a context
   switch before it returns to userspace)

 - engine->irq_seqno() is not suitable for use from an softirq/hardirq
   context as we may require long waits (100-250us) to ensure the seqno
   write is posted before we read it from the CPU

A signaling framework is a requirement for enabling dma-fences.

v2: Move to a signaling framework based upon the waiter.
v3: Track the first-signal to avoid having to walk the rbtree everytime.
v4: Mark the signaler thread as RT priority to reduce latency in the
indirect wakeups.
v5: Make failure to allocate the thread fatal.
v6: Rename kthreads to i915/signal:%u

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467390209-3576-16-git-send-email-chris@chris-wilson.co.uk

1 files changed, 2 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 4875c92cc64c..f21657f8c102 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2755,7 +2755,8 @@ i915_gem_init_seqno(struct drm_i915_private *dev_priv, u32 seqno)
 
 	/* If the seqno wraps around, we need to clear the breadcrumb rbtree */
 	if (!i915_seqno_passed(seqno, dev_priv->next_seqno)) {
-		while (intel_kick_waiters(dev_priv))
+		while (intel_kick_waiters(dev_priv) ||
+		       intel_kick_signalers(dev_priv))
 			yield();
 	}
 
@@ -3219,12 +3220,6 @@ i915_gem_retire_requests_ring(struct intel_engine_cs *engine)
 		i915_gem_object_retire__read(obj, engine->id);
 	}
 
-	if (unlikely(engine->trace_irq_req &&
-		     i915_gem_request_completed(engine->trace_irq_req))) {
-		engine->irq_put(engine);
-		i915_gem_request_assign(&engine->trace_irq_req, NULL);
-	}
-
 	WARN_ON(i915_verify_lists(engine->dev));
 }