Add support for checking before-the-fact whether an IRQ is

already registered or not. The x86 PCI layer wants this for
its availability testing.

Doing a request_irq()/free_irq() pair to check this condition
like we used to do can lock the machine if the irq happens to
be screaming.

arch/i386/kernel/irq.c

@@ -505,6 +505,20 @@ asmlinkage unsigned int do_IRQ(struct pt_regs regs)
 	return 1;
 }
 
+int can_request_irq(unsigned int irq, unsigned long irqflags)
+{
+	struct irqaction *action;
+
+	if (irq >= NR_IRQS)
+		return 0;
+	action = irq_desc[irq].action;
+	if (action) {
+		if (irqflags & action->flags & SA_SHIRQ)
+			action = NULL;
+	}
+	return !action;
+}
+
 /**
  *	request_irq - allocate an interrupt line
  *	@irq: Interrupt line to allocate

arch/i386/pci/irq.c

@@ -695,11 +695,6 @@ static struct irq_info *pirq_get_info(struct pci_dev *dev)
 	return NULL;
 }
 
-static irqreturn_t pcibios_test_irq_handler(int irq, void *dev_id, struct pt_regs *regs)
-{
-	return IRQ_NONE;
-}
-
 static int pcibios_lookup_irq(struct pci_dev *dev, int assign)
 {
 	u8 pin;
@@ -761,11 +756,8 @@ static int pcibios_lookup_irq(struct pci_dev *dev, int assign)
 		for (i = 0; i < 16; i++) {
 			if (!(mask & (1 << i)))
 				continue;
-			if (pirq_penalty[i] < pirq_penalty[newirq] &&
-			    !request_irq(i, pcibios_test_irq_handler, SA_SHIRQ, "pci-test", dev)) {
-				free_irq(i, dev);
+			if (pirq_penalty[i] < pirq_penalty[newirq] && can_request_irq(i, SA_SHIRQ))
 				newirq = i;
-			}
 		}
 	}
 	DBG(" -> newirq=%d", newirq);

include/asm-i386/irq.h

@@ -24,6 +24,7 @@ extern void disable_irq(unsigned int);
 extern void disable_irq_nosync(unsigned int);
 extern void enable_irq(unsigned int);
 extern void release_x86_irqs(struct task_struct *);
+extern int can_request_irq(unsigned int, unsigned long flags);
 
 #ifdef CONFIG_X86_LOCAL_APIC
 #define ARCH_HAS_NMI_WATCHDOG		/* See include/linux/nmi.h */