LowLatency2: Check semaphore value before signalling

1 files changed, 41 insertions, 0 deletions

diff --git a/src/strategies/low_latency2/semaphore_signal.hh b/src/strategies/low_latency2/semaphore_signal.hh
new file mode 100644
index 0000000..e1e1439
--- /dev/null
+++ b/src/strategies/low_latency2/semaphore_signal.hh
@@ -0,0 +1,41 @@
+#ifndef SEMAPHORE_SIGNAL_HH_
+#define SEMAPHORE_SIGNAL_HH_
+
+#include "device_context.hh"
+
+#include <cstdint>
+#include <vulkan/vulkan.h>
+
+// The VK_NV_low_latency2 extension supports a monotonically increasing
+// semaphore value. Monotonically increasing != strictly increasing. We have to
+// support a sequence with repeating values like 0, 1, 1, 1, 1, 2, 3. Vulkan
+// timeline semaphores do NOT support signalling <= its current value. While the
+// frame pacing isn't going to do anything in the case of repeating values (we
+// expect a global frame counter), it can happen in the case of swapchain
+// recreation or incomplete frames. This tiny class wraps a timeline semaphore
+// and associated value. It double checks that we haven't already signalled the
+// value.
+
+// TODO we _might_ want to make it so the destructor calls .signal(). This
+// would make it impossible to drop semaphores and cause hangs. However,
+// there are only a few places this can happen and I want to keep things
+// explicit for now.
+
+namespace low_latency {
+
+class SemaphoreSignal {
+  private:
+    const VkSemaphore semaphore{};
+    const std::uint64_t value{};
+
+  public:
+    SemaphoreSignal(const VkSemaphore& semaphore, const std::uint64_t& value);
+    ~SemaphoreSignal();
+
+  public:
+    void signal(const DeviceContext& device_context) const;
+};
+
+}; // namespace low_latency
+
+#endif