VK_EXT_metal_objects: vkCreateSemaphore() always creates timeline semaphore when exportObjectType == VK_EXPORT_METAL_OBJECT_TYPE_METAL_SHARED_EVENT_BIT_EXT

[bangnoise]

I'm not sure what the intended use is of exporting a MTLSharedEvent from a binary semaphore, because the underlying mechanism to manage the MTLSharedEvent's increasing value is not exposed, rendering the MTLSharedEvent next to useless.

That said, if one does try to create an exportable binary semaphore, a timeline semaphore is erroneously created in MVKDevice::createSemaphore() whenever VkExportMetalObjectCreateInfoEXT::exportObjectType is VK_EXPORT_METAL_OBJECT_TYPE_METAL_SHARED_EVENT_BIT_EXT.

[billhollings]

a timeline semaphore is erroneously created

Hmmm...a timeline semaphore and its embedded MTLSharedEvent should behave like binaries...unless manipulated by the app setting timeline values or invoking timeline behavior. Perhaps that's not happening accurately, and we may need to rework some things.

What is the effect of this happening, and where do behavioral errors appear in your use case?

[bangnoise]

When a MVKTimelineSemaphoreMTLEvent is used as a binary semaphore, the value ultimately passed to -encodeSignalEvent:value: is always 0, meaning they are always signalled.