From d75705689125df2134b29accdbd7028c76cf994e Mon Sep 17 00:00:00 2001
From: Connor McLaughlin <stenzek@gmail.com>
Date: Sat, 5 Dec 2020 15:59:47 +1000
Subject: [PATCH] GPU: Implement dot clock timer (Timer 0)

Fixes softlock in Evil Dead - Hail to the King.
---
 src/core/gpu.cpp              | 27 ++++++++++++++++++++++-----
 src/core/gpu.h                |  2 ++
 src/core/save_state_version.h |  2 +-
 3 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/src/core/gpu.cpp b/src/core/gpu.cpp
index 65d5406d5..229549792 100644
--- a/src/core/gpu.cpp
+++ b/src/core/gpu.cpp
@@ -91,6 +91,7 @@ void GPU::SoftReset()
   m_crtc_state.regs.horizontal_display_range = 0xC60260;
   m_crtc_state.regs.vertical_display_range = 0x3FC10;
   m_crtc_state.fractional_ticks = 0;
+  m_crtc_state.fractional_dot_ticks = 0;
   m_crtc_state.current_tick_in_scanline = 0;
   m_crtc_state.current_scanline = 0;
   m_crtc_state.in_hblank = false;
@@ -172,6 +173,7 @@ bool GPU::DoState(StateWrapper& sw, bool update_display)
   sw.Do(&m_crtc_state.fractional_ticks);
   sw.Do(&m_crtc_state.current_tick_in_scanline);
   sw.Do(&m_crtc_state.current_scanline);
+  sw.DoEx(&m_crtc_state.fractional_dot_ticks, 46, 0);
   sw.Do(&m_crtc_state.in_hblank);
   sw.Do(&m_crtc_state.in_vblank);
   sw.Do(&m_crtc_state.interlaced_field);
@@ -679,15 +681,21 @@ TickCount GPU::GetPendingCommandTicks() const
 void GPU::UpdateCRTCTickEvent()
 {
   // figure out how many GPU ticks until the next vblank or event
-  const TickCount lines_until_vblank =
+  TickCount lines_until_event =
     (m_crtc_state.current_scanline >= m_crtc_state.vertical_display_end ?
        (m_crtc_state.vertical_total - m_crtc_state.current_scanline + m_crtc_state.vertical_display_end) :
        (m_crtc_state.vertical_display_end - m_crtc_state.current_scanline));
-  const TickCount lines_until_event = g_timers.IsExternalIRQEnabled(HBLANK_TIMER_INDEX) ?
-                                        std::min(g_timers.GetTicksUntilIRQ(HBLANK_TIMER_INDEX), lines_until_vblank) :
-                                        lines_until_vblank;
-  const TickCount ticks_until_event =
+  if (g_timers.IsExternalIRQEnabled(HBLANK_TIMER_INDEX))
+    lines_until_event = std::min(lines_until_event, g_timers.GetTicksUntilIRQ(HBLANK_TIMER_INDEX));
+
+  TickCount ticks_until_event =
     lines_until_event * m_crtc_state.horizontal_total - m_crtc_state.current_tick_in_scanline;
+  if (g_timers.IsExternalIRQEnabled(DOT_TIMER_INDEX))
+  {
+    const TickCount dots_until_irq = g_timers.GetTicksUntilIRQ(DOT_TIMER_INDEX);
+    const TickCount ticks_until_irq = (dots_until_irq * m_crtc_state.dot_clock_divider) - m_crtc_state.fractional_dot_ticks;
+    ticks_until_event = std::min(ticks_until_event, std::max<TickCount>(ticks_until_irq, 0));
+  }
 
 #if 0
   const TickCount ticks_until_hblank =
@@ -715,6 +723,15 @@ void GPU::CRTCTickEvent(TickCount ticks)
   {
     const TickCount gpu_ticks = SystemTicksToCRTCTicks(ticks, &m_crtc_state.fractional_ticks);
     m_crtc_state.current_tick_in_scanline += gpu_ticks;
+
+    if (g_timers.IsUsingExternalClock(DOT_TIMER_INDEX))
+    {
+      m_crtc_state.fractional_dot_ticks += gpu_ticks;
+      const TickCount dots = m_crtc_state.fractional_dot_ticks / m_crtc_state.dot_clock_divider;
+      m_crtc_state.fractional_dot_ticks = m_crtc_state.fractional_dot_ticks % m_crtc_state.dot_clock_divider;
+      if (dots > 0)
+        g_timers.AddTicks(DOT_TIMER_INDEX, dots);
+    }
   }
 
   if (m_crtc_state.current_tick_in_scanline < m_crtc_state.horizontal_total)
diff --git a/src/core/gpu.h b/src/core/gpu.h
index 4c8ab4acd..bc2dd43a5 100644
--- a/src/core/gpu.h
+++ b/src/core/gpu.h
@@ -501,6 +501,8 @@ protected:
     TickCount current_tick_in_scanline;
     u32 current_scanline;
 
+    TickCount fractional_dot_ticks; // only used when timer0 is enabled
+
     bool in_hblank;
     bool in_vblank;
 
diff --git a/src/core/save_state_version.h b/src/core/save_state_version.h
index d12cfa696..b31e12a80 100644
--- a/src/core/save_state_version.h
+++ b/src/core/save_state_version.h
@@ -2,7 +2,7 @@
 #include "types.h"
 
 static constexpr u32 SAVE_STATE_MAGIC = 0x43435544;
-static constexpr u32 SAVE_STATE_VERSION = 45;
+static constexpr u32 SAVE_STATE_VERSION = 46;
 static constexpr u32 SAVE_STATE_MINIMUM_VERSION = 42;
 
 static_assert(SAVE_STATE_VERSION >= SAVE_STATE_MINIMUM_VERSION);