Correct usage of DisplayTask::SetSignal

I managed to figure this one out eventually.

Seemingly on nvidia GPU systems the SetSignal API doesn't do anything and instead a bunch of what is done via the WinRT APIs is in fact done by NvAPI. Testing on an intel system the APIs worked as I would have expected

It does however require a pro-range GPU that supports their PresentBarrier feature

Rough steps:

1. Remove the display from desktop into a "Managed" state using https://www.nvidia.com/en-gb/drivers/ddisplay-utility/
2. Sample App needs to be updated to use Dx12 / 11on12
3. Enumerate Dedicated displays using NvAPI_DISP_GetNvManagedDedicatedDisplays
4. Acquire displays NvAPI_DISP_AcquireDedicatedDisplay
5. Open the shared handle from acquiring the display as IDisplayDeviceInterop and then convert to a winrt::DisplaySource
6. Get display info for acquired display using NvAPI_Disp_GetDisplayIdInfo
7. Find the matching winrt Target of that display by using winrt::DisplayManager::GetCurrentTargets() and comparing the NV_DISPLAY_ID_INFO_DATA adapterId, targetId with the DisplayTarget adapter LUID & AdapterRelativeId
8. Create Primary instances using matched DisplayTarget
9. Create scanout instances using interopped DisplaySource
10. Create a present barrier using NvAPI_D3D12_CreateDDisplayPresentBarrierClient DisplaySource::SourceId()
11. Create D3D12 fences and pass them to NvAPI_D3D12_RegisterPresentBarrierResources
12. Join the present barrier NvAPI_JoinPresentBarrier
13. Those fences can now be waited on using SetEventOnCompletion
14. When the app is done - make sure you release the display NvAPI_DISP_ReleaseDedicatedDisplay

Microsoft support were unable to give any further answers as to whether this is expected as the D3D team did not respond to their enquiries - but seemingly this is the way it's done on nvidia

Hello @Peter Kinsey-James ,

The documentation states that the signaled fence value is DisplayTaskResult.PresentId.

The fence value that is signaled is the value that's returned from DisplayTaskPool.TryExecuteTask in the DisplayTaskResult.PresentId property (it's very probable that it will increase by 1 with each Present).

SetSignal does not signal a value you choose. If you wait for your own counter (or 1), the fence can look like it never signals.

Use TryExecuteTask, read PresentId, and wait for that exact value on the underlying D3D fence.

I hope this clarifies, and happy holidays.