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audio/mixfs: fix loopback

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

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

@@ -15,8 +15,9 @@

 struct Stream

 {

 	int	used;

+	int	mode;

 	int	run;

-	int	rd;

+	ulong	rp;

 	ulong	wp;

 	QLock;

 	Rendez;

@@ -64,9 +65,9 @@

 		qlock(s);

 		if(s->used == 0 && s->run == 0){

 			s->used = 1;

+			s->mode = r->ifcall.mode;

 			qunlock(s);

-			s->rd = (r->ifcall.mode&OWRITE) == 0;

 			r->fid->aux = s;

 			respond(r, nil);

 			return;

@@ -105,13 +106,21 @@

 		for(s = streams; s < streams+nelem(streams); s++){

 			qlock(s);

 			if(s->run){

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

-				if(n <= 0 && (s->used == 0 || sweep))

-					s->run = 0;

-				else if(n < m)

-					m = n;

-				if(n < NDELAY)

-					rwakeup(s);

+				if(s->mode & OWRITE){

+					n = (long)(s->wp - mixrp);

+					if(n <= 0 && (s->used == 0 || sweep))

+						s->run = 0;

+					else if(n < m)

+						m = n;

+					if(n < NDELAY)

+						rwakeup(s);

+				} else {

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

+					if(n > NBUF && (s->used == 0 || sweep))

+						s->run = 0;

+					if(n >= NDELAY)

+						rwakeup(s);

+				}

 			}

 			qunlock(s);

 		}

@@ -189,26 +198,29 @@

 	qlock(s);

 	while(n > 0){

 		if(s->run == 0){

-			s->wp = mixrp;

+			s->rp = mixrp;

 			s->run = 1;

 		}

-		m = NBUF-1 - (long)(s->wp - mixrp);

-

-		if(m <= 0){

+		m = (long)(mixrp - s->rp);

+		if(m < NDELAY){

 			s->run = 1;

 			rsleep(s);

 			continue;

 		}

+		if(m > NBUF){

+			m = NBUF;

+			s->rp = mixrp - m;

+		}

 		if(m > n)

 			m = n;

 		for(i=0; i<m; i++){

 			for(j=0; j<NCHAN; j++){

-				v = lbbuf[s->wp % NBUF][j];

+				v = lbbuf[s->rp % NBUF][j];

 				*p++ = v & 0xFF;

 				*p++ = v >> 8;

 			}

-			s->wp++;

+			s->rp++;

 		}

 		n -= m;

@@ -299,7 +311,7 @@

 	Stream *s;

 	for(s=streams; s < streams+nelem(streams); s++){

-		s->used = s->run = s->rd = 0;

+		s->used = s->run = 0;

 		s->Rendez.l = &s->QLock;

 	}

 	proccreate(audioproc, nil, 16*1024);