games/gb: various HDMA fixes

H-blank DMA should only transfer 16 bytes per h-blank, rather than
waiting for the first h-blank and then transferring the whole size.

HDMAC should read 0xff when the transfer is finished, and 0 in the
high bit when the transfer is ongoing. Also, if 0 is written in the
high bit, the current transfer should be aborted.

Introduce two flags, DMAREADY and DMAHBLANK rather than special
constants 1 and -1. If dma is non-zero, there is an ongoing DMA. If
DMAREADY is set, the next chunk is ready to transfer.

Reference: https://gbdev.io/pandocs/#ff55-hdma5-cgb-mode-only-new-dma-length-mode-start

Tested with pokemon crystal.

What was happening is that when the game was loading N background tiles
into vram (each 16 bytes, so one per h-blank), it did something like
this:
- start an hdma transfer for N+1 tiles
- after the Nth tile is transferred, it would read HDMA5, clear the
high bit, then write it back to abort the transfer.

games/gb would instead transfer all N+1 tiles at once, overwriting one
extra tile with whatever was 1 past the end of the source array, and
then would interpret the cancel request as the start of a new transfer
of 16 bytes, which would copy an additional tile past the end. The end
result is that every transfer would end up copying N+2 tiles instead
of just N, overwriting certain tiles with whatever was after the end
of the source data.

sys/src/games/gb/dat.h

@@ -8,7 +8,7 @@ extern MBC3Timer timer;
 extern uchar vram[16384];
 extern int nrom, nback, nbackbank;
 extern u32int pal[64];
-extern s8int dma;
+extern u8int dma;
 extern u32int divclock;
 
 extern Event *elist;
@@ -114,6 +114,9 @@ enum {
 	FEATBAT = 2,
 	FEATTIM = 4,
 
+	DMAREADY = 1,
+	DMAHBLANK = 2,
+	
 	INIT = -1,
 	SAVE = -2,
 	RSTR = -3,

sys/src/games/gb/gb.c

@@ -308,7 +308,7 @@ threadmain(int argc, char **argv)
 			qlock(&pauselock);
 			qunlock(&pauselock);
 		}
-		if(dma > 0)
+		if(dma & DMAREADY)
 			t = dmastep();
 		else
 			t = step();

sys/src/games/gb/mem.c

@@ -15,7 +15,7 @@ int nrom, nback, nbackbank;
 u32int divclock;
 int prish;
 MBC3Timer timer, timerl;
-s8int dma;
+u8int dma;
 u32int white;
 u32int moncols[4];
 
@@ -175,7 +175,14 @@ regwrite(u8int a, u8int v)
 	case HDMAC:
 		if((mode & COL) == 0)
 			goto ff;
-		dma = (v & 0x80) != 0 ? -1 : 1;
+		if(v & 0x80){
+			v &= 0x7f;
+			dma = DMAHBLANK;
+		}else if(dma){
+			v |= 0x80;
+			dma = 0;
+		}else
+			dma = DMAREADY;
 		break;
 	case NR10: v |= 0x80; goto snd;
 	case NR14: case NR24: v |= 0x38; goto snd;
@@ -534,6 +541,7 @@ meminit(void)
 	reg[VBK] = 0xfe;
 	reg[SVBK] = 0xf8;
 	reg[IF] = 0xe0;
+	reg[HDMAC] = 0xff;
 }
 
 void
@@ -583,12 +591,9 @@ dmastep(void)
 	reg[HDMADL] += 16;
 	if((reg[HDMADL] & 0xf0) == 0)
 		reg[HDMADH]++;
-	if((reg[HDMAC] & 0x7f) == 0)
+	if(--reg[HDMAC] == 0xff)
 		dma = 0;
-	else{
-		reg[HDMAC]--;
-		if((reg[HDMAC] & 0x80) != 0)
-			dma = 1;
-	}
+	else if(dma & DMAHBLANK)
+		dma &= ~DMAREADY;
 	return 64;
 }

sys/src/games/gb/ppu.c

@@ -338,8 +338,8 @@ hblanktick(void *)
 			reg[IF] |= IRQLCDS;
 		t = hblclock + 456 * 2 - clock;
 		addevent(&evhblank, t < 0 ? 456 * 2 : t);
-		if(dma < 0)
-			dma = 1;
+		if(dma & DMAHBLANK)
+			dma |= DMAREADY;
 		break;
 	}
 }