Vulkan: Add an offscreen mode to VulkanGraphicsContext

For when there's nothing to present to. Instead of a surface and swapchain,
it renders into images of its own through the VulkanPresentation interface
libretro uses, picking the graphics queue without a surface. Acquiring and
presenting signal and wait on the frame's semaphores with empty submits.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

Common/GPU/Vulkan/VulkanGraphicsContext.cpp

@@ -55,6 +55,7 @@
 #include "Core/FrameTiming.h"
 #include "Common/GPU/Vulkan/VulkanLoader.h"
 #include "Common/GPU/Vulkan/VulkanContext.h"
+#include "Common/GPU/Vulkan/VulkanPresentation.h"
 
 #include "Common/GPU/thin3d.h"
 #include "Common/GPU/thin3d_create.h"
@@ -70,6 +71,94 @@ static const bool g_validate_ = true;
 static const bool g_validate_ = false;
 #endif
 
+using namespace PPSSPP_VK;
+
+// Stands in for the swapchain in offscreen mode: a few images that frames are rendered into and left
+// in. Acquiring and presenting only signal and wait on the frame's semaphores, with empty submits, so
+// the rest of the frame synchronization works as with a real swapchain.
+class VulkanOffscreenPresentation : public VulkanPresentation {
+public:
+	VulkanOffscreenPresentation(VkFormat format, VkExtent2D extent) : format_(format), extent_(extent) {}
+
+	bool Create(VulkanContext *vulkan) {
+		VkDevice device = vulkan->GetDevice();
+		for (Image &img : images_) {
+			VkImageCreateInfo info{ VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO };
+			info.imageType = VK_IMAGE_TYPE_2D;
+			info.format = format_;
+			info.extent = { extent_.width, extent_.height, 1 };
+			info.mipLevels = 1;
+			info.arrayLayers = 1;
+			info.samples = VK_SAMPLE_COUNT_1_BIT;
+			info.tiling = VK_IMAGE_TILING_OPTIMAL;
+			info.usage = VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT | VK_IMAGE_USAGE_TRANSFER_SRC_BIT | VK_IMAGE_USAGE_TRANSFER_DST_BIT | VK_IMAGE_USAGE_SAMPLED_BIT;
+			info.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
+			if (vkCreateImage(device, &info, nullptr, &img.image) != VK_SUCCESS)
+				return false;
+
+			VkMemoryRequirements memreq;
+			vkGetImageMemoryRequirements(device, img.image, &memreq);
+			VkMemoryAllocateInfo alloc{ VK_STRUCTURE_TYPE_MEMORY_ALLOCATE_INFO };
+			alloc.allocationSize = memreq.size;
+			if (!vulkan->MemoryTypeFromProperties(memreq.memoryTypeBits, VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT, &alloc.memoryTypeIndex))
+				return false;
+			if (vkAllocateMemory(device, &alloc, nullptr, &img.memory) != VK_SUCCESS)
+				return false;
+			if (vkBindImageMemory(device, img.image, img.memory, 0) != VK_SUCCESS)
+				return false;
+		}
+		return true;
+	}
+
+	void Destroy(VulkanContext *vulkan) override {
+		VkDevice device = vulkan->GetDevice();
+		for (Image &img : images_) {
+			if (img.image)
+				vkDestroyImage(device, img.image, nullptr);
+			if (img.memory)
+				vkFreeMemory(device, img.memory, nullptr);
+			img = {};
+		}
+	}
+
+	VkResult AcquireNextImage(VulkanContext *vulkan, VkSemaphore signalSemaphore, uint32_t *imageIndex) override {
+		// The queue runs in order, so an image is free again by the time later work reaches it.
+		*imageIndex = next_;
+		next_ = (next_ + 1) % IMAGE_COUNT;
+		VkSubmitInfo submit{ VK_STRUCTURE_TYPE_SUBMIT_INFO };
+		submit.signalSemaphoreCount = 1;
+		submit.pSignalSemaphores = &signalSemaphore;
+		return vkQueueSubmit(vulkan->GetGraphicsQueue(), 1, &submit, VK_NULL_HANDLE);
+	}
+
+	VkResult QueuePresent(VulkanContext *vulkan, VkQueue queue, uint32_t imageIndex, VkSemaphore waitSemaphore) override {
+		// Nothing to show it on, so just consume the semaphore.
+		VkPipelineStageFlags stage = VK_PIPELINE_STAGE_ALL_COMMANDS_BIT;
+		VkSubmitInfo submit{ VK_STRUCTURE_TYPE_SUBMIT_INFO };
+		submit.waitSemaphoreCount = 1;
+		submit.pWaitSemaphores = &waitSemaphore;
+		submit.pWaitDstStageMask = &stage;
+		return vkQueueSubmit(queue, 1, &submit, VK_NULL_HANDLE);
+	}
+
+	uint32_t GetImageCount() const override { return IMAGE_COUNT; }
+	VkImage GetImage(uint32_t index) const override { return images_[index].image; }
+	VkExtent2D GetExtent() const override { return extent_; }
+	VkFormat GetFormat() const override { return format_; }
+	VkImageLayout GetPresentLayout() const override { return VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL; }
+
+private:
+	static constexpr int IMAGE_COUNT = 2;
+	struct Image {
+		VkImage image = VK_NULL_HANDLE;
+		VkDeviceMemory memory = VK_NULL_HANDLE;
+	};
+	Image images_[IMAGE_COUNT];
+	VkFormat format_;
+	VkExtent2D extent_;
+	uint32_t next_ = 0;
+};
+
 bool VulkanGraphicsContext::InitAPI(void *wnd, std::string *deviceName, std::string *errorMessage) {
 	*errorMessage = "N/A";
 	_dbg_assert_(deviceName);
@@ -116,20 +205,35 @@ bool VulkanGraphicsContext::InitAPI(void *wnd, std::string *deviceName, std::str
 		vulkan_ = nullptr;
 		return false;
 	}
+	// Normally the queue is picked along with the surface, which must be able to present from it.
+	if (offscreenWidth_ > 0 && !vulkan_->ChooseGraphicsQueueWithoutSurface()) {
+		*errorMessage = "No graphics queue";
+		return false;
+	}
 	return true;
 }
 
