CODEWITHSUNDEEP
vulkan
Antoine Morrier
Antoine Morrier

Reputation: 4076

HOST_WRITE useful or validation layers wrong?

In the specs about HOST_WRITE_BIT it is write :

For host writes to be seen by subsequent command buffer operations, a pipeline barrier from a source of VK_ACCESS_HOST_WRITE_BIT and VK_PIPELINE_STAGE_HOST_BIT to a destination of the relevant device pipeline stages and access types must be performed. Note that such a barrier is performed implicitly upon each command buffer submission, so an explicit barrier is only rarely needed

However, when you transition (through a vkCmdPipelineBarrier (so in a commandBuffer)) one image from PREINITIALIZED layout with a srcAccessMask to 0 instead of HOST_WRITE_BIT, you get an error :

validation layer: Source AccessMask 0 [None] must have required access bit 16384 [VK_ACCESS_HOST_WRITE_BIT] when layout is VK_IMAGE_LAYOUT_PREINITIALIZED, unless the app has previously added a barrier for this transition.

Is there an error from the specs? From the validation layers? Is the barrier we are talking about is a pure execution barrier and not a memory one? Am I missing something?

Upvotes: 2

Views: 277

Answers (1)

krOoze
krOoze

Reputation: 13306

My private opinion is: validation layers bug.

It simply checks layout vs access flags and does not seem to be aware of this corner case: https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/blob/master/layers/core_validation.cpp#L9005

There's also a case when you reconsider and do not host write to the VK_IMAGE_LAYOUT_PREINITIALIZED Image at all (and thus need no barrier), isn't it?

I believe the layer message is a WARNING, not ERROR. It may mean it is "only" a heuristic and some false positives are expected (until they improve the layers, which seem posible, but not so trivial for this case).

They even only lately corrected possibility for 0 access flags for the presentation, so it is not far fetched they would (with similar mind) forget something like that in the layers.

I would report an Issue there. They do not bite and worse that can happen is that some Khronos insider more knowledgable than me will explain why you are wrong.

That being said, perhaps the VK_PIPELINE_STAGE_HOST_BIT is unnecessary too (and TOP should suffice)?

Upvotes: 3

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