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sgi: cleanup timer code

- no need to splhi() in timerset, always called with
  interrupts off.

- make timerset always update the period (next == 0)

- remove period update in fastticks(), simplify
  delta calculation.

--- a/sys/src/9/sgi/clock.c

+++ b/sys/src/9/sgi/clock.c

@@ -83,7 +83,8 @@

 	m->maxperiod = Basetickfreq / HZ;

 	m->minperiod = Basetickfreq / (100*HZ);

-	wrcompare(rdcount()+m->maxperiod);

+	m->lastcount = rdcount();

+	wrcompare(m->lastcount+m->maxperiod);

 	intron(INTR7);

 }

@@ -128,12 +129,7 @@

 	/* avoid reentry on interrupt or trap, to prevent recursion */

 	x = splhi();

 	count = rdcount();

-	if(rdcompare() - count > m->maxperiod)

-		wrcompare(count+m->maxperiod);

-	if (count < m->lastcount)		/* wrapped around? */

-		delta = count + ((1ull<<32) - m->lastcount);

-	else

-		delta = count - m->lastcount;

+	delta = count - m->lastcount;

 	m->lastcount = count;

 	m->fastticks += delta;

 	splx(x);

@@ -150,18 +146,16 @@

 void

 timerset(Tval next)

 {

-	int x;

 	long period;

 	if(next == 0)

-		return;

-	x = splhi();			/* don't let us get scheduled */

-	period = next - fastticks(nil);

-	if(period > m->maxperiod - m->minperiod)

 		period = m->maxperiod;

-	else if(period < m->minperiod)

-		period = m->minperiod;

+	else {

+		period = next - fastticks(nil);

+		if(period > m->maxperiod)

+			period = m->maxperiod;

+		else if(period < m->minperiod)

+			period = m->minperiod;

+	}

 	wrcompare(rdcount()+period);

-	splx(x);

-

 }