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render/vulkan: clear blend image on first use

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The two-pass blend image is created with VK_IMAGE_LAYOUT_UNDEFINED, so
on its first use loadOp=LOAD loads uninitialized memory. This oughtn't
be an issue, as we render onto it before we read it. These renders are
blends, so even opaque content is rendered with reference to an uninit
dst. This too ought to be fine: src*1 + dst*0 = src for all finite dst.

But the blend image pixfmt is VK_FORMAT_R16G16B16A16_SFLOAT, so uninit
pixels can be NaN, inf, or -inf, and now src*1 + dst*0 = NaN/inf/-inf.
This is bad enough assuming the uninitialized blend image holds random
bytes (2048/65536 values are not finite), even worse on any driver/GPU
with a framebuffer compression scheme that so happens to reliably read
NaNs from any uninitialized compressed image...

Most Mesa drivers happen not to do this perfectly valid thing, so this
is only reliably a problem (afaict) for honeykrisp i.e. AGX i.e. Asahi
Linux i.e. Apple Silicon, where after an upgrade to wlroots 0.20, sway
renders a black screen forever, unless you get quite lucky spamming VT
switches, in which case there's flickery garbage on exactly one of the
two swapchain buffers.

The blend image persists across frames, so it suffices to clear before
first real use. Rather than clear by hand, make a loadOp=CLEAR variant
of the render pass and use it for that first frame only. Adding a pass
sounds heavy, but render pass compatibility ignores loadOp and layouts
such that the new pass reuses the pipelines and framebuffer, and costs
one VkRenderPass object but not the usual pipeline/shader (re)compile.
This commit is contained in:
twilligon 2026-06-09 00:43:41 +00:00 • committed by Félix Poisot
parent 63318d28b1
commit 5d150267e2
3 changed files with 40 additions and 27 deletions
Download patch file Download diff file Expand all files Collapse all files

57
render/vulkan/pass.c
View file

@ -429,33 +429,32 @@ static bool render_pass_submit(struct wlr_render_pass *wlr_pass) {
}
if (pass->two_pass) {
// The render pass changes the blend image layout from
// color attachment to read only, so on each frame, before
// the render pass starts, we change it back
VkImageLayout blend_src_layout = VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL;
if (!render_buffer->two_pass.blend_transitioned) {
blend_src_layout = VK_IMAGE_LAYOUT_UNDEFINED;
// On the first frame the clear render pass transitions the blend
// image from undefined and we just mark it transitioned. On every
// frame after, the previous frame left it read-only, so we change
// it back to a color attachment before the render pass starts
if (render_buffer->two_pass.blend_transitioned) {
VkImageMemoryBarrier blend_acq_barrier = {
.sType = VK_STRUCTURE_TYPE_IMAGE_MEMORY_BARRIER,
.srcQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED,
.dstQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED,
.image = render_buffer->two_pass.blend_image,
.oldLayout = VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL,
.newLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL,
.srcAccessMask = VK_ACCESS_SHADER_READ_BIT,
.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
.subresourceRange = {
.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT,
.layerCount = 1,
.levelCount = 1,
},
};
vkCmdPipelineBarrier(stage_cb->vk, VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
0, 0, NULL, 0, NULL, 1, &blend_acq_barrier);
} else {
render_buffer->two_pass.blend_transitioned = true;
}
VkImageMemoryBarrier blend_acq_barrier = {
.sType = VK_STRUCTURE_TYPE_IMAGE_MEMORY_BARRIER,
.srcQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED,
.dstQueueFamilyIndex = VK_QUEUE_FAMILY_IGNORED,
.image = render_buffer->two_pass.blend_image,
.oldLayout = blend_src_layout,
.newLayout = VK_IMAGE_LAYOUT_COLOR_ATTACHMENT_OPTIMAL,
.srcAccessMask = VK_ACCESS_SHADER_READ_BIT,
.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
.subresourceRange = {
.aspectMask = VK_IMAGE_ASPECT_COLOR_BIT,
.layerCount = 1,
.levelCount = 1,
},
};
vkCmdPipelineBarrier(stage_cb->vk, VK_PIPELINE_STAGE_FRAGMENT_SHADER_BIT,
VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
0, 0, NULL, 0, NULL, 1, &blend_acq_barrier);
}
// acquire render buffer before rendering
@ -1334,11 +1333,15 @@ struct wlr_vk_render_pass *vulkan_begin_render_pass(struct wlr_vk_renderer *rend
int height = buffer->wlr_buffer->height;
VkRect2D rect = { .extent = { width, height } };
bool blend_first_use = pass->two_pass && !buffer->two_pass.blend_transitioned;
VkClearValue clear_value = {0};
VkRenderPassBeginInfo rp_info = {
.sType = VK_STRUCTURE_TYPE_RENDER_PASS_BEGIN_INFO,
.renderArea = rect,
.clearValueCount = 0,
.renderPass = render_setup->render_pass,
.clearValueCount = blend_first_use ? 1 : 0,
.pClearValues = blend_first_use ? &clear_value : NULL,
.renderPass = blend_first_use ?
render_setup->render_pass_clear : render_setup->render_pass,
.framebuffer = buffer_out->framebuffer,
};
vkCmdBeginRenderPass(cb->vk, &rp_info, VK_SUBPASS_CONTENTS_INLINE);