powerpc/time: improve decrementer clockevent processing

The stop/shutdown op should not use decrementer_set_next_event because
that sets decrementers_next_tb to now + decrementer_max, which means a
decrementer interrupt that occurs after that time will call the
clockevent event handler unexpectedly. Set next_tb to ~0 here to prevent
any clock event call. Init all clockevents to stopped.

Then the decrementer clockevent device always has event_handler set and
applicable because we know the clock event device was not stopped. So
make this call unconditional to show that it is always called. next_tb
need not be set to ~0 before the event handler is called because it will
stop the clockevent device if there is no other timer.

Finally, the timer broadcast interrupt should not modify next_tb because
it is not involved with the local decrementer clockevent on this CPU.

This doesn't fix a known bug, just tidies the code.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220124143930.3923442-3-npiggin@gmail.com

arch/powerpc/kernel/time.c

@@ -107,7 +107,12 @@ struct clock_event_device decrementer_clockevent = {
 };
 EXPORT_SYMBOL(decrementer_clockevent);
 
-DEFINE_PER_CPU(u64, decrementers_next_tb);
+/*
+ * This always puts next_tb beyond now, so the clock event will never fire
+ * with the usual comparison, no need for a separate test for stopped.
+ */
+#define DEC_CLOCKEVENT_STOPPED ~0ULL
+DEFINE_PER_CPU(u64, decrementers_next_tb) = DEC_CLOCKEVENT_STOPPED;
 EXPORT_SYMBOL_GPL(decrementers_next_tb);
 static DEFINE_PER_CPU(struct clock_event_device, decrementers);
 
@@ -645,9 +650,7 @@ DEFINE_INTERRUPT_HANDLER_ASYNC(timer_interrupt)
 
 	now = get_tb();
 	if (now >= *next_tb) {
-		*next_tb = ~(u64)0;
-		if (evt->event_handler)
-			evt->event_handler(evt);
+		evt->event_handler(evt);
 		__this_cpu_inc(irq_stat.timer_irqs_event);
 	} else {
 		now = *next_tb - now;
@@ -666,9 +669,6 @@ EXPORT_SYMBOL(timer_interrupt);
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 void timer_broadcast_interrupt(void)
 {
-	u64 *next_tb = this_cpu_ptr(&decrementers_next_tb);
-
-	*next_tb = ~(u64)0;
 	tick_receive_broadcast();
 	__this_cpu_inc(irq_stat.broadcast_irqs_event);
 }
@@ -894,7 +894,9 @@ static int decrementer_set_next_event(unsigned long evt,
 
 static int decrementer_shutdown(struct clock_event_device *dev)
 {
-	decrementer_set_next_event(decrementer_max, dev);
+	__this_cpu_write(decrementers_next_tb, DEC_CLOCKEVENT_STOPPED);
+	set_dec_or_work(decrementer_max);
+
 	return 0;
 }