[RFC] drm/amdgpu: avoid scheduling on fence status query

[Andres Rodriguez]

When amdgpu_cs_wait_ioctl is called with a timeout of zero, the caller
is just interested in the current status of the fence.

The default implementation of dma_fence_wait_timeout on an unsignaled
fence will always call schedule_timeout(), even if the timeout is zero.
This may result in significant overhead for clients that heavily use
this interface.

This patch avoids the dma_fence_wait_timeout overhead by directly
checking the fence status.

Signed-off-by: Andres Rodriguez <andresx7 at gmail.com>
---

I'm not sure if we should be working around this issue at the amdgpu
level, or at fixing the dma_fence_default_wait level instead.

Source2 games like dota2 are affected by this overhead. This patch
improves dota2 perf on a i7-6700k+RX480 system from 72fps->81fps.

Patch is for drm-next-4.12-wip since this branch is where we operate
on dma_fences directly.

 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c    | 5 ++++-
 drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 5 +++++
 drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h  | 1 +
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
index ec71b93..67a5c9f 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
@@ -1168,7 +1168,10 @@ int amdgpu_cs_wait_ioctl(struct drm_device *dev, void *data,
 	if (IS_ERR(fence))
 		r = PTR_ERR(fence);
 	else if (fence) {
-		r = dma_fence_wait_timeout(fence, true, timeout);
+		if (timeout)
+			r = dma_fence_wait_timeout(fence, true, timeout);
+		else
+			r = amdgpu_fence_test_signaled(fence);
 		dma_fence_put(fence);
 	} else
 		r = 1;
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
index 7b60fb7..779a382 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
@@ -122,6 +122,11 @@ static u32 amdgpu_fence_read(struct amdgpu_ring *ring)
 	return seq;
 }
 
+bool amdgpu_fence_test_signaled(struct dma_fence *fence)
+{
+	return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags);
+}
+
 /**
  * amdgpu_fence_emit - emit a fence on the requested ring
  *
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
index 944443c..6bbd31d 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.h
@@ -84,6 +84,7 @@ int amdgpu_fence_driver_start_ring(struct amdgpu_ring *ring,
 				   unsigned irq_type);
 void amdgpu_fence_driver_suspend(struct amdgpu_device *adev);
 void amdgpu_fence_driver_resume(struct amdgpu_device *adev);
+bool amdgpu_fence_test_signaled(struct dma_fence *fence);
 int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **fence);
 void amdgpu_fence_process(struct amdgpu_ring *ring);
 int amdgpu_fence_wait_empty(struct amdgpu_ring *ring);
-- 
2.9.3