Proof of concept fix for Async GPU Readbacks

drivers/vulkan/vulkan_context.cpp

@@ -42,6 +42,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <cstdint>
 
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
 #define APP_SHORT_NAME "GodotEngine"
@@ -1794,6 +1795,22 @@ Error VulkanContext::_create_semaphores() {
 		/*pNext*/ nullptr,
 		/*flags*/ 0,
 	};
+	// Create compute semaphore that can be polled to determine whether
+	// a given compute block has finished.
+    // Every time a compute call finishes, we'll increment the value
+    // of this semaphore by one, so we can check the status
+    VkSemaphoreTypeCreateInfo timelineCreateInfo = {
+        /*sType*/ VK_STRUCTURE_TYPE_SEMAPHORE_TYPE_CREATE_INFO,
+        /*pNext*/ nullptr,
+        /*semaphoreType*/ VK_SEMAPHORE_TYPE_TIMELINE,
+        /*initialValue*/ 0,
+    };
+
+	VkSemaphoreCreateInfo computeSemaphoreCreateInfo = {
+		/*sType*/ VK_STRUCTURE_TYPE_SEMAPHORE_CREATE_INFO,
+		/*pNext*/ &timelineCreateInfo,
+		/*flags*/ 0,
+	};
 
 	// Create fences that we can use to throttle if we get too far
 	// ahead of the image presents.
@@ -1816,6 +1833,9 @@ Error VulkanContext::_create_semaphores() {
 	}
 	frame_index = 0;
 
+    err = vkCreateSemaphore(device, &computeSemaphoreCreateInfo, nullptr, &compute_semaphore[0]);
+		ERR_FAIL_COND_V(err, ERR_CANT_CREATE);
+
 	// Get Memory information and properties.
 	vkGetPhysicalDeviceMemoryProperties(gpu, &memory_properties);
 
@@ -2781,16 +2801,26 @@ void VulkanContext::local_device_push_command_buffers(RID p_local_device, const
 	LocalDevice *ld = local_device_owner.get_or_null(p_local_device);
 	ERR_FAIL_COND(ld->waiting);
 
+    compute_semaphore_signal_value += 1;
+
+    VkTimelineSemaphoreSubmitInfo signal_info;
+    signal_info.sType = VK_STRUCTURE_TYPE_TIMELINE_SEMAPHORE_SUBMIT_INFO;
+    signal_info.pNext = nullptr;
+    signal_info.waitSemaphoreValueCount = 0;
+    signal_info.signalSemaphoreValueCount = 1;
+    signal_info.pSignalSemaphoreValues = &compute_semaphore_signal_value;
+
 	VkSubmitInfo submit_info;
 	submit_info.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;
-	submit_info.pNext = nullptr;
+	submit_info.pNext = &signal_info;
 	submit_info.pWaitDstStageMask = nullptr;
 	submit_info.waitSemaphoreCount = 0;
 	submit_info.pWaitSemaphores = nullptr;
 	submit_info.commandBufferCount = p_count;
 	submit_info.pCommandBuffers = (const VkCommandBuffer *)p_buffers;
-	submit_info.signalSemaphoreCount = 0;
-	submit_info.pSignalSemaphores = nullptr;
+	submit_info.signalSemaphoreCount = 1;
+	submit_info.pSignalSemaphores = &compute_semaphore[0];
+
 
 	VkResult err = vkQueueSubmit(ld->queue, 1, &submit_info, VK_NULL_HANDLE);
 	if (err == VK_ERROR_OUT_OF_HOST_MEMORY) {
@@ -2813,8 +2843,18 @@ void VulkanContext::local_device_sync(RID p_local_device) {
 
 	vkDeviceWaitIdle(ld->device);
 	ld->waiting = false;
+
 }
 
+bool VulkanContext::local_device_check_status(RID p_local_device) {
+	LocalDevice *ld = local_device_owner.get_or_null(p_local_device);
+	ERR_FAIL_COND_V(!ld->waiting, false);
+    uint64_t out = 0;
+    vkGetSemaphoreCounterValue(ld->device, compute_semaphore[0], &out);
+    return out >= compute_semaphore_signal_value;
+}
+
+
 void VulkanContext::local_device_free(RID p_local_device) {
 	LocalDevice *ld = local_device_owner.get_or_null(p_local_device);
 	memdelete(ld->driver);
@@ -2901,6 +2941,7 @@ VulkanContext::~VulkanContext() {
 				vkDestroySemaphore(device, image_ownership_semaphores[i], nullptr);
 			}
 		}
+        vkDestroySemaphore(device, compute_semaphore[0], nullptr);
 		if (inst_initialized && is_instance_extension_enabled(VK_EXT_DEBUG_UTILS_EXTENSION_NAME)) {
 			DestroyDebugUtilsMessengerEXT(inst, dbg_messenger, nullptr);
 		}

