[PA-RISC] Fix time.c for new do_timer() calling convention

do_timer now wants to know how many ticks have elapsed.  Now that we
have to calculate that, we can eliminate some of the clever code that
avoided having to calculate that.  Also add some more documentation.
I'd like to thank Grant Grundler for helping me with this.

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>

arch/parisc/kernel/time.c

@@ -38,11 +38,28 @@ static unsigned long clocktick __read_mostly;	/* timer cycles per tick */
 extern void smp_do_timer(struct pt_regs *regs);
 #endif
 
+/*
+ * We keep time on PA-RISC Linux by using the Interval Timer which is
+ * a pair of registers; one is read-only and one is write-only; both
+ * accessed through CR16.  The read-only register is 32 or 64 bits wide,
+ * and increments by 1 every CPU clock tick.  The architecture only
+ * guarantees us a rate between 0.5 and 2, but all implementations use a
+ * rate of 1.  The write-only register is 32-bits wide.  When the lowest
+ * 32 bits of the read-only register compare equal to the write-only
+ * register, it raises a maskable external interrupt.  Each processor has
+ * an Interval Timer of its own and they are not synchronised.  
+ *
+ * We want to generate an interrupt every 1/HZ seconds.  So we program
+ * CR16 to interrupt every @clocktick cycles.  The it_value in cpu_data
+ * is programmed with the intended time of the next tick.  We can be
+ * held off for an arbitrarily long period of time by interrupts being
+ * disabled, so we may miss one or more ticks.
+ */
 irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 {
 	unsigned long now;
 	unsigned long next_tick;
-	unsigned long cycles_elapsed;
+	unsigned long cycles_elapsed, ticks_elapsed;
 	unsigned long cycles_remainder;
 	unsigned int cpu = smp_processor_id();
 
@@ -67,11 +84,14 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 		 * of the more expensive div/mul method
 		 */
 		cycles_remainder = cycles_elapsed;
+		ticks_elapsed = 1;
 		while (cycles_remainder > cpt) {
 			cycles_remainder -= cpt;
+			ticks_elapsed++;
 		}
 	} else {
 		cycles_remainder = cycles_elapsed % cpt;
+		ticks_elapsed = 1 + cycles_elapsed / cpt;
 	}
 
 	/* Can we differentiate between "early CR16" (aka Scenario 1) and
@@ -81,18 +101,7 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 	 * cycles after the IT fires. But it's arbitrary how much time passes
 	 * before we call it "late". I've picked one second.
 	 */
-/* aproximate HZ with shifts. Intended math is "(elapsed/clocktick) > HZ" */
-#if HZ == 1000
-	if (cycles_elapsed > (cpt << 10) )
-#elif HZ == 250
-	if (cycles_elapsed > (cpt << 8) )
-#elif HZ == 100
-	if (cycles_elapsed > (cpt << 7) )
-#else
-#warn WTF is HZ set to anyway?
-	if (cycles_elapsed > (HZ * cpt) )
-#endif
-	{
+	if (ticks_elapsed > HZ) {
 		/* Scenario 3: very long delay?  bad in any case */
 		printk (KERN_CRIT "timer_interrupt(CPU %d): delayed!"
 			" cycles %lX rem %lX "
@@ -136,7 +145,7 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 #endif
 	if (cpu == 0) {
 		write_seqlock(&xtime_lock);
-		do_timer(regs);
+		do_timer(ticks_elapsed);
 		write_sequnlock(&xtime_lock);
 	}