[PATCH] [POWERPC] Add IRQSTACKS support on ppc32

[Paul Mackerras]

Kumar Gala writes:

> * 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.

Mostly looks great.  One minor nit is that this ifdef could disappear
entirely now:

> @@ -352,7 +356,7 @@ void __init init_IRQ(void)
>  {
>  	if (ppc_md.init_IRQ)
>  		ppc_md.init_IRQ();
> -#ifdef CONFIG_PPC64
> +#ifdef CONFIG_IRQSTACKS
>  	irq_ctx_init();
>  #endif

Since it looks pretty much exactly like what 64-bit already does, and
it's under a config option, I withdraw my objection to it going into
2.6.26.

Paul.