CODEWITHSUNDEEP
vulkan
Alexander Guyer
Alexander Guyer

Reputation: 2203

Is it possible to synchronize an automatic layout transition with a swapchain image acquisition via pWaitDstStageMask=TOP_OF_PIPE?

It is necessary to synchronize automatic layout transitions performed by render passes with the acquisition of a swapchain image via the semaphore provided in vkAcquireNextImageKHR. Vulkan Tutorial states, "We could change the waitStages for the imageAvailableSemaphore to VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT to ensure that the render passes don't begin until the image is available" (Context: waitStages is an array of VkPipelineStageFlags supplied as pWaitDstStageMask to the VkSubmitInfo for the queue submission which performs the render pass and, consequently, the automatic layout transition). It then opts for an alternative (common) solution, which is to use an external subpass dependency instead.

In fact, I've only ever seen such synchronization done with a subpass dependency; I've never seen anyone actually do it by setting pWaitDstStageMask=TOP_OF_PIPE for the VkSubmitInfo corresponding to the queue submission which performs the automatic layout transition, and I'm not certain why this would even work. The specs provide certain guarantees about automatic layout transitions being synchronized with subpass dependencies, so I understand why they would work, but in order to do this sort of synchronization by waiting on a semaphore in a VkSubmitInfo, it is first necessary that image layout transitions even have pipeline stages to begin with. I am not aware of any pipeline stages which image layout transitions go through; I believed them to be entirely atomic operations which occur in between source-available operations and destination-visible operations in a memory dependency (e.g. subpass dependency, memory barrier, etc.), as described by the specs.

Do automatic image layout transitions go through pipeline stages? If so, which stages do they go through (perhaps I'm missing something in the specs)? If not, how could setting pWaitDstStageMask=TOP_OF_PIPE in a VkSubmitInfo waiting on the image acquisition semaphore have any synchronization effect on the automatic image layout transition?

Upvotes: 1

Views: 437

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474136

You may be misunderstanding what the tutorial is saying.

Layout transitions do not happen within a pipeline stage. They, like all aspects of an execution dependency, happen between pipeline stages. A dependency states that, after the source stage completes but before the destination stage begins, the dependency's operations will happen.

So if the destination stage of an incoming external subpass dependency is the top of the pipe, then it is saying that the layout transition as part of the dependency will happen before the execution of any pipeline stages in the target subpass.

Remember that the default external subpass dependency for each attachment has the top of the pipe as its source scope. If the semaphore wait dependency's destination stage is the top of the pipe, then the source scope of the external subpass dependency is in the destination scope of the semaphore wait. And therefore, the external subpass dependency happens-after the semaphore wait dependency.

And, as previously stated, layout transitions happen between the source scope and the destination scope. So until the source scope, and everything it depends on, finishes execution, the layout transition will not happen.

Upvotes: 3

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