[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>

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index c43e847a4b8fd94f659a197454b18677f161020e..9d642d820fe9f35b227960baafd7f3626e579b7c 100644 (file)
--- 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;