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Commit 43875cc0 authored by Paul Mackerras's avatar Paul Mackerras
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[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: default avatarPaul Mackerras <paulus@samba.org>
parent 465ccab9

arch/powerpc/kernel/time.c

+3 −7
Original line number Diff line number Diff line
@@ -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);

}

include/asm-powerpc/time.h

+13 −7
Original line number Diff line number Diff line
@@ -176,23 +176,29 @@ static inline unsigned int get_dec(void) 
#endif

}



/*

 * Note: Book E and 4xx processors differ from other PowerPC processors

 * in when the decrementer generates its interrupt: on the 1 to 0

 * transition for Book E/4xx, but on the 0 to -1 transition for others.

 */

static inline void set_dec(int val)

{

#if defined(CONFIG_40x)

	mtspr(SPRN_PIT, val);

#elif defined(CONFIG_8xx_CPU6)

	set_dec_cpu6(val);

	set_dec_cpu6(val - 1);

#else

#ifndef CONFIG_BOOKE

	--val;

#endif

#ifdef CONFIG_PPC_ISERIES

	int cur_dec;



	if (firmware_has_feature(FW_FEATURE_ISERIES) &&

			get_lppaca()->shared_proc) {

		get_lppaca()->virtual_decr = val;

		cur_dec = get_dec();

		if (cur_dec > val)

		if (get_dec() > val)

			HvCall_setVirtualDecr();

	} else

		return;

	}

#endif

	mtspr(SPRN_DEC, val);

#endif /* not 40x or 8xx_CPU6 */