[PARISC] Rewrite timer_interrupt() and gettimeoffset() using "unsigned" math.

It's just a bit easier to follow and timer code is complex enough.

So far, only tested on A500-5x (64-bit SMP), ie: gettimeoffset() code
hasn't been tested at all.

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

1 files changed, 96 insertions, 44 deletions

diff --git a/arch/parisc/kernel/time.c b/arch/parisc/kernel/time.c
index 47831c2cd093..fd425e1abe66 100644
--- a/arch/parisc/kernel/time.c
+++ b/arch/parisc/kernel/time.c
@@ -32,8 +32,8 @@
 
 #include <linux/timex.h>
 
-static long clocktick __read_mostly;    /* timer cycles per tick */
-static long halftick __read_mostly;
+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);
@@ -41,34 +41,77 @@ extern void smp_do_timer(struct pt_regs *regs);
 
 irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
 {
-        long now;
-        long next_tick;
-        int nticks;
+        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();
 
         profile_tick(CPU_PROFILING, regs);
 
-        now = mfctl(16);
-        /* initialize next_tick to time at last clocktick */
+        /* Initialize next_tick to the expected tick time. */
         next_tick = cpu_data[cpu].it_value;
 
-        /* since time passes between the interrupt and the mfctl()
-         * above, it is never true that last_tick + clocktick == now.  If we
-         * never miss a clocktick, we could set next_tick = last_tick + clocktick
-         * but maybe we'll miss ticks, hence the loop.
-         *
-         * Variables are *signed*.
+        /* Get current interval timer.
+         * CR16 reads as 64 bits in CPU wide mode.
+         * CR16 reads as 32 bits in CPU narrow mode.
          */
+        now = mfctl(16);
 
-        nticks = 0;
-        while((next_tick - now) < halftick) {
-                next_tick += clocktick;
-                nticks++;
+        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.
+                 */
+                cycles_elapsed = ~cycles_elapsed;   /* off by one cycle - don't care */
         }
+
+        ticks_elapsed += cycles_elapsed / clocktick;
+        cycles_remainder = cycles_elapsed % clocktick;
+
+        /* 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 (ticks_elapsed > HZ) {
+                /* Scenario 3: very long delay?  bad in any case */
+                printk (KERN_CRIT "timer_interrupt(CPU %d): delayed! run ntpdate"
+                        " 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 */
+        }
+
+
+        /* 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 + (clocktick - cycles_remainder);
+
+        /* 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;
 
-        while (nticks--) {
+        /* Now that we are done mucking with unreliable delivery of interrupts,
+         * go do system house keeping.
+         */
+        while (ticks_elapsed--) {
 #ifdef CONFIG_SMP
                 smp_do_timer(regs);
 #else
@@ -121,21 +164,41 @@ gettimeoffset (void)
          *    Once parisc-linux learns the cr16 difference between processors,
          *    this could be made to work.
          */
-        long last_tick;
-        long elapsed_cycles;
+        unsigned long now;
+        unsigned long prev_tick;
+        unsigned long next_tick;
+        unsigned long elapsed_cycles;
+        unsigned long usec;
 
-        /* it_value is the intended time of the next tick */
-        last_tick = cpu_data[smp_processor_id()].it_value;
+        next_tick = cpu_data[smp_processor_id()].it_value;
+        now = mfctl(16);        /* Read the hardware interval timer.  */
 
-        /* Subtract one tick and account for possible difference between
-         * when we expected the tick and when it actually arrived.
-         * (aka wall vs real)
-         */
-        last_tick -= clocktick * (jiffies - wall_jiffies + 1);
-        elapsed_cycles = mfctl(16) - last_tick;
+        prev_tick = next_tick - clocktick;
+
+        /* 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.
+                 */
+                elapsed_cycles = ~elapsed_cycles;
+        }
 
-        /* the precision of this math could be improved */
-        return elapsed_cycles / (PAGE0->mem_10msec / 10000);
+        if (elapsed_cycles > (HZ * clocktick)) {
+                /* 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);
+        }
+
+        /* 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);
+        return usec;
 #else
         return 0;
 #endif
@@ -146,6 +209,7 @@ do_gettimeofday (struct timeval *tv)
 {
         unsigned long flags, seq, usec, sec;
 
+        /* Hold xtime_lock and adjust timeval.  */
         do {
                 seq = read_seqbegin_irqsave(&xtime_lock, flags);
                 usec = gettimeoffset();
@@ -153,25 +217,13 @@ do_gettimeofday (struct timeval *tv)
                 usec += (xtime.tv_nsec / 1000);
         } while (read_seqretry_irqrestore(&xtime_lock, seq, flags));
 
-        if (unlikely(usec > LONG_MAX)) {
-                /* This can happen if the gettimeoffset adjustment is
-                 * negative and xtime.tv_nsec is smaller than the
-                 * adjustment */
-                printk(KERN_ERR "do_gettimeofday() spurious xtime.tv_nsec of %ld\n", usec);
-                usec += USEC_PER_SEC;
-                --sec;
-                /* This should never happen, it means the negative
-                 * time adjustment was more than a second, so there's
-                 * something seriously wrong */
-                BUG_ON(usec > LONG_MAX);
-        }
-
-
+        /* Move adjusted usec's into sec's.  */
         while (usec >= USEC_PER_SEC) {
                 usec -= USEC_PER_SEC;
                 ++sec;
         }
 
+        /* Return adjusted result.  */
         tv->tv_sec = sec;
         tv->tv_usec = usec;
 }