 bool VulkanGraphicsContext::InitSurface(WindowSystem winsys, void *data1, void *data2, std::string *errorMessage) {
-	// Don't proceed on failure - without a surface there's no present mode, no swapchain and no queue,
-	// so everything below would just fail in more confusing ways further down (it used to assert deep
-	// inside the thin3d context constructor). Let the caller fall back to another backend instead.
-	VkResult res = vulkan_->InitSurface(winsys, data1, data2);
-	if (res != VK_SUCCESS) {
-		*errorMessage = vulkan_->InitError();
-		if (errorMessage->empty()) {
-			*errorMessage = StringFromFormat("Failed to initialize Vulkan surface: %s", VulkanResultToString(res));
+	if (offscreenWidth_ > 0) {
+		auto presentation = std::make_unique<VulkanOffscreenPresentation>(VK_FORMAT_B8G8R8A8_UNORM, VkExtent2D{ (uint32_t)offscreenWidth_, (uint32_t)offscreenHeight_ });
+		if (!presentation->Create(vulkan_)) {
+			presentation->Destroy(vulkan_);
+			*errorMessage = "Failed to create the offscreen images";
+			return false;
+		}
+		vulkan_->SetPresentation(std::move(presentation));
+	} else {
+		// Don't proceed on failure - without a surface there's no present mode, no swapchain and no queue,
+		// so everything below would just fail in more confusing ways further down (it used to assert deep
+		// inside the thin3d context constructor). Let the caller fall back to another backend instead.
+		VkResult res = vulkan_->InitSurface(winsys, data1, data2);
+		if (res != VK_SUCCESS) {
+			*errorMessage = vulkan_->InitError();
+			if (errorMessage->empty()) {
+				*errorMessage = StringFromFormat("Failed to initialize Vulkan surface: %s", VulkanResultToString(res));
+			}
+			return false;
 		}
-		return false;
 	}
 
 	bool useMultiThreading = g_Config.bRenderMultiThreading;
@@ -139,17 +243,19 @@ bool VulkanGraphicsContext::InitSurface(WindowSystem winsys, void *data1, void *
 
 	draw_ = Draw::T3DCreateVulkanContext(vulkan_, useMultiThreading);
 
