[PATCH] x86_64: Add option to disable timer check

This works around the too fast timer seen on some ATI boards.

I don't feel confident enough about it yet to enable it by default, but give
users the option.

Patch and debugging from Christopher Allen Wing <wingc@engin.umich.edu>, with
minor tweaks (renamed the option and documented it)
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/x86_64/boot-options.txt

@@ -25,6 +25,9 @@ APICs
 
    noapictimer	 Don't set up the APIC timer
 
+   no_timer_check Don't check the IO-APIC timer. This can work around
+		 problems with incorrect timer initialization on some boards.
+
 Early Console
 
    syntax: earlyprintk=vga

arch/x86_64/kernel/io_apic.c

@@ -42,6 +42,8 @@
 
 int sis_apic_bug; /* not actually supported, dummy for compile */
 
+static int no_timer_check;
+
 static DEFINE_SPINLOCK(ioapic_lock);
 
 /*
@@ -1601,7 +1603,7 @@ static inline void check_timer(void)
 		 * Ok, does IRQ0 through the IOAPIC work?
 		 */
 		unmask_IO_APIC_irq(0);
-		if (timer_irq_works()) {
+		if (!no_timer_check && timer_irq_works()) {
 			nmi_watchdog_default();
 			if (nmi_watchdog == NMI_IO_APIC) {
 				disable_8259A_irq(0);
@@ -1671,6 +1673,13 @@ static inline void check_timer(void)
 	panic("IO-APIC + timer doesn't work! Try using the 'noapic' kernel parameter\n");
 }
 
+static int __init notimercheck(char *s)
+{
+	no_timer_check = 1;
+	return 1;
+}
+__setup("no_timer_check", notimercheck);
+
 /*
  *
  * IRQ's that are handled by the PIC in the MPS IOAPIC case.