From 58b0ae84b56996304b6ad373e4f6cff2cf6bdd41 Mon Sep 17 00:00:00 2001
From: ReinUsesLisp <reinuseslisp@airmail.cc>
Date: Thu, 10 Sep 2020 03:43:30 -0300
Subject: renderer_vulkan: Make unconditional use of VK_KHR_timeline_semaphore

This reworks how host<->device synchronization works on the Vulkan
backend. Instead of "protecting" resources with a fence and signalling
these as free when the fence is known to be signalled by the host GPU,
use timeline semaphores.

Vulkan timeline semaphores allow use to work on a subset of D3D12
fences. As far as we are concerned, timeline semaphores are a value set
by the host or the device that can be waited by either of them.

Taking advantange of this, we can have a monolithically increasing
atomic value for each submission to the graphics queue. Instead of
protecting resources with a fence, we simply store the current logical
tick (the atomic value stored in CPU memory). When we want to know if a
resource is free, it can be compared to the current GPU tick.

This greatly simplifies resource management code and the free status of
resources should have less false negatives.

To workaround bugs in validation layers, when these are attached there's
a thread waiting for timeline semaphores.
---
 .../renderer_vulkan/vk_master_semaphore.h          | 70 ++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 src/video_core/renderer_vulkan/vk_master_semaphore.h

(limited to 'src/video_core/renderer_vulkan/vk_master_semaphore.h')

diff --git a/src/video_core/renderer_vulkan/vk_master_semaphore.h b/src/video_core/renderer_vulkan/vk_master_semaphore.h
new file mode 100644
index 000000000..0e93706d7
--- /dev/null
+++ b/src/video_core/renderer_vulkan/vk_master_semaphore.h
@@ -0,0 +1,70 @@
+// Copyright 2020 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <atomic>
+#include <thread>
+
+#include "common/common_types.h"
+#include "video_core/renderer_vulkan/wrapper.h"
+
+namespace Vulkan {
+
+class VKDevice;
+
+class MasterSemaphore {
+public:
+    explicit MasterSemaphore(const VKDevice& device);
+    ~MasterSemaphore();
+
+    /// Returns the current logical tick.
+    [[nodiscard]] u64 CurrentTick() const noexcept {
+        return current_tick;
+    }
+
+    /// Returns the timeline semaphore handle.
+    [[nodiscard]] VkSemaphore Handle() const noexcept {
+        return *semaphore;
+    }
+
+    /// Returns true when a tick has been hit by the GPU.
+    [[nodiscard]] bool IsFree(u64 tick) {
+        return gpu_tick >= tick;
+    }
+
+    /// Advance to the logical tick.
+    void NextTick() noexcept {
+        ++current_tick;
+    }
+
+    /// Refresh the known GPU tick
+    void Refresh() {
+        gpu_tick = semaphore.GetCounter();
+    }
+
+    /// Waits for a tick to be hit on the GPU
+    void Wait(u64 tick) {
+        // No need to wait if the GPU is ahead of the tick
+        if (IsFree(tick)) {
+            return;
+        }
+        // Update the GPU tick and try again
+        Refresh();
+        if (IsFree(tick)) {
+            return;
+        }
+        // If none of the above is hit, fallback to a regular wait
+        semaphore.Wait(tick);
+    }
+
+private:
+    vk::Semaphore semaphore;           ///< Timeline semaphore.
+    std::atomic<u64> gpu_tick{0};      ///< Current known GPU tick.
+    std::atomic<u64> current_tick{1};  ///< Current logical tick.
+    std::atomic<bool> shutdown{false}; ///< True when the object is being destroyed.
+    std::thread debug_thread;          ///< Debug thread to workaround validation layer bugs.
+};
+
+} // namespace Vulkan
-- 
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