APB timer is used on Moorestown platforms but not on a standard PC.
If APB timer code is compiled in but not initialized at run-time due
to lack of FW reported SFI table, kernel would panic when the non-boot
CPUs are offlined and notifier is called.

https://bugzilla.kernel.org/show_bug.cgi?id=15786

This patch ensures CPU hotplug notifier for APB timer is only registered
when the APBT timer block is initialized.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
---
 arch/x86/kernel/apb_timer.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/apb_timer.c b/arch/x86/kernel/apb_timer.c
index ff469e4..a353475 100644
--- a/arch/x86/kernel/apb_timer.c
+++ b/arch/x86/kernel/apb_timer.c
@@ -429,7 +429,7 @@ static int apbt_cpuhp_notify(struct notifier_block *n,
 
 static __init int apbt_late_init(void)
 {
-	if (disable_apbt_percpu)
+	if (disable_apbt_percpu || !apb_timer_block_enabled)
 		return 0;
 	/* This notifier should be called after workqueue is ready */
 	hotcpu_notifier(apbt_cpuhp_notify, -20);
-- 
1.5.6.5

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