nx2:vi2: pace presents by their swap interval

Present word 12 holds (swap interval << 16) | refresh rate. Presents were
always held to the next 60 Hz vblank, so Fire Emblem: Fortune's Weave,
which sends 0x2003c (interval 2) after its boot logos, ran at 60 fps and
double speed instead of 30.

Pace each present by its interval and rate (interval clamped to 1-4, rate
60 if outside 30-240 Hz) and log when the word changes.
EDEN_NX2_SWAP_INTERVAL=<n> forces the interval.

Assisted-by: Claude Opus 5.5 (debugging, implementation and drafting)
Signed-off-by: duck912 <350528965+Duck912@users.noreply.github.com>

1 files changed, 20 insertions, 4 deletions

diff --git a/src/core/hle/service/nx2_gsv.h b/src/core/hle/service/nx2_gsv.h
index 7ef4c2e0e3..1760ee0573 100644
--- a/src/core/hle/service/nx2_gsv.h
+++ b/src/core/hle/service/nx2_gsv.h
@@ -1501,7 +1501,7 @@ private:
         const auto waited = std::chrono::steady_clock::now();
         Composite(w[5], ~w[22], w[24], w[7]);
         const auto composited = std::chrono::steady_clock::now();
-        PaceToVblank();
+        PaceToVblank(w[12]);
         const auto paced = std::chrono::steady_clock::now();
         // Where each frame's time goes on the present thread, averaged per 600 presents: the
         // game is blocked here while Eden's GPU thread catches up (fence) and composites.
@@ -1557,15 +1557,31 @@ private:
     // present faster than that. Every slot is reported free here: hold each present until the
     // next vblank instead (title 4 ran at up to 160 fps, its timing and audio with it).
     std::chrono::steady_clock::time_point m_next_vblank{};
+    u32 m_swap_word{};
 
-    void PaceToVblank() {
+    // timing: present word 12, (swap interval << 16) | refresh rate. Fire Emblem: Fortune's Weave
+    // sends 0x1003c during its boot logos and 0x2003c (two vblanks per frame, 30 fps) after; paced
+    // at every vblank it ran at double speed. EDEN_NX2_SWAP_INTERVAL=<n> forces the interval.
+    void PaceToVblank(u32 timing) {
         if (!Settings::values.use_speed_limit.GetValue() || Settings::SpeedLimit() == 0) {
             return;
         }
-        const auto interval = std::chrono::nanoseconds{100'000'000'000LL / 60 / Settings::SpeedLimit()};
+        static const u32 forced_interval = [] {
+            const char* value = std::getenv("EDEN_NX2_SWAP_INTERVAL");
+            return value ? static_cast<u32>(std::strtoul(value, nullptr, 10)) : 0U;
+        }();
+        const u32 swap_interval =
+            forced_interval != 0 ? forced_interval : std::clamp<u32>(timing >> 16, 1, 4);
+        const u32 refresh_rate = (timing & 0xffff) >= 30 && (timing & 0xffff) <= 240 ? timing & 0xffff : 60;
+        if (timing != std::exchange(m_swap_word, timing)) {
+            LOG_INFO(Service, "nx2:vi2 present #{} swap interval {} at {} Hz (word 12 = {:#x})",
+                     m_presents, swap_interval, refresh_rate, timing);
+        }
+        const auto interval = std::chrono::nanoseconds{
+            100'000'000'000LL * swap_interval / refresh_rate / Settings::SpeedLimit()};
         const auto now = std::chrono::steady_clock::now();
         if (now > m_next_vblank + interval) {
-            // Running slower than 60 fps: show at once and don't bank the lost time.
+            // Running slower than the swap interval: show at once and don't bank the lost time.
             m_next_vblank = now;
         } else if (now < m_next_vblank) {
             std::this_thread::sleep_until(m_next_vblank);