单个子通道的子通道依赖性。如何？

Question

所以我有一个渲染通道和一个直接绘制到帧缓冲区的子通道。规范不强制我使用依赖项——如果我省略它们，实现会隐式插入它们（尽管我不明白为什么它使用 srcStageMask = VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT 作为第一个子通道——这个阶段意味着最开始，即不要不要等待任何事情）。

但与 Vulkan 一样 - 最好是明确的。这就是混淆 - 多个来源使用不同的子通道。

Sdk 的立方体示例根本没有使用它们。

Vulkan 教程只使用一个：

dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
dependency.dstSubpass = 0;
dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.srcAccessMask = 0;
dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT | VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;

这里为什么srcAccessMask为零？

API 没有秘密使用两个：

dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
dependency.dstSubpass = 0;
dependency.srcStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
dependency.srcAccessMask = VK_ACCESS_MEMORY_READ_BIT;
dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;

和

dependency.srcSubpass = 0;
dependency.dstSubpass = VK_SUBPASS_EXTERNAL;
dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
dependency.dstStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
dependency.dstAccessMask = VK_ACCESS_MEMORY_READ_BIT;

不太清楚为什么 srcStageMask VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT 在第一个子通道中 - 不是这个阶段应该用于执行依赖，但在这里我们需要内存依赖？关于为什么 dstStageMask 的相同问题 VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT在第二个subpass中吗？

Khronos同步实例使用一个：

dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
dependency.dstSubpass = 0;
dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.srcAccessMask = 0;
dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;

为什么srcAccessMask是0？

这是我对两个依赖项的尝试：

    dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
    dependency.dstSubpass = 0;
    dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // need to wait until
presentation is finished reading the image
    dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT;
    dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // we are writing to
the image in this stage
    dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;

and

    dependency.srcSubpass = 0;
    dependency.dstSubpass = VK_SUBPASS_EXTERNAL;
    dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // we are writing to
the image in this stage
    dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
    dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // presentation reads
image in this stage (is it?)
    dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT;

这一切都非常令人困惑。如您所见，多个有能力的消息来源有不同的看法。使用哪一个？如何理解这些依赖？

Answer 1

天哪，这是一个大多数人都不太了解的话题。许多错误信息被假定并扩散开来。规范示例的适当位置是 wikipage in the github repo。

可以找到关于交换链图像 acquire/present 的相关部分 here。

这是许多同步细节被正确确定之前的旧示例。
目的是同步信号量使用以等待 vkAcquireNextImage 的结果与渲染。这是一个读后写的危险，信号量将包括内存可见性同步。所以 srcAccessMask 是没有必要的。
同样，障碍是为了与信号量同步。将命令缓冲区提交到队列时，您可以设置哪些阶段等待哪些信号量。在这种情况下，他们使用管道底部而不是 VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT
与 2 相同的答案，唯一不同的是 dstAccessMask

我在上面链接的 wiki 页面使用了以下内容：

/* Only need a dependency coming in to ensure that the first
   layout transition happens at the right time.
   Second external dependency is implied by having a different
   finalLayout and subpass layout. */
VkSubpassDependency dependency = {
    .srcSubpass = VK_SUBPASS_EXTERNAL,
    .dstSubpass = 0,
    // .srcStageMask needs to be a part of pWaitDstStageMask in the WSI semaphore.
    .srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
    .dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
    .srcAccessMask = 0,
    .dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
    .dependencyFlags = 0};

/* Normally, we would need an external dependency at the end as well since we are changing layout in finalLayout,
   but since we are signalling a semaphore, we can rely on Vulkan's default behavior,
   which injects an external dependency here with
   dstStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT,
   dstAccessMask = 0. */

您混淆了第二个障碍的子通道索引。尽管如评论中所述，在 renderpass 的末尾已经存在一个隐式屏障，它反过来与您用来与当前同步的信号量或栅栏同步。所以不需要显式依赖。

单个子通道的子通道依赖性。如何？

Subpass dependencies for a single subpass. How to?

vulkan