reads from /dev/acpievent return acpi events.  currently the
only available event is power.  the purpose of this is so
power button -> acpi event -> fshalt -> off

Reference: /n/atom/patch/applied2013/acpieventq
Date: Tue Oct 22 10:22:24 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/devacpi.c	Tue Oct 22 10:21:30 2013
+++ /sys/src/nix/k10/devacpi.c	Tue Oct 22 10:21:31 2013
@@ -19,6 +19,9 @@
 	Lock;
 	int	init;
 	void	(*powerbutton)(void);
+
+	u32int	eventopen;
+	Queue	*event;
 };
 
 /* internal */
@@ -58,7 +61,7 @@
 static Dirtab acpidir[]={
 	".",		{Qdir, 0, QTDIR},	0,	DMDIR|0555,
 	"acpictl",	{Qctl},			0,	0666,
-	"acpievent",	{Qevent},	0,	0444,
+	"acpievent",	{Qevent, 0, QTEXCL},	0,	DMEXCL|0440,
 };
 
 static	Gpe*	gpes;	/* General purpose events */
@@ -286,6 +289,7 @@
 {
 	int i;
 	uint sts, en;
+	Queue *q;
 
 	for(i = 0; i < ngpe; i++)
 		if(getgpests(i)){
@@ -305,8 +309,9 @@
 	}
 	setpm1sts(sts);
 	if(sts&Epowerbtn){
-		aconf.powerbutton();
-	//	qiwrite(event.q, "power");
+		if((q = aconf.event) == nil
+		|| qiwrite(q, "powerbutton\n", 12) == -1)
+			aconf.powerbutton();
 	}
 }
 
@@ -418,12 +423,38 @@
 static Chan*
 acpiopen(Chan *c, int omode)
 {
-	return devopen(c, omode, acpidir, nelem(acpidir), acpigen);
+	c = devopen(c, omode, acpidir, nelem(acpidir), acpigen);
+	switch((uint)c->qid.path){
+	case Qevent:
+		if(tas32(&aconf.eventopen) != 0){
+			c->flag &= ~COPEN;
+			error(Einuse);
+		}
+		if(aconf.event == nil){
+			aconf.event = qopen(8*1024, Qmsg, 0, 0);
+			if(aconf.event == nil){
+				c->flag &= ~COPEN;
+				error(Enomem);
+			}
+			qnoblock(aconf.event, 1);
+		}else
+			qreopen(aconf.event);
+		break;
+	}
+	return c;
 }
 
 static void
-acpiclose(Chan *)
+acpiclose(Chan *c)
 {
+	switch((uint)c->qid.path){
+	case Qevent:
+		if(c->flag & COPEN){
+			aconf.eventopen = 0;
+			qhangup(aconf.event, nil);
+		}
+		break;
+	}
 }
 
 static long
@@ -453,8 +484,7 @@
 		return readstr(off, a, n, buf);
 		
 	case Qevent:
-		s = "";
-		return readstr(off, a, n, s);
+		return qread(aconf.event, a, n);
 	}
 	error(Eperm);
 	return -1;