[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/setup_32.c

@@ -16,6 +16,7 @@
 #include <linux/root_dev.h>
 #include <linux/cpu.h>
 #include <linux/console.h>
+#include <linux/lmb.h>
 
 #include <asm/io.h>
 #include <asm/prom.h>
@@ -229,6 +230,24 @@ int __init ppc_init(void)
 
 arch_initcall(ppc_init);
 
+#ifdef CONFIG_IRQSTACKS
+static void __init irqstack_early_init(void)
+{
+	unsigned int i;
+
+	/* interrupt stacks must be in lowmem, we get that for free on ppc32
+	 * as the lmb is limited to lowmem by LMB_REAL_LIMIT */
+	for_each_possible_cpu(i) {
+		softirq_ctx[i] = (struct thread_info *)
+			__va(lmb_alloc(THREAD_SIZE, THREAD_SIZE));
+		hardirq_ctx[i] = (struct thread_info *)
+			__va(lmb_alloc(THREAD_SIZE, THREAD_SIZE));
+	}
+}
+#else
+#define irqstack_early_init()
+#endif
+
 /* Warning, IO base is not yet inited */
 void __init setup_arch(char **cmdline_p)
 {
@@ -286,6 +305,8 @@ void __init setup_arch(char **cmdline_p)
 	init_mm.end_data = (unsigned long) _edata;
 	init_mm.brk = klimit;
 
+	irqstack_early_init();
+
 	/* set up the bootmem stuff with available memory */
 	do_init_bootmem();
 	if ( ppc_md.progress ) ppc_md.progress("setup_arch: bootmem", 0x3eab);