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games/snes: flush screen in parallel to audio (fixes buffer underruns on x200s)

the x200s is too slow on a single core to keep up without
audio buffer underruns, so the idea is to flush screen
in parallel to witing audio samples in a separate process.

with the proc, we also can keep updating the screen on resize
when paused.

--- a/sys/src/games/snes/snes.c

+++ b/sys/src/games/snes/snes.c

@@ -13,6 +13,7 @@

 int ppuclock, spcclock, dspclock, stimerclock, saveclock, msgclock, paused, perfclock, cpupause;

 Mousectl *mc;

+Channel *flushc;

 QLock pauselock;

 u32int keys;

 int savefd, scale, profile, mouse, loadreq, savereq;

@@ -185,6 +186,68 @@

 }

 void

+screenproc(void *)

+{

+	extern uchar pic[256*240*2*3];

+	Mouse m;

+	Point p;

+

+	enum { AMOUSE, ARESIZE, AFLUSH, AEND };

+	Alt a[AEND+1] = {

+		{ mc->c,	&m,	CHANRCV },

+		{ mc->resizec,	nil,	CHANRCV },

+		{ flushc,	nil,	CHANRCV },

+		{ nil,		nil,	CHANEND }

+	};

+

+	for(;;){

+		switch(alt(a)){

+		case AMOUSE:

+			if(mouse && ptinrect(m.xy, picr)){

+				p = subpt(m.xy, picr.min);

+				p.x /= scale;

+				p.y /= scale;

+				keys = keys & 0xff3f0000 | p.x | p.y << 8;

+				if((m.buttons & 1) != 0)

+					keys |= 1<<22;

+				if((m.buttons & 4) != 0)

+					keys |= 1<<23;

+				if((m.buttons & 2) != 0)

+					lastkeys = keys;

+			}

+			break;

+		case ARESIZE:

+			if(getwindow(display, Refnone) < 0)

+				sysfatal("resize failed: %r");

+			screeninit();

+			/* wet floor */

+		case AFLUSH:

+			if(scale == 1){

+				loadimage(tmp, tmp->r, pic, 256*239*2);

+				draw(screen, picr, tmp, nil, ZP);

+			} else {

+				Rectangle r;

+				uchar *s;

+				int w;

+

+				s = pic;

+				r = picr;

+				w = 256*2*scale;

+				while(r.min.y < picr.max.y){

+					loadimage(tmp, tmp->r, s, w);

+					s += w;

+					r.max.y = r.min.y+scale;

+					draw(screen, r, tmp, nil, ZP);

+					r.min.y = r.max.y;

+				}

+			}

+			flushimage(display, 1);

+			break;

+		}

+	}

+}

+

+void

 timing(void)

 {

 	static vlong old;

@@ -242,14 +305,16 @@

 		threadexitsall("usage");

 	}

 	loadrom(argv[0]);

-	if(initdraw(nil, nil, nil) < 0)

+	if(initdraw(nil, nil, argv0) < 0)

 		sysfatal("initdraw: %r");

+	flushc = chancreate(sizeof(ulong), 1);

 	mc = initmouse(nil, screen);

 	if(mc == nil)

 		sysfatal("initmouse: %r");

 	screeninit();

-	loadbat(argv[0]);

 	proccreate(keyproc, 0, 8192);

+	proccreate(screenproc, 0, 8192);

+	loadbat(argv[0]);

 	cpureset();

 	memreset();

 	spcreset();

@@ -314,48 +379,7 @@

 void

 flush(void)

 {

-	extern uchar pic[256*240*2*3];

-	Mouse m;

-	Point p;

-

-	if(nbrecvul(mc->resizec) > 0){

-		if(getwindow(display, Refnone) < 0)

-			sysfatal("resize failed: %r");

-		screeninit();

-	}

-	while(nbrecv(mc->c, &m) > 0)

-		if(mouse && ptinrect(m.xy, picr)){

-			p = subpt(m.xy, picr.min);

-			p.x /= scale;

-			p.y /= scale;

-			keys = keys & 0xff3f0000 | p.x | p.y << 8;

-			if((m.buttons & 1) != 0)

-				keys |= 1<<22;

-			if((m.buttons & 4) != 0)

-				keys |= 1<<23;

-			if((m.buttons & 2) != 0)

-				lastkeys = keys;

-		}

-	if(scale == 1){

-		loadimage(tmp, tmp->r, pic, 256*239*2);

-		draw(screen, picr, tmp, nil, ZP);

-	} else {

-		Rectangle r;

-		uchar *s;

-		int w;

-

-		s = pic;

-		r = picr;

-		w = 256*2*scale;

-		while(r.min.y < picr.max.y){

-			loadimage(tmp, tmp->r, s, w);

-			s += w;

-			r.max.y = r.min.y+scale;

-			draw(screen, r, tmp, nil, ZP);

-			r.min.y = r.max.y;

-		}

-	}

-	flushimage(display, 1);

+	sendul(flushc, 1);	/* flush screen */

 	audioout();

 }