[POWERPC] Fix off-by-one error in setting decrementer on Book E/4xx (v2)

The decrementer in Book E and 4xx processors interrupts on the
transition from 1 to 0, rather than on the 0 to -1 transition as on
64-bit server and 32-bit "classic" (6xx/7xx/7xxx) processors.  At the
moment we subtract 1 from the count of how many decrementer ticks are
required before the next interrupt before putting it into the
decrementer, which is correct for server/classic processors, but could
possibly cause the interrupt to happen too early on Book E and 4xx if
the timebase/decrementer frequency is low.

This fixes the problem by making set_dec subtract 1 from the count for
server and classic processors, instead of having the callers subtract
1.  Since set_dec already had a bunch of ifdefs to handle different
processor types, there is no net increase in ugliness. :)

Note that calling set_dec(0) may not generate an interrupt on some
processors.  To make sure that decrementer_set_next_event always calls
set_dec with an interval of at least 1 tick, we set min_delta_ns of
the decrementer_clockevent to correspond to 2 ticks (2 rather than 1
to compensate for truncations in the conversions between ticks and
ns).

This also removes a redundant call to set the decrementer to
0x7fffffff - it was already set to that earlier in timer_interrupt.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 3 insertions, 7 deletions

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 9eb3284deac4..99ebcd3884d2 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -586,7 +586,7 @@ void timer_interrupt(struct pt_regs * regs)
                 /* not time for this event yet */
                 now = per_cpu(decrementer_next_tb, cpu) - now;
                 if (now <= DECREMENTER_MAX)
-                        set_dec((unsigned int)now - 1);
+                        set_dec((int)now);
                 return;
         }
         old_regs = set_irq_regs(regs);
@@ -611,8 +611,6 @@ void timer_interrupt(struct pt_regs * regs)
 
         if (evt->event_handler)
                 evt->event_handler(evt);
-        else
-                evt->set_next_event(DECREMENTER_MAX, evt);
 
 #ifdef CONFIG_PPC_ISERIES
         if (firmware_has_feature(FW_FEATURE_ISERIES) && hvlpevent_is_pending())
@@ -836,9 +834,6 @@ static int decrementer_set_next_event(unsigned long evt,
                                       struct clock_event_device *dev)
 {
         __get_cpu_var(decrementer_next_tb) = get_tb_or_rtc() + evt;
-        /* The decrementer interrupts on the 0 -> -1 transition */
-        if (evt)
-                --evt;
         set_dec(evt);
         return 0;
 }
@@ -871,7 +866,8 @@ void init_decrementer_clockevent(void)
                                              decrementer_clockevent.shift);
         decrementer_clockevent.max_delta_ns =
                 clockevent_delta2ns(DECREMENTER_MAX, &decrementer_clockevent);
-        decrementer_clockevent.min_delta_ns = 1000;
+        decrementer_clockevent.min_delta_ns =
+                clockevent_delta2ns(2, &decrementer_clockevent);
 
         register_decrementer_clockevent(cpu);
 }