[PATCH] 2.4.18, 2.5.5: I/O APIC through-8259A mode IRQ 0 routing

 There is a problem with the through-8259A mode for IRQ 0 on I/O APIC
systems.  Depending on correctness of an MP table, IRQ 0 routing is either
not registered at all or registered at a wrong pin.  As a result the 8254
timer IRQ only works by an accident (it's edge-triggered and never
disabled/enabled so it happens to survive this incorrect configuration).
A visible effect is you can't change the affinity for IRQ 0.

 Following is a patch that fixes both cases referred to above.  The code
looks obvious but it was additionally run-time tested just in case.  The
issue is serious -- please apply the patch ASAP.  As no changes were done
to io_apic.c since the development fork, the patch applies cleanly both to
2.4 and to 2.5.

 Credit goes to Joe for discovering the affinity problem and providing a
fix proposal (incorporated in the final one).

  Maciej

arch/i386/kernel/io_apic.c

@@ -67,7 +67,7 @@ static struct irq_pin_list {
  * shared ISA-space IRQs, so we have to support them. We are super
  * fast in the common case, and fast for shared ISA-space IRQs.
  */
-static void add_pin_to_irq(unsigned int irq, int apic, int pin)
+static void __init add_pin_to_irq(unsigned int irq, int apic, int pin)
 {
 	static int first_free_entry = NR_IRQS;
 	struct irq_pin_list *entry = irq_2_pin + irq;
@@ -85,6 +85,26 @@ static void add_pin_to_irq(unsigned int irq, int apic, int pin)
 	entry->pin = pin;
 }
 
+/*
+ * Reroute an IRQ to a different pin.
+ */
+static void __init replace_pin_at_irq(unsigned int irq,
+				      int oldapic, int oldpin,
+				      int newapic, int newpin)
+{
+	struct irq_pin_list *entry = irq_2_pin + irq;
+
+	while (1) {
+		if (entry->apic == oldapic && entry->pin == oldpin) {
+			entry->apic = newapic;
+			entry->pin = newpin;
+		}
+		if (!entry->next)
+			break;
+		entry = irq_2_pin + entry->next;
+	}
+}
+
 #define __DO_ACTION(R, ACTION, FINAL)					\
 									\
 {									\
@@ -1533,6 +1553,10 @@ static inline void check_timer(void)
 		setup_ExtINT_IRQ0_pin(pin2, vector);
 		if (timer_irq_works()) {
 			printk("works.\n");
+			if (pin1 != -1)
+				replace_pin_at_irq(0, 0, pin1, 0, pin2);
+			else
+				add_pin_to_irq(0, 0, pin2);
 			if (nmi_watchdog == NMI_IO_APIC) {
 				setup_nmi();
 				check_nmi_watchdog();