drm/i915: Don't emit semaphore wait if wrap happened

If wrap just happened we need to prevent emitting waits for
pre wrap values. Detect this and emit no-ops instead.

v2: Use olr > seqno to detect wrap instead of *seqno == 0
as suggested by Chris Wilson.

v3: Use last used seqno to detect the wraparound. From
Chris Wilson

v4: Fixed unnecessary last_seqno assigment

References: https://bugs.freedesktop.org/show_bug.cgi?id=57967
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 21 insertions, 5 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 0d03dc649c5e..69bbe7bb9f6d 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -596,6 +596,13 @@ gen6_add_request(struct intel_ring_buffer *ring)
         return 0;
 }
 
+static inline bool i915_gem_has_seqno_wrapped(struct drm_device *dev,
+                                              u32 seqno)
+{
+        struct drm_i915_private *dev_priv = dev->dev_private;
+        return dev_priv->last_seqno < seqno;
+}
+
 /**
  * intel_ring_sync - sync the waiter to the signaller on seqno
  *
@@ -626,11 +633,20 @@ gen6_ring_sync(struct intel_ring_buffer *waiter,
         if (ret)
                 return ret;
 
-        intel_ring_emit(waiter,
-                        dw1 | signaller->semaphore_register[waiter->id]);
-        intel_ring_emit(waiter, seqno);
-        intel_ring_emit(waiter, 0);
-        intel_ring_emit(waiter, MI_NOOP);
+        /* If seqno wrap happened, omit the wait with no-ops */
+        if (likely(!i915_gem_has_seqno_wrapped(waiter->dev, seqno))) {
+                intel_ring_emit(waiter,
+                                dw1 |
+                                signaller->semaphore_register[waiter->id]);
+                intel_ring_emit(waiter, seqno);
+                intel_ring_emit(waiter, 0);
+                intel_ring_emit(waiter, MI_NOOP);
+        } else {
+                intel_ring_emit(waiter, MI_NOOP);
+                intel_ring_emit(waiter, MI_NOOP);
+                intel_ring_emit(waiter, MI_NOOP);
+                intel_ring_emit(waiter, MI_NOOP);
+        }
         intel_ring_advance(waiter);
 
         return 0;