x86/apic, doc: Justification for disabling IO APIC before Local APIC

Since erratum AVR31 in "Intel Atom Processor C2000 Product Family
Specification Update" is now published, I added a justification
comment for disabling IO APIC before Local APIC, as changed in commit:

522e66464467 x86/apic: Disable I/O APIC before shutdown of the local APIC

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1386202069-51515-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index da3c599584a3..c752cb43e52f 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -558,6 +558,17 @@ void native_machine_shutdown(void)
 {
         /* Stop the cpus and apics */
 #ifdef CONFIG_X86_IO_APIC
+        /*
+         * Disabling IO APIC before local APIC is a workaround for
+         * erratum AVR31 in "Intel Atom Processor C2000 Product Family
+         * Specification Update". In this situation, interrupts that target
+         * a Logical Processor whose Local APIC is either in the process of
+         * being hardware disabled or software disabled are neither delivered
+         * nor discarded. When this erratum occurs, the processor may hang.
+         *
+         * Even without the erratum, it still makes sense to quiet IO APIC
+         * before disabling Local APIC.
+         */
         disable_IO_APIC();
 #endif