x86/apic: Remove the SET_APIC_ID(x) macro

The SET_APIC_ID() macro obfusates the code. Remove it to increase
readability and add a comment to the apic struct to document that the
callback is required on 64-bit.
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/1488971270-14359-1-git-send-email-douly.fnst@cn.fujitsu.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/apic.h

@@ -252,12 +252,6 @@ static inline int x2apic_enabled(void) { return 0; }
 #define	x2apic_supported()	(0)
 #endif /* !CONFIG_X86_X2APIC */
 
-#ifdef CONFIG_X86_64
-#define	SET_APIC_ID(x)		(apic->set_apic_id(x))
-#else
-
-#endif
-
 /*
  * Copyright 2004 James Cleverdon, IBM.
  * Subject to the GNU Public License, v.2
@@ -299,6 +293,7 @@ struct apic {
 	int (*phys_pkg_id)(int cpuid_apic, int index_msb);
 
 	unsigned int (*get_apic_id)(unsigned long x);
+	/* Can't be NULL on 64-bit */
 	unsigned long (*set_apic_id)(unsigned int id);
 
 	int (*cpu_mask_to_apicid_and)(const struct cpumask *cpumask,

arch/x86/kernel/apic/apic.c

@@ -2249,7 +2249,7 @@ void __init apic_set_eoi_write(void (*eoi_write)(u32 reg, u32 v))
 static void __init apic_bsp_up_setup(void)
 {
 #ifdef CONFIG_X86_64
-	apic_write(APIC_ID, SET_APIC_ID(boot_cpu_physical_apicid));
+	apic_write(APIC_ID, apic->set_apic_id(boot_cpu_physical_apicid));
 #else
 	/*
 	 * Hack: In case of kdump, after a crash, kernel might be booting