[PATCH 2/3] arm: introduce IRQ stacks

[Maninder Singh]

This patch adds code for switching to IRQ stack.
IRQ stack and Kernel SVC stack have below design.

IRQ STACK:-
                    ------------ IRQ stack top
                    |          |
                    ------------
                    .          .
                    .          .
                    .          .
                    ------------
                    |    sp    | <- irq_stack_base + 0x8
                    ------------
                    |    fp    | <- irq_stack_base + 0x4
                    ------------
                    |tinfo_ptr | /* pointer to thread info */
irq_stack_ptr -->   ------------ IRQ stack base

Kernel SVC stack:-
                    ------------  Kernel stack top
                    |          |
                    ------------
                    .          .
                    .          .
                    .          .
                    ------------
                    |          |
                    |          |
                    ------------
                    |tinfo_ptr |  /* pointer to thread info */
                    ------------ Kernel stack base

Co-developed-by: Vaneet Narang <v.narang@xxxxxxxxxxx>
Signed-off-by: Vaneet Narang <v.narang@xxxxxxxxxxx>
Signed-off-by: Maninder Singh <maninder1.s@xxxxxxxxxxx>
---
 arch/arm/include/asm/assembler.h |  8 ++++++++
 arch/arm/include/asm/irq.h       |  6 ++++++
 arch/arm/kernel/entry-armv.S     | 41 +++++++++++++++++++++++++++++++++++++++-
 arch/arm/kernel/irq.c            | 24 +++++++++++++++++++++++
 4 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 8512bdc..82ee6ee 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -212,6 +212,14 @@
 #endif
 	.endm
 
+	.macro	this_cpu_ptr, sym, reg, tmp
+	ldr	\reg, =\sym
+#if defined(CONFIG_SMP) && !defined(CONFIG_CPU_V6)
+	mrc	p15, 0, \tmp, cr13, c0, 4
+	add	\reg, \reg, \tmp
+#endif
+	.endm
+
 /*
  * Increment/decrement the preempt count.
  */
diff --git a/arch/arm/include/asm/irq.h b/arch/arm/include/asm/irq.h
index 46d4114..f3299ab 100644
--- a/arch/arm/include/asm/irq.h
+++ b/arch/arm/include/asm/irq.h
@@ -22,10 +22,16 @@
 #define NO_IRQ	((unsigned int)(-1))
 #endif
 
+#define IRQ_STACK_SIZE	THREAD_SIZE
+
 #ifndef __ASSEMBLY__
 struct irqaction;
 struct pt_regs;
 
+#ifdef CONFIG_IRQ_STACK
+DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
+#endif
+
 extern void asm_do_IRQ(unsigned int, struct pt_regs *);
 void handle_IRQ(unsigned int, struct pt_regs *);
 void init_IRQ(void);
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 55a47df..13a5889 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -32,6 +32,43 @@
 #include "entry-header.S"
 #include <asm/entry-macro-multi.S>
 #include <asm/probes.h>
+#ifdef CONFIG_IRQ_STACK
+#include <asm/irq.h>
+#endif
+
+	.macro  irq_stack_entry
+#ifdef CONFIG_IRQ_STACK
+	mov	r6, sp	/* preserve sp */
+
+	this_cpu_ptr irq_stack_ptr, r7, r8
+	ldr	r7, [r7]
+	mov	r8, r7
+
+	/*
+	 * Compare sp with base of IRQ stack.
+	 * if the top ~(#THREAD_SIZE_ORDER + PAGE_SHIFT) bits match,
+	 * we are on a irq stack.
+	 */
+	eor	r8, r8, sp
+	lsrs	r8, #THREAD_SIZE_ORDER + PAGE_SHIFT
+	beq	9998f
+
+	/*
+	 * store thread info pointer on IRQ stack and
+	 * switch to the irq stack.
+	 */
+	get_thread_info r8
+	stm	r7, {r8, fp, sp}
+	add	sp, r7, #IRQ_STACK_SIZE
+9998:
+#endif
+        .endm
+
+	.macro	irq_stack_exit
+#ifdef CONFIG_IRQ_STACK
+	mov	sp, r6	/* restore sp */
+#endif
+       .endm
 
 /*
  * Interrupt handling.
@@ -41,11 +78,13 @@
 	ldr	r1, =handle_arch_irq
 	mov	r0, sp
 	badr	lr, 9997f
+	irq_stack_entry
 	ldr	pc, [r1]
+9997:
+	irq_stack_exit
 #else
 	arch_irq_handler_default
 #endif
-9997:
 	.endm
 
 	.macro	pabt_helper
diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
index 698b6f6..79872e5 100644
--- a/arch/arm/kernel/irq.c
+++ b/arch/arm/kernel/irq.c
@@ -43,6 +43,15 @@
 
 unsigned long irq_err_count;
 
+#ifdef CONFIG_IRQ_STACK
+DEFINE_PER_CPU(unsigned long *, irq_stack_ptr);
+
+/*
+ * irq_stack must be IRQ_STACK_SIZE(THREAD_SIZE) aligned,
+ */
+DEFINE_PER_CPU(unsigned long [IRQ_STACK_SIZE/sizeof(long)], irq_stack) __aligned(IRQ_STACK_SIZE);
+#endif
+
 int arch_show_interrupts(struct seq_file *p, int prec)
 {
 #ifdef CONFIG_FIQ
@@ -55,6 +64,20 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 	return 0;
 }
 
+#ifdef CONFIG_IRQ_STACK
+static void init_irq_stacks(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		per_cpu(irq_stack_ptr, cpu) = per_cpu(irq_stack, cpu);
+}
+#else
+static inline void init_irq_stacks(void)
+{
+}
+#endif
+
 /*
  * handle_IRQ handles all hardware IRQ's.  Decoded IRQs should
  * not come via this function.  Instead, they should provide their
@@ -79,6 +102,7 @@ void __init init_IRQ(void)
 {
 	int ret;
 
+	init_irq_stacks();
 	if (IS_ENABLED(CONFIG_OF) && !machine_desc->init_irq)
 		irqchip_init();
 	else
-- 
1.9.1