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@kernel.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
LKML-Reference: <4CFDF785.7010401@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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 @@
= 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);
