[PATCH -v2] x86, ioapic: Don't write io_apic ID if it is not changed

[Yinghai Lu <yinghai@kernel.org>]

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

-v2: update to recent setup_ioapic_ids_from_mpc() split..

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

---
 arch/x86/kernel/apic/io_apic.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 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
@@ -2006,9 +2006,12 @@ void __init setup_ioapic_ids_from_mpc_no
 						= 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);