add stats for ata busy interrupts and those associated with no drive (spurious interrupts ?).
also added a fix derrived from 9atom code (sdide.c) which sends an acknowledge to the interface
in the spurious case and reduced the inmber of interrupts/sec by an order of magnitude. 

here is my /dev/sdctl with the fix applied, sdE and sdF are a
mirror'ed pair on the same IRQ.

	sdE ata port 24F0 ctl 2808 irq 10 busy 563 nildrive 313424 ok 540356
	sdF ata port 24F8 ctl 280C irq 10 busy 283 nildrive 539719 ok 314341

-Steve

Notes:
Thu Sep 29 16:55:05 EDT 2011 geoff
    changed a little from the patch.


Reference: /n/sources/patch/applied/ata-high-int-rate
Date: Tue Sep 27 10:19:52 CES 2011
Signed-off-by: steve@quintile.net
Reviewed-by: geoff

--- /sys/src/9/pc/sdata.c	Tue Sep 27 10:13:08 2011
+++ /sys/src/9/pc/sdata.c	Tue Sep 27 10:12:59 2011
@@ -285,6 +285,7 @@
 	Drive*	drive[2];
 
 	Prd*	prdt;			/* physical region descriptor table */
+	void	(*irqack)(Ctlr*);
 
 	QLock;				/* current command */
 	Drive*	curdrive;
@@ -292,6 +293,10 @@
 	Rendez;
 	int	done;
 
+	ulong intnil;
+	ulong intbusy;
+	ulong intok;
+
 	Lock;				/* register access */
 } Ctlr;
 
@@ -329,6 +334,12 @@
 	int	status;
 	int	error;
 	int	flags;			/* internal flags */
+
+	ulong intcd;
+	ulong intio;
+	ulong intzero;
+	ulong intother;
+
 } Drive;
 
 enum {					/* internal flags */
@@ -937,8 +948,10 @@
 {
 	Ctlr *ctlr = sdev->ctlr;
 
-	return seprint(p, e, "%s ata port %X ctl %X irq %d\n", 
-		    	       sdev->name, ctlr->cmdport, ctlr->ctlport, ctlr->irq);
+	return seprint(p, e, "%s ata port %X ctl %X irq %d "
+				"busy %lud nildrive %lud ok %lud\n", 
+				sdev->name, ctlr->cmdport, ctlr->ctlport, ctlr->irq,
+				ctlr->intbusy, ctlr->intnil, ctlr->intok);
 }
 
 static SDev*
@@ -1209,10 +1222,12 @@
 	cmdport = ctlr->cmdport;
 	switch(inb(cmdport+Ir) & (/*Rel|*/Io|Cd)){
 	case Cd:
+		drive->intcd++;
 		outss(cmdport+Data, drive->pktcmd, drive->pkt/2);
 		break;
 
 	case 0:
+		drive->intzero++;
 		len = (inb(cmdport+Bytehi)<<8)|inb(cmdport+Bytelo);
 		if(drive->data+len > drive->limit){
 			atanop(drive, 0);
@@ -1223,6 +1238,7 @@
 		break;
 
 	case Io:
+		drive->intio++;
 		len = (inb(cmdport+Bytehi)<<8)|inb(cmdport+Bytelo);
 		if(drive->data+len > drive->limit){
 			atanop(drive, 0);
@@ -1233,11 +1249,16 @@
 		break;
 
 	case Io|Cd:
+		drive->intio++;
+		drive->intcd++;
 		if(drive->pktdma)
 			atadmainterrupt(drive, drive->dlen);
 		else
 			ctlr->done = 1;
 		break;
+	default:
+		drive->intother++;
+		break;
 	}
 }
 
@@ -1758,6 +1779,16 @@
 }
 
 static void
+ichirqack(Ctlr *ctlr)
+{
+	int bmiba;
+
+	if(bmiba = ctlr->bmiba)
+		outb(bmiba+Bmisx, inb(bmiba+Bmisx));
+}
+
+
+static void
 atainterrupt(Ureg*, void* arg)
 {
 	Ctlr *ctlr;
@@ -1771,17 +1802,23 @@
 		iunlock(ctlr);
 		if(DEBUG & DbgBsy)
 			print("IBsy+");
+		ctlr->intbusy++;
 		return;
 	}
 	cmdport = ctlr->cmdport;
 	status = inb(cmdport+Status);
 	if((drive = ctlr->curdrive) == nil){
+		if(ctlr->irqack != nil)
+			ctlr->irqack(ctlr);
 		iunlock(ctlr);
 		if((DEBUG & DbgINL) && ctlr->command != Cedd)
 			print("Inil%2.2uX+", ctlr->command);
+		ctlr->intnil++;
 		return;
 	}
 
+	ctlr->intok++;
+
 	if(status & Err)
 		drive->error = inb(cmdport+Error);
 	else switch(drive->command){
@@ -1837,6 +1874,8 @@
 		ctlr->done = 1;
 		break;
 	}
+	if(ctlr->irqack != nil)
+		ctlr->irqack(ctlr);
 	iunlock(ctlr);
 
 	if(drive->error){
@@ -1856,9 +1895,11 @@
 {
 	Ctlr *ctlr;
 	Pcidev *p;
+	void (*irqack)(Ctlr*);
 	SDev *legacy[2], *sdev, *head, *tail;
 	int channel, ispc87415, maxio, pi, r, span;
 
+	irqack = nil;
 	legacy[0] = legacy[1] = head = tail = nil;
 	if(sdev = ataprobe(0x1F0, 0x3F4, IrqATA0)){
 		head = tail = sdev;
@@ -2048,6 +2089,7 @@
 		case (0x2920<<16)|0x8086:	/* 82801(IB)/IR/IH/IO SATA IDE (ICH9) */
 		case (0x3a20<<16)|0x8086:	/* 82801JI (ICH10) */
 		case (0x3a26<<16)|0x8086:	/* 82801JI (ICH10) */
+			irqack = ichirqack;
 			break;
 		}
 
@@ -2080,6 +2122,7 @@
 			ctlr->pcidev = p;
 			ctlr->maxio = maxio;
 			ctlr->span = span;
+			ctlr->irqack = irqack;
 			if(!(pi & 0x80))
 				continue;
 			ctlr->bmiba = (p->mem[4].bar & ~0x01) + channel*8;
@@ -2200,6 +2243,8 @@
 		n += snprint(p+n, l-n, " lba48always %s",
 			(drive->flags&Lba48always) ? "on" : "off");
 	n += snprint(p+n, l-n, "\n");
+	n += snprint(p+n, l-n, "interrupts zero %lud io %lud ctrldata %lud other %lud\n",
+			drive->intzero, drive->intio, drive->intcd, drive->intother);
 	if(drive->sectors){
 		n += snprint(p+n, l-n, "geometry %lld %d",
 			drive->sectors, drive->secsize);