[POWERPC] Add IRQSTACKS support on ppc32

This makes it possible to use separate stacks for hard and soft IRQs
on 32-bit powerpc as well as on 64-bit.  The code for 32-bit is just
the 32-bit analog of the 64-bit code.

* Added allocation and initialization of the irq stacks.  We limit the
  stacks to be in lowmem for ppc32.
* Implemented ppc32 versions of call_do_softirq() and call_handle_irq()
  to switch the stack pointers
* Reworked how we do stack overflow detection.  We now keep around the
  limit of the stack in the thread_struct and compare against the limit
  to see if we've overflowed.  We can now use this on ppc64 if desired.

[ paulus@samba.org: Fixed bug on 6xx where we need to reload r9 with the
  thread_info pointer. ]

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -307,6 +307,7 @@ void do_IRQ(struct pt_regs *regs)
 		if (curtp != irqtp) {
 			struct irq_desc *desc = irq_desc + irq;
 			void *handler = desc->handle_irq;
+			unsigned long saved_sp_limit = current->thread.ksp_limit;
 			if (handler == NULL)
 				handler = &__do_IRQ;
 			irqtp->task = curtp->task;
@@ -319,7 +320,10 @@ void do_IRQ(struct pt_regs *regs)
 				(irqtp->preempt_count & ~SOFTIRQ_MASK) |
 				(curtp->preempt_count & SOFTIRQ_MASK);
 
+			current->thread.ksp_limit = (unsigned long)irqtp +
+				_ALIGN_UP(sizeof(struct thread_info), 16);
 			call_handle_irq(irq, desc, irqtp, handler);
+			current->thread.ksp_limit = saved_sp_limit;
 			irqtp->task = NULL;
 
 
@@ -352,9 +356,7 @@ void __init init_IRQ(void)
 {
 	if (ppc_md.init_IRQ)
 		ppc_md.init_IRQ();
-#ifdef CONFIG_PPC64
 	irq_ctx_init();
-#endif
 }
 
 
@@ -383,11 +385,15 @@ void irq_ctx_init(void)
 static inline void do_softirq_onstack(void)
 {
 	struct thread_info *curtp, *irqtp;
+	unsigned long saved_sp_limit = current->thread.ksp_limit;
 
 	curtp = current_thread_info();
 	irqtp = softirq_ctx[smp_processor_id()];
 	irqtp->task = curtp->task;
+	current->thread.ksp_limit = (unsigned long)irqtp +
+				    _ALIGN_UP(sizeof(struct thread_info), 16);
 	call_do_softirq(irqtp);
+	current->thread.ksp_limit = saved_sp_limit;
 	irqtp->task = NULL;
 }