sched/idle: Move the cpuidle entry point to the generic idle loop

In order to integrate cpuidle with the scheduler, we must have a better
proximity in the core code with what cpuidle is doing and not delegate
such interaction to arch code.

Architectures implementing arch_cpu_idle() should simply enter
a cheap idle mode in the absence of a proper cpuidle driver.

In both cases i.e. whether it is a cpuidle driver or the default
arch_cpu_idle(), the calling convention expects IRQs to be disabled
on entry and enabled on exit. There is a warning in place already but
let's add a forced IRQ enable here as well.  This will allow for
removing the forced IRQ enable some implementations do locally and
allowing for the warning to trig.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Olof Johansson <olof@lixom.net>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.LFD.2.11.1401291526320.1652@knanqh.ubzr
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/cpu/idle.c

@@ -3,6 +3,7 @@
  */
 #include <linux/sched.h>
 #include <linux/cpu.h>
+#include <linux/cpuidle.h>
 #include <linux/tick.h>
 #include <linux/mm.h>
 #include <linux/stackprotector.h>
@@ -95,8 +96,10 @@ static void cpu_idle_loop(void)
 				if (!current_clr_polling_and_test()) {
 					stop_critical_timings();
 					rcu_idle_enter();
-					arch_cpu_idle();
-					WARN_ON_ONCE(irqs_disabled());
+					if (cpuidle_idle_call())
+						arch_cpu_idle();
+					if (WARN_ON_ONCE(irqs_disabled()))
+						local_irq_enable();
 					rcu_idle_exit();
 					start_critical_timings();
 				} else {