1 files changed, 29 insertions, 83 deletions

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index 325f30d82b64..4215f5596c8b 100644
--- a/arch/parisc/kernel/time.c
+++ b/arch/parisc/kernel/time.c
@@ -59,10 +59,9 @@ static unsigned long clocktick __read_mostly;	/* timer cycles per tick */
  */
 irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
 {
-	unsigned long now, now2;
+	unsigned long now;
 	unsigned long next_tick;
-	unsigned long cycles_elapsed, ticks_elapsed = 1;
-	unsigned long cycles_remainder;
+	unsigned long ticks_elapsed = 0;
 	unsigned int cpu = smp_processor_id();
 	struct cpuinfo_parisc *cpuinfo = &per_cpu(cpu_data, cpu);
 
@@ -71,102 +70,49 @@ irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
 
 	profile_tick(CPU_PROFILING);
 
-	/* Initialize next_tick to the expected tick time. */
+	/* Initialize next_tick to the old expected tick time. */
 	next_tick = cpuinfo->it_value;
 
-	/* Get current cycle counter (Control Register 16). */
-	now = mfctl(16);
-
-	cycles_elapsed = now - next_tick;
-
-	if ((cycles_elapsed >> 6) < cpt) {
-		/* use "cheap" math (add/subtract) instead
-		 * of the more expensive div/mul method
-		 */
-		cycles_remainder = cycles_elapsed;
-		while (cycles_remainder > cpt) {
-			cycles_remainder -= cpt;
-			ticks_elapsed++;
-		}
-	} else {
-		/* TODO: Reduce this to one fdiv op */
-		cycles_remainder = cycles_elapsed % cpt;
-		ticks_elapsed += cycles_elapsed / cpt;
-	}
-
-	/* convert from "division remainder" to "remainder of clock tick" */
-	cycles_remainder = cpt - cycles_remainder;
-
-	/* Determine when (in CR16 cycles) next IT interrupt will fire.
-	 * We want IT to fire modulo clocktick even if we miss/skip some.
-	 * But those interrupts don't in fact get delivered that regularly.
-	 */
-	next_tick = now + cycles_remainder;
+	/* Calculate how many ticks have elapsed. */
+	do {
+		++ticks_elapsed;
+		next_tick += cpt;
+		now = mfctl(16);
+	} while (next_tick - now > cpt);
 
+	/* Store (in CR16 cycles) up to when we are accounting right now. */
 	cpuinfo->it_value = next_tick;
 
-	/* Program the IT when to deliver the next interrupt.
-	 * Only bottom 32-bits of next_tick are writable in CR16!
-	 */
-	mtctl(next_tick, 16);
+	/* Go do system house keeping. */
+	if (cpu == 0)
+		xtime_update(ticks_elapsed);
+
+	update_process_times(user_mode(get_irq_regs()));
 
-	/* Skip one clocktick on purpose if we missed next_tick.
+	/* Skip clockticks on purpose if we know we would miss those.
 	 * The new CR16 must be "later" than current CR16 otherwise
 	 * itimer would not fire until CR16 wrapped - e.g 4 seconds
 	 * later on a 1Ghz processor. We'll account for the missed
-	 * tick on the next timer interrupt.
+	 * ticks on the next timer interrupt.
+	 * We want IT to fire modulo clocktick even if we miss/skip some.
+	 * But those interrupts don't in fact get delivered that regularly.
 	 *
 	 * "next_tick - now" will always give the difference regardless
 	 * if one or the other wrapped. If "now" is "bigger" we'll end up
 	 * with a very large unsigned number.
 	 */
-	now2 = mfctl(16);
-	if (next_tick - now2 > cpt)
-		mtctl(next_tick+cpt, 16);
+	while (next_tick - mfctl(16) > cpt)
+		next_tick += cpt;
 
-#if 1
-/*
- * GGG: DEBUG code for how many cycles programming CR16 used.
- */
-	if (unlikely(now2 - now > 0x3000)) 	/* 12K cycles */
-		printk (KERN_CRIT "timer_interrupt(CPU %d): SLOW! 0x%lx cycles!"
-			" cyc %lX rem %lX "
-			" next/now %lX/%lX\n",
-			cpu, now2 - now, cycles_elapsed, cycles_remainder,
-			next_tick, now );
-#endif
-
-	/* Can we differentiate between "early CR16" (aka Scenario 1) and
-	 * "long delay" (aka Scenario 3)? I don't think so.
-	 *
-	 * Timer_interrupt will be delivered at least a few hundred cycles
-	 * after the IT fires. But it's arbitrary how much time passes
-	 * before we call it "late". I've picked one second.
-	 *
-	 * It's important NO printk's are between reading CR16 and
-	 * setting up the next value. May introduce huge variance.
-	 */
-	if (unlikely(ticks_elapsed > HZ)) {
-		/* Scenario 3: very long delay?  bad in any case */
-		printk (KERN_CRIT "timer_interrupt(CPU %d): delayed!"
-			" cycles %lX rem %lX "
-			" next/now %lX/%lX\n",
-			cpu,
-			cycles_elapsed, cycles_remainder,
-			next_tick, now );
-	}
-
-	/* Done mucking with unreliable delivery of interrupts.
-	 * Go do system house keeping.
+	/* Program the IT when to deliver the next interrupt.
+	 * Only bottom 32-bits of next_tick are writable in CR16!
+	 * Timer interrupt will be delivered at least a few hundred cycles
+	 * after the IT fires, so if we are too close (<= 500 cycles) to the
+	 * next cycle, simply skip it.
 	 */
-
-	if (!--cpuinfo->prof_counter) {
-		cpuinfo->prof_counter = cpuinfo->prof_multiplier;
-		update_process_times(user_mode(get_irq_regs()));
-	}
-
-	if (cpu == 0)
-		xtime_update(ticks_elapsed);
+	if (next_tick - mfctl(16) <= 500)
+		next_tick += cpt;
+	mtctl(next_tick, 16);
 
 	return IRQ_HANDLED;
 }