powerpc/smp: Document irq enable/disable after migrating IRQs

This code was until recently completely undocumented and even now the comment is
not very verbose.

We've already had one patch sent to remove the IRQ enable/disable because it's
"paradoxical and unnecessary". So document it thoroughly to save anyone else
from puzzling over it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/smp.c

@@ -615,7 +615,14 @@ int generic_cpu_disable(void)
 	/* Update affinity of all IRQs previously aimed at this CPU */
 	irq_migrate_all_off_this_cpu();
 
-	/* Give the CPU time to drain in-flight ones */
+	/*
+	 * Depending on the details of the interrupt controller, it's possible
+	 * that one of the interrupts we just migrated away from this CPU is
+	 * actually already pending on this CPU. If we leave it in that state
+	 * the interrupt will never be EOI'ed, and will never fire again. So
+	 * temporarily enable interrupts here, to allow any pending interrupt to
+	 * be received (and EOI'ed), before we take this CPU offline.
+	 */
 	local_irq_enable();
 	mdelay(1);
 	local_irq_disable();