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audio/mixfs: set and forward a more sensible audio delay amount

remove the hardcoded constant which does not honor what the underlying
driver sets or uses.  instead, set a default value ourselves, the same
as what the drivers already use, and update it on change.  this fixes
issues with the emulators where the delay being too small caused a
high amount of jitter in the time it took to write the audio buffer,
severely impacting performance.

--- a/sys/src/cmd/audio/mixfs/mixfs.c

+++ b/sys/src/cmd/audio/mixfs/mixfs.c

@@ -8,7 +8,7 @@

 enum {

 	NBUF = 8*1024,

-	NDELAY = 512,	/* ~11.6ms */

+	NDELAY = 1764,	/* 25.0ms */

 	NQUANTA = 64,	/* ~1.45ms */

 	NCHAN = 2,

 	ABUF = NBUF*NCHAN*2,

@@ -50,6 +50,7 @@

 Pcmdesc	fmt;

 Lock	fmtlock;

 int	fmtchanged;

+int delay;

 int

 s16(uchar *p)

@@ -184,6 +185,8 @@

 		name = smprint("%.*svolume%s", (int)(p - name), name, p+5);

 		volfd = open(name, ORDWR);

 		free(name);

+		if(volfd >= 0)

+			fprint(volfd, "delay %d\n", delay);

 		updfmt();

 	}

 	return 0;

@@ -268,6 +271,7 @@

 	bufconv = nil;

 	sweep = 0;

 	rate = fmt.rate;

+	delay = NDELAY;

 	for(;;){

 		m = NBUF;

@@ -280,7 +284,7 @@

 						s->run = 0;

 					else if(n < m)

 						m = n;

-					if(n < NDELAY)

+					if(n < delay)

 						rwakeup(s);

 				} else {

 					n = (long)(mixrp - s->rp);

@@ -486,6 +490,14 @@

 			vol64k[0] = 65.536 * (exp(volume[0] * 0.0690876) - 1.0);

 			vol64k[1] = 65.536 * (exp(volume[1] * 0.0690876) - 1.0);

 		}else if(volfd >= 0){

+			if(nf > 1 && strcmp(f[0], "delay") == 0){

+				n = atoi(f[1]);

+				if(n < NQUANTA || n > NBUF){

+					responderror(r);

+					return;

+				}

+				delay = n;

+			}

 			if(write(volfd, r->ifcall.data, r->ifcall.count) < 0){

 				responderror(r);

 				return;

@@ -536,7 +548,7 @@

 		n -= m;

 	}

-	if((long)(s->wp - mixrp) >= NDELAY && !s->flush){

+	if((long)(s->wp - mixrp) >= delay && !s->flush){

 		s->run = 1;

 		rsleep(s);

 	}

-- 

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