sh: Decouple 4k and soft/hardirq stacks.

While using separate IRQ stacks can cut down on stack consumption,
many users can also use 4k stacks directly without the additional
need of separate stacks for soft and hardirqs.

With this split, we support the same rationale for 4KSTACKS as
m68knommu, with the IRQSTACKS abstraction as per ppc64.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/Kconfig.debug

@@ -86,6 +86,14 @@ config 4KSTACKS
 	  on the VM subsystem for higher order allocations. This option
 	  will also use IRQ stacks to compensate for the reduced stackspace.
 
+config IRQSTACKS
+	bool "Use separate kernel stacks when processing interrupts"
+	depends on DEBUG_KERNEL
+	help
+	  If you say Y here the kernel will use separate kernel stacks
+	  for handling hard and soft interrupts.  This can help avoid
+	  overflowing the process kernel stacks.
+
 config SH_KGDB
 	bool "Include KGDB kernel debugger"
 	select FRAME_POINTER

arch/sh/kernel/irq.c

@@ -69,7 +69,7 @@ unlock:
 }
 #endif
 
-#ifdef CONFIG_4KSTACKS
+#ifdef CONFIG_IRQSTACKS
 /*
  * per-CPU IRQ handling contexts (thread information and stack)
  */
@@ -85,7 +85,7 @@ static union irq_ctx *softirq_ctx[NR_CPUS] __read_mostly;
 asmlinkage int do_IRQ(unsigned int irq, struct pt_regs *regs)
 {
 	struct pt_regs *old_regs = set_irq_regs(regs);
-#ifdef CONFIG_4KSTACKS
+#ifdef CONFIG_IRQSTACKS
 	union irq_ctx *curctx, *irqctx;
 #endif
 
@@ -109,7 +109,7 @@ asmlinkage int do_IRQ(unsigned int irq, struct pt_regs *regs)
 
 	irq = irq_demux(evt2irq(irq));
 
-#ifdef CONFIG_4KSTACKS
+#ifdef CONFIG_IRQSTACKS
 	curctx = (union irq_ctx *)current_thread_info();
 	irqctx = hardirq_ctx[smp_processor_id()];
 
@@ -157,7 +157,7 @@ asmlinkage int do_IRQ(unsigned int irq, struct pt_regs *regs)
 	return 1;
 }
 
-#ifdef CONFIG_4KSTACKS
+#ifdef CONFIG_IRQSTACKS
 static char softirq_stack[NR_CPUS * THREAD_SIZE]
 		__attribute__((__section__(".bss.page_aligned")));
 

include/asm-sh/irq.h

@@ -41,7 +41,7 @@ static inline int generic_irq_demux(int irq)
 #define irq_canonicalize(irq)	(irq)
 #define irq_demux(irq)		sh_mv.mv_irq_demux(irq)
 
-#ifdef CONFIG_4KSTACKS
+#ifdef CONFIG_IRQSTACKS
 extern void irq_ctx_init(int cpu);
 extern void irq_ctx_exit(int cpu);
 # define __ARCH_HAS_DO_SOFTIRQ