genirq: Don't suspend nested_thread irqs over system suspend

Nested IRQs can only fire when the parent irq fires.  So when the
parent is suspended, there is no need to suspend the child irq.

Suspending nested irqs can cause a problem is they are suspended or
resumed in the wrong order.  If an interrupt fires while the parent is
active but the child is suspended, then the interrupt will not be
acknowledged properly and so an interrupt storm can result.  This is
particularly likely if the parent is resumed before the child, and the
interrupt was raised during suspend.

Ensuring correct ordering would be possible, but it is simpler to just
never suspend nested interrupts.

Signed-off-by: NeilBrown <neil@brown.name>
Cc: GTA04 owners <gta04-owner@goldelico.com>
Cc: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Link: http://lkml.kernel.org/r/20150517151934.2393e8f8@notabene.brown
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/pm.c

@@ -123,6 +123,8 @@ void suspend_device_irqs(void)
 		unsigned long flags;
 		bool sync;
 
+		if (irq_settings_is_nested_thread(desc))
+			continue;
 		raw_spin_lock_irqsave(&desc->lock, flags);
 		sync = suspend_device_irq(desc, irq);
 		raw_spin_unlock_irqrestore(&desc->lock, flags);
@@ -163,6 +165,8 @@ static void resume_irqs(bool want_early)
 
 		if (!is_early && want_early)
 			continue;
+		if (irq_settings_is_nested_thread(desc))
+			continue;
 
 		raw_spin_lock_irqsave(&desc->lock, flags);
 		resume_irq(desc, irq);