[tip:x86/apic-cleanups] x86, ioapic: Avoid writing io_apic id if already correct

[tip-bot for Yinghai Lu]

Commit-ID:  60d79fd99ff3b9c692b260a4d53a203f537c052a
Gitweb:     http://git.kernel.org/tip/60d79fd99ff3b9c692b260a4d53a203f537c052a
Author:     Yinghai Lu <yinghai@xxxxxxxxxx>
AuthorDate: Tue, 7 Dec 2010 00:59:49 -0800
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 9 Dec 2010 21:52:05 +0100

x86, ioapic: Avoid writing io_apic id if already correct

For 32bit mptable path, setup_ids_from_mpc() always writes the io_apic
id register, even there is no change needed.

Skip the write, when readout and mptable match.

Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
LKML-Reference: <4CFDF785.7010401@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/kernel/apic/io_apic.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 4abf08a..8a02150 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2007,9 +2007,12 @@ void __init setup_ioapic_ids_from_mpc_nocheck(void)
 						= 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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