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Subject[PATCH] x86: Don't write io_apic ID if it is not changed

For 32bit mptable path, setup_ids_from_mpc() always write io apic id
register, even there is no change needed.

So try to do that when they are different bewteen reading out and mptable

Signed-off-by: Yinghai Lu <yinghai@kernel.org>

---
arch/x86/kernel/apic/io_apic.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

Index: linux-2.6/arch/x86/kernel/apic/io_apic.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/apic/io_apic.c
+++ linux-2.6/arch/x86/kernel/apic/io_apic.c
@@ -2125,7 +2125,6 @@ void __init setup_ioapic_ids_from_mpc(vo
physids_or(phys_id_present_map, phys_id_present_map, tmp);
}

-
/*
* We need to adjust the IRQ routing table
* if the ID changed.
@@ -2137,9 +2136,12 @@ void __init setup_ioapic_ids_from_mpc(vo
= mp_ioapics[apic_id].apicid;

/*
- * Read the right value from the MPC table and
- * write it into the ID register.
+ * Update the ID register according to the right value from
+ * the MPC table if they are different.
*/
+ if (mp_ioapics[apic_id].apicid == reg_00.bits.ID)
+ continue;
+
apic_printk(APIC_VERBOSE, KERN_INFO
"...changing IO-APIC physical APIC ID to %d ...",
mp_ioapics[apic_id].apicid);

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