From 16d1464d19ed308e4066280090128bd32dcf18ac Mon Sep 17 00:00:00 2001
From: Arisotura <thetotalworm@gmail.com>
Date: Sat, 5 Jul 2025 00:14:11 +0200
Subject: [PATCH] camera: make timings more realistic (fixes rolling in some
 games)

---
 src/DSi_Camera.cpp | 33 ++++++++++++++++++++++++---------
 src/DSi_Camera.h   |  4 +++-
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/src/DSi_Camera.cpp b/src/DSi_Camera.cpp
index a5248af3..38251fd6 100644
--- a/src/DSi_Camera.cpp
+++ b/src/DSi_Camera.cpp
@@ -34,8 +34,11 @@ using Platform::LogLevel;
 // namely, how long cameras take to process frames
 // camera IRQ is fired at roughly 15FPS with default config
 
-const u32 DSi_CamModule::kIRQInterval = 1120000; // ~30 FPS
-const u32 DSi_CamModule::kTransferStart = 60000;
+// camera IRQ marks camera VBlank
+// each scanline takes roughly 3173 cycles
+const u32 DSi_CamModule::kIRQInterval = 2234248; // ~15 FPS
+const u32 DSi_CamModule::kScanlineTime = 3173;
+const u32 DSi_CamModule::kTransferStart = DSi_CamModule::kIRQInterval - (DSi_CamModule::kScanlineTime * 480);
 
 
 DSi_CamModule::DSi_CamModule(melonDS::DSi& dsi) : DSi(dsi)
@@ -127,11 +130,11 @@ void DSi_CamModule::TransferScanline(u32 line)
     int maxlen = 512 - BufferWritePos;
 
     u32 tmpbuf[512];
-    int datalen = CurCamera->TransferScanline(tmpbuf, 512);
+    int lines_next;
+    int datalen = CurCamera->TransferScanline(tmpbuf, 512, lines_next);
     u32 numscan;
 
-    // TODO: must be tweaked such that each block has enough time to transfer
-    u32 delay = datalen*4 + 16;
+    u32 delay = lines_next * kScanlineTime;
 
     int copystart = 0;
     int copylen = datalen;
@@ -438,6 +441,7 @@ void DSi_Camera::Reset()
     // default state is preview mode (checkme)
     MCURegs[0x2104] = 3;
 
+    InternalY = 0;
     TransferY = 0;
     memset(FrameBuffer, 0, (640*480/2)*sizeof(u32));
 }
@@ -458,6 +462,7 @@ bool DSi_Camera::IsActivated() const
 
 void DSi_Camera::StartTransfer()
 {
+    InternalY = 0;
     TransferY = 0;
 
     u8 state = MCURegs[0x2104];
@@ -491,9 +496,11 @@ bool DSi_Camera::TransferDone() const
     return TransferY >= FrameHeight;
 }
 
-int DSi_Camera::TransferScanline(u32* buffer, int maxlen)
+int DSi_Camera::TransferScanline(u32* buffer, int maxlen, int& nlines)
 {
-    if (TransferY >= FrameHeight)
+    nlines = 0;
+
+    if ((TransferY >= FrameHeight) || (InternalY >= 480))
         return 0;
 
     if (FrameWidth > 640 || FrameHeight > 480 ||
@@ -509,7 +516,7 @@ int DSi_Camera::TransferScanline(u32* buffer, int maxlen)
     // TODO: non-YUV pixel formats and all
 
     int retlen = FrameWidth >> 1;
-    int sy = (TransferY * 480) / FrameHeight;
+    int sy = InternalY;
     if (FrameReadMode & (1<<1))
         sy = 479 - sy;
 
@@ -538,7 +545,15 @@ int DSi_Camera::TransferScanline(u32* buffer, int maxlen)
         }
     }
 
-    TransferY++;
+    // determine how many scanlines we're skipping until the next scanline
+    int oldy = TransferY;
+    do
+    {
+        InternalY++;
+        TransferY = (InternalY * FrameHeight) / 480;
+        nlines++;
+    }
+    while ((TransferY == oldy) && (InternalY < 480));
 
     return retlen;
 }
diff --git a/src/DSi_Camera.h b/src/DSi_Camera.h
index 5a626f56..61934bc8 100644
--- a/src/DSi_Camera.h
+++ b/src/DSi_Camera.h
@@ -44,7 +44,7 @@ public:
     bool TransferDone() const;
 
     // lengths in words
-    int TransferScanline(u32* buffer, int maxlen);
+    int TransferScanline(u32* buffer, int maxlen, int& nlines);
 
     void Acquire() override;
     u8 Read(bool last) override;
@@ -77,6 +77,7 @@ private:
 
     u16 FrameWidth, FrameHeight;
     u16 FrameReadMode, FrameFormat;
+    int InternalY;
     int TransferY;
     u32 FrameBuffer[640*480/2]; // YUYV framebuffer, two pixels per word
 };
@@ -124,6 +125,7 @@ private:
     DSi_Camera* CurCamera;
 
     static const u32 kIRQInterval;
+    static const u32 kScanlineTime;
     static const u32 kTransferStart;
 };