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devkbd: poll pc keyboard before blocking on kbd.q

the keyboard stops sending interrupts when its fifo gets full,
which can happen on boot when keys get mashed while interrupts
are still disabled. to work arround this, call the keyboard
interrupt handler when kbd.q is starved before blocking.

--- a/sys/src/9/pc/devkbd.c

+++ b/sys/src/9/pc/devkbd.c

@@ -277,6 +277,14 @@

 	iunlock(&i8042lock);

 }

+static void

+kbdpoll(void)

+{

+	if(nokbd || qlen(kbd.q) > 0)

+		return;

+	i8042intr(0, 0);

+}

+

 static Chan *

 kbdattach(char *spec)

 {

@@ -324,20 +332,22 @@

 static Block*

 kbdbread(Chan *c, long n, ulong off)

 {

-	if(c->qid.path == Qscancode)

+	if(c->qid.path == Qscancode){

+		kbdpoll();

 		return qbread(kbd.q, n);

-	else

-		return devbread(c, n, off);

+	}

+	return devbread(c, n, off);

 }

 static long

 kbdread(Chan *c, void *a, long n, vlong)

 {

-	if(c->qid.path == Qscancode)

+	if(c->qid.path == Qscancode){

+		kbdpoll();

 		return qread(kbd.q, a, n);

+	}

 	if(c->qid.path == Qdir)

 		return devdirread(c, a, n, kbdtab, nelem(kbdtab), devgen);

-

 	error(Egreg);

 	return 0;

 }