Somehow, somewhere, say something about synchronization in Ray Tracing

[Tobski]

The way we've ended up making synchronization work for Vulkan ray tracing is not entirely obvious from the spec alone, and it's probably worth a bit of the guide to spell it out. I believe how it's intended to work is as follows:

• For trace or query calls in a shader, use VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_KHR with the relevant shader stage(s) for the acceleration structure
• For accesses to the shader binding table in the ray tracing pipeline, use , VK_ACCESS_SHADER_READ_BIT with VK_PIPELINE_STAGE_RAY_TRACING_SHADER_BIT_KHR
• For builds you use VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR with access bits corresponding to which resource is being accessed:
  • Destination AS uses VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_KHR
  • Source AS (e.g. for updates) use VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_KHR
  • Scratch buffers need both VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_KHR and VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_KHR
  • Vertex/Index/Instance/Transform buffers use VK_ACCESS_SHADER_READ_BIT
• For acceleration structure copy commands you also use VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR, again with access flags dependent on sources
  • Destination AS uses VK_ACCESS_ACCELERATION_STRUCTURE_WRITE_BIT_KHR
  • Source AS uses VK_ACCESS_ACCELERATION_STRUCTURE_READ_BIT_KHR
  • Destination buffer uses VK_ACCESS_TRANSFER_WRITE_BIT
  • Source AS uses VK_ACCESS_TRANSFER_READ_BIT
• For indirect trace calls, the indirect buffer is VK_PIPELINE_STAGE_DRAW_INDIRECT_BIT with VK_ACCESS_INDIRECT_COMMAND_READ_BIT
• For indirect AS builds, the indirect buffer is VK_PIPELINE_STAGE_ACCELERATION_STRUCTURE_BUILD_BIT_KHR with VK_ACCESS_INDIRECT_COMMAND_READ_BIT

Of additional note is that VK_PIPELINE_STAGE_TRANSFER_BIT does not work for acceleration structure copy commands, breaking with other copy operations.

CC @dgkoch for confirmation

[dgkoch]

The "Resource Usage and Synchronization" section in https://www.khronos.org/blog/vulkan-ray-tracing-final-specification-release says a lot of this, but it's not necessarily the most discoverable place for it either.

I haven't reviewed the above yet.

[dgkoch]

See #217 for PR to add this

[dgkoch]

Closed by #217