Firstly: Context is VR with HP Reverb G2, WMR runtime, DX12.
We're seeing some unexplained behaviour across developer machines when working with OpenXR. It looks as thought the OpenXR runtime is changing the way it presents depending on the machine setting for preferred GPU.
More specifically, we noticed that depending on the machines setting for preferred GPU, we are seeing a different method used when XrEndFrame is called. This is a big deal as the different method results in a blank screen being drawn into our current renderTarget!
The difference is that when the preferred device is an Nvidia GPU, xrEndFrame looks like this in PIX (in a graphics queue that is separate to our main render):
Index Global ID Name EOP to EOP Duration (ns) Execution Start Time (ns)
2 8063 Signal(pFence:obj#20, Value:62)
3 8064 Wait(pFence:obj#36, Value:31)
5 8065 CopyTextureRegion(pDst:{pResource:obj#4083, Type:D3D12_TEXTURE_COPY_TYPE_SUBRESOURCE_INDEX, SubresourceIndex:0}, DstX:0, DstY:0, DstZ:0, pSrc:{pResource:obj#4084, Type:D3D12_TEXTURE_COPY_TYPE_SUBRESOURCE_INDEX, SubresourceIndex:0}, pSrcBox:{left:0, top:0, front:0, right:2088, bottom:2036, back:1})
6 8066 CopyTextureRegion(pDst:{pResource:obj#4083, Type:D3D12_TEXTURE_COPY_TYPE_SUBRESOURCE_INDEX, SubresourceIndex:1}, DstX:0, DstY:0, DstZ:0, pSrc:{pResource:obj#4085, Type:D3D12_TEXTURE_COPY_TYPE_SUBRESOURCE_INDEX, SubresourceIndex:0}, pSrcBox:{left:0, top:0, front:0, right:2088, bottom:2036, back:1})
8 8067 Signal(pFence:obj#20, Value:63)
9 8068 Signal(pFence:obj#21, Value:31)
and when it isn't, (i.e. somehow maybe picking up Intel onboard?) it looks like this:
Index Global ID Name EOP to EOP Duration (ns) Execution Start Time (ns)
0 8064 Wait(pFence:obj#45, Value:21)
2 8065 ClearRenderTargetView(RenderTargetView:res#4008, ColorRGBA:{Element:0, Element:0, Element:0, Element:0}, NumRects:0, pRects:nullptr)
15 8066 DrawIndexedInstanced(IndexCountPerInstance:4, InstanceCount:2, StartIndexLocation:0, BaseVertexLocation:0, StartInstanceLocation:0)
17 8067 Signal(pFence:obj#22, Value:23)
18 8068 Signal(pFence:obj#23, Value:21)
The latter is clearing the current renderTargetView and drawing a quad over the top that is the dimensions of the headset display.
Yet- we've checked the rendering code and it is definitely not selecting the Intel graphics device. However the second behaviour goes away if we set 'preferred graphics processor' to nvidia gpu in nvidia control panel.
We can also see that the above behaviour is the result of a call to XrEndFrame, and that our rendering code is identical otherwise.
Any clue as to what part of the runtime might be looking at or influenced by this setting?
Unfortunately (fortuitously?) we found we need to work on the rendering code to be able to swap runtimes to say SteamVR, so right now we can't swap out the runtime.
Obviously we have a workaround, which is to set the preferred device. But understanding how/why this issue is occurring would be great.
So this was finally tracked down to an error on our part.
In our case we were using xrGetD3D12GraphicsRequirementsKHR to get the minimum graphics requirements for openxr.
This has an adapterLuid identifier in the structure XrGraphicsRequirementsD3D12KHR which we should have been using to select the GPU in the graphics API, but weren't.