[PARISC] remove halftick and copy clocktick to local var (gcc can optimize usage)

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

1 files changed, 31 insertions, 15 deletions

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index fd425e1abe66..c43e847a4b8f 100644
--- a/arch/parisc/kernel/time.c
+++ b/arch/parisc/kernel/time.c
@@ -33,7 +33,6 @@
 #include <linux/timex.h>
 
 static unsigned long clocktick __read_mostly;   /* timer cycles per tick */
-static unsigned long halftick __read_mostly;
 
 #ifdef CONFIG_SMP
 extern void smp_do_timer(struct pt_regs *regs);
@@ -48,6 +47,9 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
         unsigned long ticks_elapsed = 1;        /* at least one elapsed */
         int cpu = smp_processor_id();
 
+        /* gcc can optimize for "read-only" case with a local clocktick */
+        unsigned long local_ct = clocktick;
+
         profile_tick(CPU_PROFILING, regs);
 
         /* Initialize next_tick to the expected tick time. */
@@ -74,8 +76,16 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
                 cycles_elapsed = ~cycles_elapsed;   /* off by one cycle - don't care */
         }
 
-        ticks_elapsed += cycles_elapsed / clocktick;
-        cycles_remainder = cycles_elapsed % clocktick;
+        if (likely(cycles_elapsed < local_ct)) {
+                /* ticks_elapsed = 1 -- We already assumed one tick elapsed. */
+                cycles_remainder = cycles_elapsed;
+        } 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);
+        }
 
         /* Can we differentiate between "early CR16" (aka Scenario 1) and
          * "long delay" (aka Scenario 3)? I don't think so.
@@ -86,14 +96,12 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
          */
         if (ticks_elapsed > HZ) {
                 /* Scenario 3: very long delay?  bad in any case */
-                printk (KERN_CRIT "timer_interrupt(CPU %d): delayed! run ntpdate"
+                printk (KERN_CRIT "timer_interrupt(CPU %d): delayed!"
                         " ticks %ld cycles %lX rem %lX"
                         " next/now %lX/%lX\n",
                         cpu,
                         ticks_elapsed, cycles_elapsed, cycles_remainder,
                         next_tick, now );
-
-                ticks_elapsed = 1;      /* hack to limit damage in loop below */
         }
 
 
@@ -101,12 +109,19 @@ irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
          * 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 + (clocktick - cycles_remainder);
+        next_tick = now + (local_ct - cycles_remainder);
+
+        /* 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;
 
         /* Program the IT when to deliver the next interrupt. */
         /* Only bottom 32-bits of next_tick are written to cr16.  */
-        mtctl(next_tick, 16);
         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.
@@ -169,35 +184,37 @@ gettimeoffset (void)
         unsigned long next_tick;
         unsigned long elapsed_cycles;
         unsigned long usec;
+        unsigned long cpuid = smp_processor_id();
+        unsigned long local_ct = clocktick;
 
-        next_tick = cpu_data[smp_processor_id()].it_value;
+        next_tick = cpu_data[cpuid].it_value;
         now = mfctl(16);        /* Read the hardware interval timer.  */
 
-        prev_tick = next_tick - clocktick;
+        prev_tick = next_tick - local_ct;
 
         /* Assume Scenario 1: "now" is later than prev_tick.  */
         elapsed_cycles = now - prev_tick;
 
         if (now < prev_tick) {
                 /* Scenario 2: CR16 wrapped!
-                 * 1's complement is close enough.
+                 * ones complement is off-by-one. Don't care.
                  */
                 elapsed_cycles = ~elapsed_cycles;
         }
 
-        if (elapsed_cycles > (HZ * clocktick)) {
+        if (elapsed_cycles > (HZ * local_ct)) {
                 /* 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, clocktick);
+                         elapsed_cycles, prev_tick, now, next_tick, local_ct);
         }
 
         /* FIXME: Can we improve the precision? Not with PAGE0. */
         usec = (elapsed_cycles * 10000) / PAGE0->mem_10msec;
 
         /* add in "lost" jiffies */
-        usec += clocktick * (jiffies - wall_jiffies);
+        usec += local_ct * (jiffies - wall_jiffies);
         return usec;
 #else
         return 0;
@@ -290,7 +307,6 @@ void __init time_init(void)
         static struct pdc_tod tod_data;
 
         clocktick = (100 * PAGE0->mem_10msec) / HZ;
-        halftick = clocktick / 2;
 
         start_cpu_itimer();     /* get CPU 0 started */