unscramble bootfile of CD-ROM XA discs during HLE bootstrap

CMakeLists.txt

@@ -207,6 +207,7 @@ set(RELIB_SOURCES
   src/guest/arm7/arm7.c
   src/guest/bios/bios.c
   src/guest/bios/flash.c
+  src/guest/bios/scramble.c
   src/guest/bios/syscalls.c
   src/guest/gdrom/cdi.c
   src/guest/gdrom/chd.c

src/guest/bios/bios.c

@@ -4,6 +4,7 @@
 #include "guest/aica/aica.h"
 #include "guest/bios/bios.h"
 #include "guest/bios/flash.h"
+#include "guest/bios/scramble.h"
 #include "guest/bios/syscalls.h"
 #include "guest/dreamcast.h"
 #include "guest/gdrom/gdrom.h"
@@ -285,6 +286,18 @@ void bios_boot(struct bios *bios) {
       return;
     }
 
+    /* CD-ROM XA discs have their binary scrambled. the bios descrambles this
+       later on in the boot, but it should be fine to descramble now */
+    struct gd_status_info stat;
+    gdrom_get_status(gd, &stat);
+
+    if (stat.format == GD_DISC_CDROM_XA) {
+      uint8_t *tmp2 = malloc(len);
+      descramble(tmp2, tmp, len);
+      free(tmp);
+      tmp = tmp2;
+    }
+
     sh4_memcpy_to_guest(dc->mem, BOOT2_ADDR, tmp, read);
     free(tmp);
   }
@@ -319,7 +332,7 @@ void bios_boot(struct bios *bios) {
     sh4_write32(dc->mem, VECTOR_SYSTEM, SYSCALL_SYSTEM);
   }
 
-  /* start executing at license screen code inside of ip.bin */
+  /* start executing at license screen code inside of IP.BIN */
   ctx->pc = 0xac008300;
 }
 

src/guest/bios/scramble.c

@@ -0,0 +1,72 @@
+#include "guest/bios/scramble.h"
+#include "core/core.h"
+
+#define MIN_CHUNK_SIZE 32
+#define MAX_CHUNK_SIZE (2048 * 1024)
+
+#define MAX_SLICES (MAX_CHUNK_SIZE / MIN_CHUNK_SIZE)
+
+static int scramble_init(int n) {
+  return n & 0xffff;
+}
+
+static int scramble_next(int *seed, int i) {
+  unsigned key;
+  *seed = (*seed * 2109 + 9273) & 0x7fff;
+  key = (*seed + 0xc000) & 0xffff;
+  return ((unsigned)i * (unsigned)key) >> 16;
+}
+
+static void descramble_chunk(int *seed, uint8_t *dst, const uint8_t *src,
+                             int size) {
+  CHECK((size % MIN_CHUNK_SIZE) == 0 && size <= MAX_CHUNK_SIZE);
+
+  /* descramble each chunk in MIN_CHUNK_SIZE slices */
+  size /= MIN_CHUNK_SIZE;
+
+  /* lookup table maps scrambled slice index to descrambled index */
+  int table[MAX_SLICES];
+
+  for (int i = 0; i < size; i++) {
+    table[i] = i;
+  }
+
+  for (int i = size - 1; i >= 0; i--) {
+    int x = scramble_next(seed, i);
+
+    /* swap table index */
+    int tmp = table[i];
+    table[i] = table[x];
+    table[x] = tmp;
+
+    /* write slice out to descrambled index */
+    memcpy(dst + MIN_CHUNK_SIZE * table[i], src, MIN_CHUNK_SIZE);
+    src += MIN_CHUNK_SIZE;
+  }
+}
+
+void descramble(uint8_t *dst, const uint8_t *src, int size) {
+  int seed = scramble_init(size);
+
+  /* descramble the data starting with the largest chunk size (2mb) */
+  int chunk_size = MAX_CHUNK_SIZE;
+
+  while (chunk_size >= MIN_CHUNK_SIZE) {
+    /* continue descrambling with the current chunk size until the remaining
+       data is too small */
+    while (size >= chunk_size) {
+      descramble_chunk(&seed, dst, src, chunk_size);
+      size -= chunk_size;
+      dst += chunk_size;
+      src += chunk_size;
+    }
+
+    /* attempt to use the the next smallest chunk size */
+    chunk_size >>= 1;
+  }
+
+  /* any remaining data isn't scrambled, just copy it */
+  if (size) {
+    memcpy(dst, src, size);
+  }
+}

src/guest/bios/scramble.h

@@ -0,0 +1,8 @@
+#ifndef SCRAMBLE_H
+#define SCRAMBLE_H
+
+#include <stdint.h>
+
+void descramble(uint8_t *dst, const uint8_t *src, int size);
+
+#endif

src/guest/bios/syscalls.c

@@ -1,6 +1,7 @@
 #include "guest/bios/syscalls.h"
 #include "guest/bios/bios.h"
 #include "guest/bios/flash.h"
+#include "guest/bios/scramble.h"
 #include "guest/gdrom/gdrom.h"
 #include "guest/holly/holly.h"
 #include "guest/memory.h"
@@ -29,6 +30,7 @@ enum {
 
 void bios_system_vector(struct bios *bios) {
   struct dreamcast *dc = bios->dc;
+  struct gdrom *gd = dc->gdrom;
   struct sh4_context *ctx = &dc->sh4->ctx;
 
   uint32_t fn = ctx->r[4];
@@ -138,6 +140,7 @@ static uint32_t bios_gdrom_send_cmd(struct bios *bios, uint32_t cmd_code,
 static void bios_gdrom_mainloop(struct bios *bios) {
   struct dreamcast *dc = bios->dc;
   struct gdrom *gd = dc->gdrom;
+  struct holly *hl = dc->holly;
 
   if (bios->status != GDC_STATUS_ACTIVE) {
     return;
@@ -265,7 +268,8 @@ static void bios_gdrom_mainloop(struct bios *bios) {
     } break;
 
     case GDC_INIT: {
-      /* this seems to always immediately follow GDROM_INIT */
+      /* sanity check in case dma transfers are made async in the future */
+      CHECK_EQ(*hl->SB_GDST, 0);
     } break;
 
     case GDC_SEEK: {