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audio/mixfs: implement Tstat to obtain buffer size, better underrun handling

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

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

@@ -8,6 +8,7 @@

 	NBUF = 8*1024,

 	NDELAY = 2048,

 	NCHAN = 2,

+	FREQ = 44100,

 };

 typedef struct Stream Stream;

@@ -83,7 +84,7 @@

 audioproc(void *)

 {

 	static uchar buf[NBUF*NCHAN*2];

-	int nsleep, fd, i, j, n, m, v;

+	int sweep, fd, i, j, n, m, v;

 	Stream *s;

 	uchar *p;

@@ -90,7 +91,7 @@

 	threadsetname("audioproc");

 	fd = -1;

-	nsleep = 0;

+	sweep = 0;

 	for(;;){

 		m = NBUF;

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

@@ -97,7 +98,7 @@

 			qlock(s);

 			if(s->run){

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

-				if(n <= 0 && nsleep > 4)

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

 					s->run = 0;

 				else if(n < m)

 					m = n;

@@ -109,21 +110,35 @@

 		m %= NBUF;

 		if(m == 0){

-			sleep(1<<nsleep);

-			if(nsleep < 7)

-				nsleep++;

-			else {

-				close(fd);

-				fd = -1;

+			int ms;

+

+			ms = 100;

+			if(fd >= 0){

+				if(sweep){

+					close(fd);

+					fd = -1;

+				} else {

+					/* attempt to sleep just shortly before buffer underrun */

+					ms = seek(fd, 0, 2);

+					if(ms > 0){

+						ms *= 800;

+						ms /= FREQ*NCHAN*2;

+					} else

+						ms = 4;

+				}

+				sweep = 1;

 			}

+			sleep(ms);

 			continue;

 		}

+		sweep = 0;

 		if(fd < 0)

-			if((fd = open("/dev/audio", OWRITE)) < 0)

-				continue;

+		if((fd = open("/dev/audio", OWRITE)) < 0){

+			fprint(2, "%s: open /dev/audio: %r\n", argv0);

+			sleep(1000);

+			continue;

+		}

-		nsleep = 0;

-

 		p = buf;

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

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

@@ -180,6 +195,28 @@

 }

 void

+fsstat(Req *r)

+{

+	Stream *s;

+

+	if(r->fid->file == nil){

+		respond(r, "bug");

+		return;

+	}

+	if(strcmp(r->fid->file->name, "audio") == 0 && (s = r->fid->aux) != nil){

+		qlock(s);

+		if(s->run){

+			r->d.length = (long)(s->wp - mixrp);

+			r->d.length *= NCHAN*2;

+		} else {

+			r->d.length = 0;

+		}

+		qunlock(s);

+	}

+	respond(r, nil);

+}

+

+void

 fsstart(Srv *)

 {

 	Stream *s;

@@ -200,6 +237,7 @@

 Srv fs = {

 	.open=		fsopen,

 	.write=		fswrite,

+	.stat=		fsstat,

 	.destroyfid=	fsclunk,

 	.start=		fsstart,

 	.end=		fsend,

-- 

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