-	VkPresentModeKHR presentMode = ConfigPresentModeToVulkan(draw_);
+	if (offscreenWidth_ == 0) {
+		VkPresentModeKHR presentMode = ConfigPresentModeToVulkan(draw_);
 
 #ifdef VK_EXT_full_screen_exclusive
-	vulkan_->SetFullScreenExclusiveMode(g_Config.bFullScreen && g_Config.bAllowFullScreenExclusive
-		? VK_FULL_SCREEN_EXCLUSIVE_ALLOWED_EXT
-		: VK_FULL_SCREEN_EXCLUSIVE_DISALLOWED_EXT);
+		vulkan_->SetFullScreenExclusiveMode(g_Config.bFullScreen && g_Config.bAllowFullScreenExclusive
+			? VK_FULL_SCREEN_EXCLUSIVE_ALLOWED_EXT
+			: VK_FULL_SCREEN_EXCLUSIVE_DISALLOWED_EXT);
 #endif
 
-	if (!vulkan_->InitSwapchain(presentMode)) {
-		*errorMessage = vulkan_->InitError();
-		return false;
+		if (!vulkan_->InitSwapchain(presentMode)) {
+			*errorMessage = vulkan_->InitError();
+			return false;
+		}
 	}
 
 	SetGPUBackend(GPUBackend::VULKAN, vulkan_->GetPhysicalDeviceProperties().properties.deviceName);
@@ -176,8 +282,13 @@ void VulkanGraphicsContext::ShutdownSurface() {
 	draw_ = nullptr;
 
 	vulkan_->WaitUntilQueueIdle();
-	vulkan_->DestroySwapchain();
-	vulkan_->DestroySurface();
+	if (VulkanPresentation *presentation = vulkan_->GetPresentation()) {
+		presentation->Destroy(vulkan_);
+		vulkan_->SetPresentation(nullptr);
+	} else {
+		vulkan_->DestroySwapchain();
+		vulkan_->DestroySurface();
+	}
 }
 
 void VulkanGraphicsContext::ShutdownAPI() {
@@ -192,6 +303,10 @@ void VulkanGraphicsContext::ShutdownAPI() {
 }
 
 void VulkanGraphicsContext::Resize() {
+	if (offscreenWidth_ > 0) {
+		// The images have a fixed size.
+		return;
+	}
 	draw_->HandleEvent(Draw::Event::LOST_BACKBUFFER, vulkan_->GetBackbufferWidth(), vulkan_->GetBackbufferHeight());
 	VkPresentModeKHR presentMode = ConfigPresentModeToVulkan(draw_);
 

Common/GPU/Vulkan/VulkanGraphicsContext.h

@@ -26,6 +26,14 @@ class VulkanRenderManager;
 class VulkanGraphicsContext : public GraphicsContext {
 public:
 	VulkanGraphicsContext() : draw_(nullptr) {}
+
+	// Renders into images of this size instead of a window's swapchain, for when there's nothing to
+	// present to (headless). Call before InitAPI. InitSurface then ignores the window system.
+	void SetOffscreen(int width, int height) {
+		offscreenWidth_ = width;
+		offscreenHeight_ = height;
+	}
+
 	bool InitAPI(void *wnd, std::string *deviceName, std::string *errorMessage) override;
 	bool InitSurface(WindowSystem winsys, void *data1, void *data2, std::string *errorMessage) override;
 	void ShutdownSurface() override;
@@ -45,4 +53,6 @@ private:
 	VulkanContext *vulkan_ = nullptr;
 	VulkanRenderManager *renderManager_ = nullptr;
 	bool windowRestored_ = false;
+	int offscreenWidth_ = 0;
+	int offscreenHeight_ = 0;
 };