Why does vulkan have a separate device handle type?

[patrick-han]

In the DeviceMangers for DX11 and DX12, the member m_NvrhiDevice is of type nvrhi::DeviceHandle whereas in Vulkan's DeviceManager it is nvrhi::vulkan::DeviceHandle.

However the tutorial when invoking createDevice in the Vulkan section uses nvrhi::DeviceHandle: https://github.com/NVIDIAGameWorks/nvrhi/blob/main/doc/Tutorial.md

I notice if I don't use nvrhi::vulkan::DeviceHandle I cannot invoke functions like queueWaitForSemaphore. And on the other hand since createValidationLayer returns nvrhi::DeviceHandle, I cannot use it for those function calls that rely on the Vulkan handle. What is the intended usage of these handles and why does Vulkan seemingly have different treatment?

[apanteleev]

nvrhi::vulkan::DeviceHandle is just a ref-count handle for nvrhi::vulkan::IDevice, which is derived from nvrhi::IDevice. There are similar derived types for other APIs, like nvrhi::d3d12::IDevice that provide additional functionality compared to regular nvrhi::IDevice. The Vulkan DeviceManager needs some of those additional functions of nvrhi::vulkan::IDevice so it uses that type of handle.

You can trivially upcast a nvrhi::vulkan::DeviceHandle into nvrhi::DeviceHandle, and to downcast, use getNativeObject like so:

nvrhi::vulkan::DeviceHandle nvrhiVulkanDevice = static_cast<nvrhi::vulkan::IDevice*>(
    deviceManager->GetDevice()->getNativeObject(nvrhi::ObjectTypes::Nvrhi_VK_Device));

This call will unwrap the validation layer, if it is present, and return a valid pointer only if the underlying implementation is actually a Vulkan device. Similar calls can be used for other backends (DX11, DX12).

[patrick-han]

Thanks!