From 690f80e15f411d8fa46a2eecdfc50ddadc8ed5e0 Mon Sep 17 00:00:00 2001
From: Sssnowyyy <78863999+Sssnowyyy@users.noreply.github.com>
Date: Mon, 2 Nov 2026 11:34:01 -0500
Subject: nx2: don't stall presents on a current-frame wait the GPU can't
 finish

0f14696044 made PresentClientBuffer also wait for the current frame's
fence whenever the previous frame's wasn't reached. Mario Kart World's
GPU waits on semaphores its CPU releases only after the present returns,
so in that state the current fence can't be reached until the present
returns, and each such present stalled up to the 4 s deadline.

Wait for the current frame by default, but give up after a few ms once a
GPU channel was seen suspended on a semaphore acquire and wait for the
previous frame instead. EDEN_NX2_FRAMES_AHEAD=1 always waits for the
previous frame (with 0f14696044's current-frame wait when the GPU is
behind), =0 always for the current one. The scheduler gains a lock-free
flag so the present thread can see suspended channels.

Assisted-by: Claude Sonnet 5.5 (implementation and commit message)
---
 src/video_core/control/scheduler.cpp | 2 ++
 src/video_core/control/scheduler.h   | 6 ++++++
 src/video_core/gpu.cpp               | 4 ++++
 src/video_core/gpu.h                 | 3 +++
 4 files changed, 15 insertions(+)

(limited to 'src/video_core')

diff --git a/src/video_core/control/scheduler.cpp b/src/video_core/control/scheduler.cpp
index dc42906752..97d2dcd482 100644
--- a/src/video_core/control/scheduler.cpp
+++ b/src/video_core/control/scheduler.cpp
@@ -48,6 +48,7 @@ void Scheduler::Push(GPU& gpu, s32 channel, CommandList&& entries) {
         std::find(pending_channels.begin(), pending_channels.end(), channel_state) ==
             pending_channels.end()) {
         pending_channels.push_back(channel_state);
+        any_pending.store(true, std::memory_order_release);
     }
     if (!pending_channels.empty()) {
         ResumePending(gpu);
@@ -77,6 +78,7 @@ void Scheduler::ResumePending(GPU& gpu) {
     std::erase_if(pending_channels, [](const auto& state) {
         return !state->payload->dma_pusher.HasPendingAcquire();
     });
+    any_pending.store(!pending_channels.empty(), std::memory_order_release);
 }
 
 void Scheduler::DeclareChannel(std::shared_ptr<ChannelState> new_channel) {
diff --git a/src/video_core/control/scheduler.h b/src/video_core/control/scheduler.h
index 9a0f1b919f..b1aaf46b60 100644
--- a/src/video_core/control/scheduler.h
+++ b/src/video_core/control/scheduler.h
@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <atomic>
 #include <memory>
 #include <mutex>
 #include <vector>
@@ -30,6 +31,10 @@ public:
     [[nodiscard]] bool HasPendingChannels() const {
         return !pending_channels.empty();
     }
+    /// HasPendingChannels for threads other than the GPU thread.
+    [[nodiscard]] bool HasPendingChannelsAnyThread() const {
+        return any_pending.load(std::memory_order_acquire);
+    }
 
     void DeclareChannel(std::shared_ptr<ChannelState> new_channel);
 
@@ -39,6 +44,7 @@ private:
     ::Common::unordered_map<s32, std::shared_ptr<ChannelState>> channels;
     std::mutex scheduling_guard;
     std::vector<std::shared_ptr<ChannelState>> pending_channels; ///< Suspended on an acquire
+    std::atomic<bool> any_pending{};
 };
 
 } // namespace Control
diff --git a/src/video_core/gpu.cpp b/src/video_core/gpu.cpp
index 53f8f063f7..a6f31359a3 100644
--- a/src/video_core/gpu.cpp
+++ b/src/video_core/gpu.cpp
@@ -505,6 +505,10 @@ bool GPU::IsAsync() const {
     return impl->is_async;
 }
 
+bool GPU::HasSuspendedChannels() const {
+    return impl->scheduler.HasPendingChannelsAnyThread();
+}
+
 bool GPU::UseNvdec() const {
     return impl->use_nvdec;
 }
diff --git a/src/video_core/gpu.h b/src/video_core/gpu.h
index 3a6546b41b..4afc89f0e6 100644
--- a/src/video_core/gpu.h
+++ b/src/video_core/gpu.h
@@ -217,6 +217,9 @@ public:
 
     [[nodiscard]] bool IsAsync() const;
 
+    /// Whether a channel is suspended on a semaphore acquire (callable from any thread).
+    [[nodiscard]] bool HasSuspendedChannels() const;
+
     [[nodiscard]] bool UseNvdec() const;
 
     void RendererFrameEndNotify();
-- 
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