arm64: don't use GENERIC_IRQ_MULTI_HANDLER

In subsequent patches we want to allow irqchip drivers to register as
FIQ handlers, with a set_handle_fiq() function. To keep the IRQ/FIQ
paths similar, we want arm64 to provide both set_handle_irq() and
set_handle_fiq(), rather than using GENERIC_IRQ_MULTI_HANDLER for the
former.

This patch adds an arm64-specific implementation of set_handle_irq().
There should be no functional change as a result of this patch.

Signed-off-by: Marc Zyngier <maz@kernel.org>
[Mark: use a single handler pointer]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Hector Martin <marcan@marcan.st>
Cc: James Morse <james.morse@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210315115629.57191-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -110,7 +110,6 @@ config ARM64
 	select GENERIC_EARLY_IOREMAP
 	select GENERIC_IDLE_POLL_SETUP
 	select GENERIC_IRQ_IPI
-	select GENERIC_IRQ_MULTI_HANDLER
 	select GENERIC_IRQ_PROBE
 	select GENERIC_IRQ_SHOW
 	select GENERIC_IRQ_SHOW_LEVEL

arch/arm64/include/asm/irq.h

@@ -8,6 +8,9 @@
 
 struct pt_regs;
 
+int set_handle_irq(void (*handle_irq)(struct pt_regs *));
+#define set_handle_irq	set_handle_irq
+
 static inline int nr_legacy_irqs(void)
 {
 	return 0;

arch/arm64/kernel/irq.c

@@ -71,6 +71,17 @@ static void init_irq_stacks(void)
 }
 #endif
 
+void (*handle_arch_irq)(struct pt_regs *) __ro_after_init;
+
+int __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
+{
+	if (handle_arch_irq)
+		return -EBUSY;
+
+	handle_arch_irq = handle_irq;
+	return 0;
+}
+
 void __init init_IRQ(void)
 {
 	init_irq_stacks();