Can Vulkan synchronization source and destination stage mask be the same?

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The following code from the Vulkan Tutorial seems to conflict with how synchronization scopes work.

// <dependency> is a subpass dependency.
dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
...

dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;

The above code is trying to set both the srcStageMask and dstStageMask to be the same pipeline stage: VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT.

According to Vulkan Specification:

If a synchronization command includes a source stage mask, its first synchronization scope only includes execution of the pipeline stages specified in that mask, ...

If a synchronization command includes a destination stage mask, its second synchronization scope only includes execution of the pipeline stages specified in that mask, ...

In other words, srcStageMask and dstStageMask create a first synchronization scope with specified stage(s) and a second one with the specified stage(s), respectively.

Also, according to the following:

... for two sets of operations, the first set must happen before the second set.

My confusion is that, since the source and destination stage are the same, the subpass dependency is requiring this pipeline stage must complete before the exact same stage starts to execute.

The color attachment output stage is already guaranteed to be finished (the first scope). How can you specify to start to execute the same finished stage again? (the second scope)

So what is this dependency is trying to say?

vulkan
1 Answers

A stage only exists within an action command that executes some portion of itself within that stage. Synchronization scopes are based on commands first. Once you have defined which commands are in the scope, stage masks can specify which stages within those commands are affected by the synchronization.

As such, all synchronization operations define a set of commands that happen before the synchronization and the set of commands that happen after. These represent the "first synchronization scope" and "second synchronization scope".

The source stage mask applies to the commands in the "first synchronization scope". The destination stage mask applies to commands in the "second synchronization scope". The commands in one scope are a distinct set from the other scope. So even if you're talking about the same pipeline stages, they're stages in different commands that execute at different times.

So what that does is exactly what it says: it creates a dependency between all executions of the color attachment stage from the source subpass (aka: the "first synchronization scope") and all executions of the color attachment stage from the destination subpass (aka: the "second synchronization scope").

user_7340690
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