From b2fee4242cd061a55e8f3bb2f6dfafbf9f87403f Mon Sep 17 00:00:00 2001
From: gm-matthew <108370479+gm-matthew@users.noreply.github.com>
Date: Sat, 19 Aug 2023 23:47:00 +0100
Subject: [PATCH] DMA device register always returns Step 1.x PCI ID Step 2.x
 games by AM3 request PCI ID this way and expect to see 0x16c311db

---
 Src/Model3/Model3.cpp |  7 +------
 Src/Model3/Real3D.cpp | 15 ++++++++-------
 Src/Model3/Real3D.h   | 18 +++++++-----------
 3 files changed, 16 insertions(+), 24 deletions(-)

diff --git a/Src/Model3/Model3.cpp b/Src/Model3/Model3.cpp
index 7b38e55..20acae9 100644
--- a/Src/Model3/Model3.cpp
+++ b/Src/Model3/Model3.cpp
@@ -2949,12 +2949,7 @@ bool CModel3::LoadGame(const Game &game, const ROMSet &rom_set)
 
   // Initialize Real3D
   int stepping = ((game.stepping[0] - '0') << 4) | (game.stepping[2] - '0');
-  uint32_t real3DPCIID = game.real3d_pci_id;
-  if (0 == real3DPCIID)
-  {
-    real3DPCIID = stepping >= 0x20 ? CReal3D::PCIID::Step2x : CReal3D::PCIID::Step1x;
-  }
-  GPU.SetStepping(stepping, real3DPCIID);
+  GPU.SetStepping(stepping);
 
   // MPEG board (if present)
   if (rom_set.get_rom("mpeg_program").size)
diff --git a/Src/Model3/Real3D.cpp b/Src/Model3/Real3D.cpp
index a15ac6b..622dcda 100644
--- a/Src/Model3/Real3D.cpp
+++ b/Src/Model3/Real3D.cpp
@@ -27,10 +27,10 @@
  *
  * PCI IDs
  * -------
- * It appears that Step 2.0 returns a different PCI ID depending on whether
- * the PCI configuration space or DMA register are accessed. For example,
- * Virtual On 2 expects 0x178611DB from the PCI configuration header but
- * 0x16C311DB from the DMA device.
+ * It appears that accessing the PCI configuration space returns the PCI ID
+ * of Mercury (0x16C311DB) on Step 1.x and the DMA device (0x178611DB) on
+ * Step 2.x, while accessing the Step 2.x DMA device register returns the
+ * PCI ID of Mercury. Step 2.x games by AM3 expect this behavior.
  *
  * To-Do List
  * ----------
@@ -628,7 +628,8 @@ void CReal3D::WriteDMARegister32(unsigned reg, uint32_t data)
   case 0x10:  // command register
     if ((data&0x20000000)) // DMA ID command
     {
-      dmaData = pciID;
+      // Games requesting PCI ID via the DMA device expect 0x16C311DB, even on step 2.x boards
+      dmaData = PCIID::Step1x;
       DebugLog("Real3D: DMA ID command issued (ATTENTION: make sure we're returning the correct value), PC=%08X, LR=%08X\n", ppc_get_pc(), ppc_get_lr());
     }
     else if ((data&0x80000000))
@@ -921,7 +922,7 @@ uint32_t CReal3D::GetASICIDCode(ASIC asic) const
   return it == m_asicID.end() ? 0 : it->second;
 }
 
-void CReal3D::SetStepping(int stepping, uint32_t pciIDValue)
+void CReal3D::SetStepping(int stepping)
 {
   step = stepping;
   if ((step!=0x10) && (step!=0x15) && (step!=0x20) && (step!=0x21))
@@ -931,7 +932,7 @@ void CReal3D::SetStepping(int stepping, uint32_t pciIDValue)
   }
 
   // Set PCI ID
-  pciID = pciIDValue;
+  pciID = stepping >= 0x20 ? PCIID::Step2x : PCIID::Step1x;
 
   // Pass to renderer
   if (Render3D != NULL)
diff --git a/Src/Model3/Real3D.h b/Src/Model3/Real3D.h
index 3013924..7eddb66 100644
--- a/Src/Model3/Real3D.h
+++ b/Src/Model3/Real3D.h
@@ -68,10 +68,11 @@ public:
   /*
    * PCI IDs
    *
-   * The CReal3D object must be configured with the desired ID. Some Step 2.x
-   * appear to defy this and expect the 1.x ID. The symptom of this is that 
-   * VBL is enabled briefly then disabled. This should be investigated further.
-   * Perhaps a different ASIC's PCI ID is being read in these situations?
+   * The CReal3D object must be configured with the PCI ID of the ASIC directly
+   * connected to the PCI slot; 0x16c311db for Step 1.x, 0x178611db for Step
+   * 2.x. Requesting PCI ID via the DMA device on Step 2.x appears to return
+   * the PCI ID of Mercury which is the ASIC connected to the PCI slot on Step
+   * 1.x, hence why some Step 2.x games appear to expect the 1.x ID.
    *
    * The vendor ID code 0x11db is Sega's.
    */
@@ -374,7 +375,7 @@ public:
   uint32_t GetASICIDCode(ASIC asic) const;
 
   /*
-   * SetStepping(stepping, pciIDValue):
+   * SetStepping(stepping):
    *
    * Sets the Model 3 hardware stepping, which also determines the Real3D
    * functionality. The default is Step 1.0. This should be called prior to 
@@ -383,13 +384,8 @@ public:
    * Parameters:
    *    stepping    0x10 for Step 1.0, 0x15 for Step 1.5, 0x20 for Step 2.0, or
    *                0x21 for Step 2.1. Anything else defaults to 1.0.
-   *    pciIDValue  The PCI ID code to return. This should be one of the PCIID
-   *                enum values otherwise games may fail to boot. Although the
-   *                PCI ID depends on stepping, there are a few games that
-   *                have to be explicitly configured with an older ID code,
-   *                which is why this parameter is exposed.
    */
-  void SetStepping(int stepping, uint32_t pciIDValue);
+  void SetStepping(int stepping);
 
   
   /*