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audio/mixfs: add support for a null sink

for computers with a faulty or non-existent audio(3) device,
you can now set dev to /dev/null to avoid writing into the
underlying /dev/audio.  it's meant to be used with
screamsend(1) to target an external receiver with a
functional audio system.

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

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

@@ -1,5 +1,6 @@

 #include <u.h>

 #include <libc.h>

+#include <tos.h>

 #include <fcall.h>

 #include <thread.h>

 #include <9p.h>

@@ -8,10 +9,11 @@

 enum {

 	NBUF = 8*1024,

-	NDELAY = 1764,	/* 25.0ms */

+	NDELAY = 1764,	/* 40.0ms */

 	NQUANTA = 64,	/* ~1.45ms */

 	NCHAN = 2,

 	ABUF = NBUF*NCHAN*2,

+	Nsec = 1000000000ULL,

 };

 #define MIN(a,b) ((a)<(b)?(a):(b))

@@ -50,6 +52,7 @@

 Pcmdesc	fmt;

 Lock	fmtlock;

 int	fmtchanged;

+int	isnulldev;

 int delay;

 int

@@ -129,7 +132,8 @@

 			/* hack: restrict to known audio device names */

 			if((strncmp(name, "/dev/audio", 10) != 0 || strchr(name+10, '/') != nil)

 			&& (strncmp(name, "#u/audio", 8) != 0 || strchr(name+8, '/') != nil)

-			&& (strncmp(name, "#A/audio", 8) != 0 || strchr(name+8, '/') != nil)){

+			&& (strncmp(name, "#A/audio", 8) != 0 || strchr(name+8, '/') != nil)

+			&& (strncmp(name, "/dev/null", 9) != 0 || strchr(name+9, '/') != nil)){

 				werrstr("name doesnt look like an audio device");

 				return -1;

 			}

@@ -169,6 +173,7 @@

 	qlock(&devlock);

 	free(devaudio);

 	devaudio = name;

+	isnulldev = 0;

 	audiofd = dup(afd, audiofd);

 	qunlock(&devlock);

@@ -188,6 +193,9 @@

 		if(volfd >= 0)

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

 		updfmt();

+	}else if(strncmp(p, "null", 4) == 0){

+		isnulldev = 1;

+		updfmt();

 	}

 	return 0;

 }

@@ -256,11 +264,40 @@

 		&& a->fmt == b->fmt;

 }

+uvlong

+nanosec(void)

+{

+	static uvlong fasthz, xstart;

+	uvlong x;

+

+	if(fasthz == ~0ULL)

+		return nsec() - xstart;

+

+	if(fasthz == 0){

+		if(_tos->cyclefreq){

+			fasthz = _tos->cyclefreq;

+			cycles(&xstart);

+		} else {

+			fasthz = ~0ULL;

+			xstart = nsec();

+		}

+		return 0;

+	}

+	cycles(&x);

+	x -= xstart;

+

+	uvlong q = x / fasthz;

+	uvlong r = x % fasthz;

+

+	return q*Nsec + r*Nsec/fasthz;

+}

+

 void

 audioproc(void *)

 {

 	static uchar buf[ABUF];

 	int sweep, i, j, n, m, v, rate;

+	uvlong t0, dt, ns;

 	ulong rp;

 	Stream *s;

 	uchar *p, *bufconv;

@@ -272,7 +309,9 @@

 	sweep = 0;

 	rate = fmt.rate;

 	delay = NDELAY;

+	ns = Nsec/pcmdescdef.rate;

+	t0 = nanosec();

 	for(;;){

 		m = NBUF;

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

@@ -323,8 +362,14 @@

 			continue;

 		}

-		if(m > NQUANTA)

-			m = NQUANTA;

+		if(isnulldev){

+			dt = nanosec() - t0;

+			m = (dt+ns-1)/ns;

+			m = MAX(NQUANTA, m);

+			m = MIN(m, NBUF);

+			t0 += dt;

+		}else

+			m = MIN(m, NQUANTA);

 		p = buf;

 		rp = mixrp;

@@ -344,6 +389,11 @@

 		mixrp = rp;

 		unlock(&rplock);

+		if(isnulldev){

+			sleep(5);

+			continue;

+		}

+

 		lock(&fmtlock);

 		if(fmtchanged){

 			fmtchanged = 0;

@@ -484,8 +534,10 @@

 				x[0] += volume[0];

 				x[1] += volume[1];

 			}

-			volume[0] = MIN(MAX(0, x[0]), 100);

-			volume[1] = MIN(MAX(0, x[1]), 100);

+			volume[0] = MAX(0, x[0]);

+			volume[0] = MIN(volume[0], 100);

+			volume[1] = MAX(0, x[1]);

+			volume[1] = MIN(volume[1], 100);

 			/* ≈60dB dynamic range; [0-100] → [0-65536] */

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

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

-- 

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