Synchronization in renderpass after copy

Vulkan
1

I’m having trouble to get this synchronization right.

First clear an image then I use it in a copy as a source, then as an attachment to draw to (with external subpass dependency)

vkCmdClearColorImage(..)

VkImageMemoryBarrier2 memoryBarrier = {
  ...
  .srcStageMask = VK_PIPELINE_STAGE_2_CLEAR_BIT,
  .srcAccessMask = VK_ACCESS_2_TRANSFER_WRITE_BIT,
  .oldLayout = VK_IMAGE_LAYOUT_TRANSFER_DST_OPTIMAL
  .dstStageMask = VK_PIPELINE_STAGE_2_COPY_BIT,
  .dstAccessMask = VK_ACCESS_2_TRANSFER_READ_BIT,
  .newLayout = VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL };

vkCmdCopyImage(..)  // as a source

VkAttachmentDescription2 attachement_info = {
  ...
  .loadOp = VK_ATTACHMENT_LOAD_OP_LOAD,
  .storeOp = VK_ATTACHMENT_STORE_OP_STORE
  .initialLayout = VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL,
  .finalLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL,
}

VkMemoryBarrier2 subpass_memoryBarrier = {
  ...
  .srcStageMask = VK_PIPELINE_STAGE_2_COPY_BIT,
  .srcAccessMask = 0,
  .dstStageMask = VK_PIPELINE_STAGE_2_COLOR_ATTACHMENT_OUTPUT_BIT
  .dstAccessMask = VK_ACCESS_2_COLOR_ATTACHMENT_WRITE_BIT
}

VkSubpassDependency2 info = {
  ...
  .pNext = &subpass_memoryBarrier,
  .srcSubpass = VK_SUBPASS_EXTERNAL,
  .dstSubpass = 0,
  .dependencyFlags = VK_DEPENDENCY_BY_REGION_BIT
};

vkCmdBeginRenderPass(..)  // sync validation error when trying to record this command:

Validation Error: [ SYNC-HAZARD-READ-AFTER-WRITE ]
Object 0: handle = 0x4d6d9a000000010c, type = VK_OBJECT_TYPE_RENDER_PASS; | MessageID = 0xe4d96472 | vkCmdBeginRenderPass():
Hazard READ_AFTER_WRITE vs. layout transition in subpass 0 for attachment 0 aspect color during load with loadOp VK_ATTACHMENT_LOAD_OP_LOAD.

2

Ok, I found out how to get rid of the sync validation error, but I don’t understand why.

If I change the external dependency barrier of the renderpass, from VK_ACCESS_2_COLOR_ATTACHMENT_WRITE_BIT to VK_ACCESS_2_COLOR_ATTACHMENT_READ_BIT, fixes the issue.

The dependency is inserted for the vkCmdDraw in the subpass. Which writes to it, but I guess VK_ATTACHMENT_LOAD_OP_LOAD also kinda makes it a read first?

What should I use if I VK_ATTACHMENT_LOAD_OP_CLEAR or VK_ATTACHMENT_LOAD_OP_DONT_CARE, do I also create a read attachment barrier?

Makes that source external dependency always VK_ACCESS_2_COLOR_ATTACHMENT_READ_BIT?