[PARISC] Further updates to timer_interrupt()

This version (relative to the current tree):
o eliminates "while (ticks_elapsed)" loop. It's not needed.
o drop "ticks_elapsed" completely from timer_interrupt().
o Estimates elapsed cycles (based on HZ) to see which kind of
  math we want to use to calculate "cycles_remainder".
o Fixes a bug where we would loose a tick if we decided
  we wanted to skip one interrupt.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

1 files changed, 65 insertions, 62 deletions

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index c43e847a4b8f..9d642d820fe9 100644
--- a/arch/parisc/kernel/time.c
+++ b/arch/parisc/kernel/time.c
@@ -43,12 +43,11 @@ 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_remainder;
-        unsigned long ticks_elapsed = 1;        /* at least one elapsed */
-        int cpu = smp_processor_id();
+        unsigned long cycles_remainder;
+        unsigned int cpu = smp_processor_id();
 
         /* gcc can optimize for "read-only" case with a local clocktick */
-        unsigned long local_ct = clocktick;
+        unsigned long cpt = clocktick;
 
         profile_tick(CPU_PROFILING, regs);
 
@@ -63,28 +62,16 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 
         cycles_elapsed = now - next_tick;
 
-        /* Determine how much time elapsed.  */
-        if (now < next_tick) {
-                /* Scenario 2: CR16 wrapped after clock tick.
-                 * 1's complement will give us the "elapse cycles".
-                 *
-                 * This "cr16 wrapped" cruft is primarily for 32-bit kernels.
-                 * So think "unsigned long is u32" when reading the code.
-                 * And yes, of course 64-bit will someday wrap, but only
-                 * every 198841 days on a 1GHz machine.
+        if ((cycles_elapsed >> 5) < cpt) {
+                /* use "cheap" math (add/subtract) instead
+                 * of the more expensive div/mul method
                  */
-                cycles_elapsed = ~cycles_elapsed;   /* off by one cycle - don't care */
-        }
-
-        if (likely(cycles_elapsed < local_ct)) {
-                /* ticks_elapsed = 1 -- We already assumed one tick elapsed. */
                 cycles_remainder = cycles_elapsed;
+                while (cycles_remainder > cpt) {
+                        cycles_remainder -= cpt;
+                }
         } else {
-                /* more than one tick elapsed. Do "expensive" math. */
-                ticks_elapsed += cycles_elapsed / local_ct;
-
-                /* Faster version of "remainder = elapsed % clocktick" */
-                cycles_remainder = cycles_elapsed - (ticks_elapsed * local_ct);
+                cycles_remainder = cycles_elapsed % cpt;
         }
 
         /* Can we differentiate between "early CR16" (aka Scenario 1) and
@@ -94,51 +81,65 @@ 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.
          */
-        if (ticks_elapsed > HZ) {
+/* 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
+        {
                 /* Scenario 3: very long delay?  bad in any case */
                 printk (KERN_CRIT "timer_interrupt(CPU %d): delayed!"
-                        " ticks %ld cycles %lX rem %lX"
+                        " cycles %lX rem %lX "
                         " next/now %lX/%lX\n",
                         cpu,
-                        ticks_elapsed, cycles_elapsed, cycles_remainder,
+                        cycles_elapsed, cycles_remainder,
                         next_tick, now );
         }
 
+        /* 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 + (local_ct - cycles_remainder);
+        next_tick = now + cycles_remainder;
+
+        cpu_data[cpu].it_value = next_tick;
 
         /* Skip one clocktick on purpose if we are likely to miss next_tick.
-         * We'll catch what we missed on the tick after that.
-         * We should never need 0x1000 cycles to read CR16, calc the
-         * new next_tick, then write CR16 back. */
-        if (!((local_ct - cycles_remainder) >> 12))
-                next_tick += local_ct;
+         * 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.
+         */
+        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.  */
-        cpu_data[cpu].it_value = next_tick;
         mtctl(next_tick, 16);
 