drivers/vulkan/vulkan_context.h

@@ -40,6 +40,7 @@
 #include "rendering_device_driver_vulkan.h"
 #include "servers/display_server.h"
 #include "servers/rendering/renderer_rd/api_context_rd.h"
+#include <cstdint>
 
 #ifdef USE_VOLK
 #include <volk.h>
@@ -133,6 +134,8 @@ class VulkanContext : public ApiContextRD {
 	VkFormat format;
 	VkSemaphore draw_complete_semaphores[FRAME_LAG];
 	VkSemaphore image_ownership_semaphores[FRAME_LAG];
+	VkSemaphore compute_semaphore[1];
+    uint64_t compute_semaphore_signal_value = 0;
 	int frame_index = 0;
 	VkFence fences[FRAME_LAG];
 	VkPhysicalDeviceMemoryProperties memory_properties;
@@ -313,6 +316,7 @@ class VulkanContext : public ApiContextRD {
 	virtual RID local_device_create() override final;
 	virtual void local_device_push_command_buffers(RID p_local_device, const RDD::CommandBufferID *p_buffers, int p_count) override final;
 	virtual void local_device_sync(RID p_local_device) override final;
+    virtual bool local_device_check_status(RID p_local_device) override final;
 	virtual void local_device_free(RID p_local_device) override final;
 
 	VkFormat get_screen_format() const;

servers/rendering/renderer_rd/api_context_rd.h

@@ -54,6 +54,7 @@ class ApiContextRD {
 	virtual RID local_device_create() = 0;
 	virtual void local_device_push_command_buffers(RID p_local_device, const RDD::CommandBufferID *p_buffers, int p_count) = 0;
 	virtual void local_device_sync(RID p_local_device) = 0;
+    virtual bool local_device_check_status(RID p_local_device) = 0;
 	virtual void local_device_free(RID p_local_device) = 0;
 
 	virtual void set_setup_buffer(RDD::CommandBufferID p_command_buffer) = 0;

servers/rendering/rendering_device.cpp

@@ -29,6 +29,7 @@
 /**************************************************************************/
 
 #include "rendering_device.h"
+#include "core/os/thread_safe.h"
 #include "rendering_device.compat.inc"
 
 #include "rendering_device_binds.h"
@@ -4659,6 +4660,12 @@ void RenderingDevice::sync() {
 	local_device_processing = false;
 }
 
+bool RenderingDevice::check_status(){
+	_THREAD_SAFE_METHOD_
+
+        return context->local_device_check_status(local_device);
+}
+
 void RenderingDevice::_free_pending_resources(int p_frame) {
 	// Free in dependency usage order, so nothing weird happens.
 	// Pipelines.
@@ -5340,6 +5347,7 @@ void RenderingDevice::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_frame_delay"), &RenderingDevice::get_frame_delay);
 	ClassDB::bind_method(D_METHOD("submit"), &RenderingDevice::submit);
 	ClassDB::bind_method(D_METHOD("sync"), &RenderingDevice::sync);
+	ClassDB::bind_method(D_METHOD("check_status"), &RenderingDevice::check_status);
 
 #ifndef DISABLE_DEPRECATED
 	ClassDB::bind_method(D_METHOD("barrier", "from", "to"), &RenderingDevice::barrier, DEFVAL(BARRIER_MASK_ALL_BARRIERS), DEFVAL(BARRIER_MASK_ALL_BARRIERS));

servers/rendering/rendering_device.h

@@ -1333,6 +1333,7 @@ class RenderingDevice : public RenderingDeviceCommons {
 
 	void submit();
 	void sync();
+    bool check_status();
 
 	enum MemoryType {
 		MEMORY_TEXTURES,