[PATCHv3 3/6] arm64: irq: rework root IRQ handler registration

[Mark Rutland]

If we accidentally unmask IRQs before we've registered a root IRQ
handler, handle_arch_irq will be NULL, and the IRQ exception handler
will branch to a bogus address.

To make this easier to debug, this patch initialises handle_arch_irq to
a default handler which will panic(), making such problems easier to
debug. When we add support for FIQ handlers, we can follow the same
approach.

When we add support for a root FIQ handler, it's possible to have root
IRQ handler without an root FIQ handler, and in theory the inverse is
also possible. To permit this, and to keep the IRQ/FIQ registration
logic similar, this patch removes the panic in the absence of a root IRQ
controller. Instead, set_handle_irq() logs when a handler is registered,
which is sufficient for debug purposes.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Tested-by: Hector Martin <marcan at marcan.st>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: James Morse <james.morse at arm.com>
Cc: Marc Zyngier <maz at kernel.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/irq.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
index ad63bd50fa7b..2fe0b535de30 100644
--- a/arch/arm64/kernel/irq.c
+++ b/arch/arm64/kernel/irq.c
@@ -71,14 +71,20 @@ static void init_irq_stacks(void)
 }
 #endif
 
-void (*handle_arch_irq)(struct pt_regs *) __ro_after_init;
+static void default_handle_irq(struct pt_regs *regs)
+{
+	panic("IRQ taken without a root IRQ handler\n");
+}
+
+void (*handle_arch_irq)(struct pt_regs *) __ro_after_init = default_handle_irq;
 
 int __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
 {
-	if (handle_arch_irq)
+	if (handle_arch_irq != default_handle_irq)
 		return -EBUSY;
 
 	handle_arch_irq = handle_irq;
+	pr_info("Root IRQ handler: %ps\n", handle_irq);
 	return 0;
 }
 
@@ -87,8 +93,6 @@ void __init init_IRQ(void)
 	init_irq_stacks();
 	init_irq_scs();
 	irqchip_init();
-	if (!handle_arch_irq)
-		panic("No interrupt controller found.");
 
 	if (system_uses_irq_prio_masking()) {
 		/*
-- 
2.11.0