-        /* Now that we are done mucking with unreliable delivery of interrupts,
-         * go do system house keeping.
+
+        /* Done mucking with unreliable delivery of interrupts.
+         * Go do system house keeping.
          */
-        while (ticks_elapsed--) {
 #ifdef CONFIG_SMP
-                smp_do_timer(regs);
+        smp_do_timer(regs);
 #else
-                update_process_times(user_mode(regs));
+        update_process_times(user_mode(regs));
 #endif
-                if (cpu == 0) {
-                        write_seqlock(&xtime_lock);
-                        do_timer(1);
-                        write_sequnlock(&xtime_lock);
-                }
+        if (cpu == 0) {
+                write_seqlock(&xtime_lock);
+                do_timer(regs);
+                write_sequnlock(&xtime_lock);
         }
-    
+
         /* check soft power switch status */
         if (cpu == 0 && !atomic_read(&power_tasklet.count))
                 tasklet_schedule(&power_tasklet);
@@ -164,14 +165,12 @@ unsigned long profile_pc(struct pt_regs *regs)
 EXPORT_SYMBOL(profile_pc);
 
 
-/*** converted from ia64 ***/
 /*
  * Return the number of micro-seconds that elapsed since the last
  * update to wall time (aka xtime).  The xtime_lock
  * must be at least read-locked when calling this routine.
  */
-static inline unsigned long
-gettimeoffset (void)
+static inline unsigned long gettimeoffset (void)
 {
 #ifndef CONFIG_SMP
         /*
@@ -185,36 +184,40 @@ gettimeoffset (void)
         unsigned long elapsed_cycles;
         unsigned long usec;
         unsigned long cpuid = smp_processor_id();
-        unsigned long local_ct = clocktick;
+        unsigned long cpt = clocktick;
 
         next_tick = cpu_data[cpuid].it_value;
         now = mfctl(16);        /* Read the hardware interval timer.  */
 
-        prev_tick = next_tick - local_ct;
+        prev_tick = next_tick - cpt;
 
         /* Assume Scenario 1: "now" is later than prev_tick.  */
         elapsed_cycles = now - prev_tick;
 
-        if (now < prev_tick) {
-                /* Scenario 2: CR16 wrapped!
-                 * ones complement is off-by-one. Don't care.
-                 */
-                elapsed_cycles = ~elapsed_cycles;
-        }
-
-        if (elapsed_cycles > (HZ * local_ct)) {
+/* aproximate HZ with shifts. Intended math is "(elapsed/clocktick) > HZ" */
+#if HZ == 1000
+        if (elapsed_cycles > (cpt << 10) )
+#elif HZ == 250
+        if (elapsed_cycles > (cpt << 8) )
+#elif HZ == 100
+        if (elapsed_cycles > (cpt << 7) )
+#else
+#warn WTF is HZ set to anyway?
+        if (elapsed_cycles > (HZ * cpt) )
+#endif
+        {
                 /* Scenario 3: clock ticks are missing. */
-                printk (KERN_CRIT "gettimeoffset(CPU %d): missing ticks!"
-                        "cycles %lX prev/now/next %lX/%lX/%lX  clock %lX\n",
-                        cpuid,
-                         elapsed_cycles, prev_tick, now, next_tick, local_ct);
+                printk (KERN_CRIT "gettimeoffset(CPU %ld): missing %ld ticks!"
+                        " cycles %lX prev/now/next %lX/%lX/%lX  clock %lX\n",
+                        cpuid, elapsed_cycles / cpt,
+                        elapsed_cycles, prev_tick, now, next_tick, cpt);
         }
 
         /* FIXME: Can we improve the precision? Not with PAGE0. */
         usec = (elapsed_cycles * 10000) / PAGE0->mem_10msec;
 
         /* add in "lost" jiffies */
-        usec += local_ct * (jiffies - wall_jiffies);
+        usec += cpt * (jiffies - wall_jiffies);
         return usec;
 #else
         return 0;