powerpc/pmac/smp: Properly NAP offlined CPU on G5

The current code soft-disables, and then goes to NAP mode which
turns interrupts on. That means that if an interrupt occurs, we
will hit the masked interrupt code path which isn't what we want,
as it will return with EE off, which will either get us out of
NAP mode, or fail to enter it (according to spec).

Instead, let's just rely on the fact that it is safe to take
decrementer interrupts on an offline CPU and leave interrupts
enabled. We can also get rid of the special case in asm for
power4_cpu_offline_powersave() and just use power4_idle().
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/machdep.h

@@ -266,7 +266,6 @@ struct machdep_calls {
 
 extern void e500_idle(void);
 extern void power4_idle(void);
-extern void power4_cpu_offline_powersave(void);
 extern void ppc6xx_idle(void);
 extern void book3e_idle(void);
 

arch/powerpc/kernel/head_64.S

@@ -536,6 +536,13 @@ _GLOBAL(pmac_secondary_start)
 	add	r13,r13,r4		/* for this processor.		*/
 	mtspr	SPRN_SPRG_PACA,r13	/* Save vaddr of paca in an SPRG*/
 
+	/* Mark interrupts soft and hard disabled (they might be enabled
+	 * in the PACA when doing hotplug)
+	 */
+	li	r0,0
+	stb	r0,PACASOFTIRQEN(r13)
+	stb	r0,PACAHARDIRQEN(r13)
+
 	/* Create a temp kernel stack for use before relocation is on.	*/
 	ld	r1,PACAEMERGSP(r13)
 	subi	r1,r1,STACK_FRAME_OVERHEAD

arch/powerpc/kernel/idle_power4.S

@@ -53,24 +53,3 @@ END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
 	isync
 	b	1b
 
-_GLOBAL(power4_cpu_offline_powersave)
-	/* Go to NAP now */
-	mfmsr	r7
-	rldicl	r0,r7,48,1
-	rotldi	r0,r0,16
-	mtmsrd	r0,1			/* hard-disable interrupts */
-	li	r0,1
-	li	r6,0
-	stb	r0,PACAHARDIRQEN(r13)	/* we'll hard-enable shortly */
-	stb	r6,PACASOFTIRQEN(r13)	/* soft-disable irqs */
-BEGIN_FTR_SECTION
-	DSSALL
-	sync
-END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
-	ori	r7,r7,MSR_EE
-	oris	r7,r7,MSR_POW@h
-	sync
-	isync
-	mtmsrd	r7
-	isync
-	blr

arch/powerpc/platforms/powermac/smp.c

@@ -916,18 +916,20 @@ static void pmac_cpu_die(void)
 	preempt_enable();
 
 	/*
-	 * hard-disable interrupts for the non-NAP case, the NAP code
-	 * needs to re-enable interrupts (but soft-disables them)
+	 * Re-enable interrupts. The NAP code needs to enable them
+	 * anyways, do it now so we deal with the case where one already
+	 * happened while soft-disabled.
+	 * We shouldn't get any external interrupts, only decrementer, and the
+	 * decrementer handler is safe for use on offline CPUs
 	 */
-	hard_irq_disable();
+	local_irq_enable();
 
 	while (1) {
 		/* let's not take timer interrupts too often ... */
 		set_dec(0x7fffffff);
 
-		/* should always be true at this point */
-		if (cpu_has_feature(CPU_FTR_CAN_NAP))
-			power4_cpu_offline_powersave();
+		/* Enter NAP mode */
+		power4_idle();
 	}
 }