1 files changed, 53 insertions, 35 deletions

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index d97d07f47a55..a79c6f9e7e2c 100644
--- a/arch/parisc/kernel/time.c
+++ b/arch/parisc/kernel/time.c
@@ -56,9 +56,9 @@ static unsigned long clocktick __read_mostly;	/* timer cycles per tick */
  */
 irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
 {
-	unsigned long now;
+	unsigned long now, now2;
 	unsigned long next_tick;
-	unsigned long cycles_elapsed, ticks_elapsed;
+	unsigned long cycles_elapsed, ticks_elapsed = 1;
 	unsigned long cycles_remainder;
 	unsigned int cpu = smp_processor_id();
 	struct cpuinfo_parisc *cpuinfo = &per_cpu(cpu_data, cpu);
@@ -71,44 +71,24 @@ irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
 	/* Initialize next_tick to the expected tick time. */
 	next_tick = cpuinfo->it_value;
 
-	/* Get current interval timer.
-	 * CR16 reads as 64 bits in CPU wide mode.
-	 * CR16 reads as 32 bits in CPU narrow mode.
-	 */
+	/* Get current cycle counter (Control Register 16). */
 	now = mfctl(16);
 
 	cycles_elapsed = now - next_tick;
 
-	if ((cycles_elapsed >> 5) < cpt) {
+	if ((cycles_elapsed >> 6) < cpt) {
 		/* use "cheap" math (add/subtract) instead
 		 * of the more expensive div/mul method
 		 */
 		cycles_remainder = cycles_elapsed;
-		ticks_elapsed = 1;
 		while (cycles_remainder > cpt) {
 			cycles_remainder -= cpt;
 			ticks_elapsed++;
 		}
 	} else {
+		/* TODO: Reduce this to one fdiv op */
 		cycles_remainder = cycles_elapsed % cpt;
-		ticks_elapsed = 1 + cycles_elapsed / cpt;
-	}
-
-	/* Can we differentiate between "early CR16" (aka Scenario 1) and
-	 * "long delay" (aka Scenario 3)? I don't think so.
-	 *
-	 * We expected timer_interrupt to 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.
-	 */
-	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 );
+		ticks_elapsed += cycles_elapsed / cpt;
 	}
 
 	/* convert from "division remainder" to "remainder of clock tick" */
@@ -122,18 +102,56 @@ irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
 
 	cpuinfo->it_value = next_tick;
 
-	/* Skip one clocktick on purpose if we are likely to miss next_tick.
-	 * We want to avoid the new next_tick being less than CR16.
-	 * If that happened, itimer wouldn't fire until CR16 wrapped.
-	 * We'll catch the tick we missed on the tick after that.
+	/* Program the IT when to deliver the next interrupt.
+	 * Only bottom 32-bits of next_tick are writable in CR16!
 	 */
-	if (!(cycles_remainder >> 13))
-		next_tick += cpt;
-
-	/* Program the IT when to deliver the next interrupt. */
-	/* Only bottom 32-bits of next_tick are written to cr16.  */
 	mtctl(next_tick, 16);
 
+	/* Skip one clocktick on purpose if we missed next_tick.
+	 * 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.
+	 *
+	 * "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);
+
+#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.