x86/apic: Simplify init_bsp_APIC() usage

Since CONFIG_X86_64 selects CONFIG_X86_LOCAL_APIC, the following
condition:

  #if defined(CONFIG_X86_64) || defined(CONFIG_X86_LOCAL_APIC)

is equivalent to:

  #if defined(CONFIG_X86_LOCAL_APIC)

... and we can eliminate that #ifdef by providing an empty
init_bsp_APIC() stub in the !CONFIG_X86_LOCAL_APIC case.

Also add some comments to explain why we call init_bsp_APIC().

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: mroos@linux.ee
Cc: ville.syrjala@linux.intel.com
Link: http://lkml.kernel.org/r/20180117073748.23905-1-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/apic.h

@@ -183,6 +183,7 @@ static inline void disable_local_APIC(void) { }
 # define setup_boot_APIC_clock x86_init_noop
 # define setup_secondary_APIC_clock x86_init_noop
 static inline void lapic_update_tsc_freq(void) { }
+static inline void init_bsp_APIC(void) { }
 static inline void apic_intr_mode_init(void) { }
 static inline void lapic_assign_system_vectors(void) { }
 static inline void lapic_assign_legacy_vector(unsigned int i, bool r) { }

arch/x86/kernel/irqinit.c

@@ -61,9 +61,14 @@ void __init init_ISA_irqs(void)
 	struct irq_chip *chip = legacy_pic->chip;
 	int i;
 
-#if defined(CONFIG_X86_64) || defined(CONFIG_X86_LOCAL_APIC)
+	/*
+	 * Try to set up the through-local-APIC virtual wire mode earlier.
+	 *
+	 * On some 32-bit UP machines, whose APIC has been disabled by BIOS
+	 * and then got re-enabled by "lapic", it hangs at boot time without this.
+	 */
 	init_bsp_APIC();
-#endif
+
 	legacy_pic->init(0);
 
 	for (i = 0; i < nr_legacy_irqs(); i++)