PM / sleep: enable suspend-to-idle even without registered suspend_ops

Suspend-to-idle (aka the "freeze" sleep state) is a system sleep state
in which all of the processors enter deepest possible idle state and
wait for interrupts right after suspending all the devices.

There is no hard requirement for a platform to support and register
platform specific suspend_ops to enter suspend-to-idle/freeze state.
Only deeper system sleep states like PM_SUSPEND_STANDBY and
PM_SUSPEND_MEM rely on such low level support/implementation.

suspend-to-idle can be entered as along as all the devices can be
suspended. This patch enables the support for suspend-to-idle even on
systems that don't have any low level support for deeper system sleep
states and/or don't register any platform specific suspend_ops.
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/suspend.h

@@ -245,6 +245,7 @@ static inline bool idle_should_freeze(void)
 	return unlikely(suspend_freeze_state == FREEZE_STATE_ENTER);
 }
 
+extern void __init pm_states_init(void);
 extern void freeze_set_ops(const struct platform_freeze_ops *ops);
 extern void freeze_wake(void);
 
@@ -279,6 +280,7 @@ static inline bool pm_resume_via_firmware(void) { return false; }
 static inline void suspend_set_ops(const struct platform_suspend_ops *ops) {}
 static inline int pm_suspend(suspend_state_t state) { return -ENOSYS; }
 static inline bool idle_should_freeze(void) { return false; }
+static inline void __init pm_states_init(void) {}
 static inline void freeze_set_ops(const struct platform_freeze_ops *ops) {}
 static inline void freeze_wake(void) {}
 #endif /* !CONFIG_SUSPEND */

kernel/power/main.c

@@ -644,6 +644,7 @@ static int __init pm_init(void)
 		return error;
 	hibernate_image_size_init();
 	hibernate_reserved_size_init();
+	pm_states_init();
 	power_kobj = kobject_create_and_add("power", NULL);
 	if (!power_kobj)
 		return -ENOMEM;

kernel/power/suspend.c

@@ -118,10 +118,18 @@ static bool valid_state(suspend_state_t state)
  */
 static bool relative_states;
 
+void __init pm_states_init(void)
+{
+	/*
+	 * freeze state should be supported even without any suspend_ops,
+	 * initialize pm_states accordingly here
+	 */
+	pm_states[PM_SUSPEND_FREEZE] = pm_labels[relative_states ? 0 : 2];
+}
+
 static int __init sleep_states_setup(char *str)
 {
 	relative_states = !strncmp(str, "1", 1);
-	pm_states[PM_SUSPEND_FREEZE] = pm_labels[relative_states ? 0 : 2];
 	return 1;
 }
 
@@ -211,7 +219,7 @@ static int platform_suspend_begin(suspend_state_t state)
 {
 	if (state == PM_SUSPEND_FREEZE && freeze_ops && freeze_ops->begin)
 		return freeze_ops->begin();
-	else if (suspend_ops->begin)
+	else if (suspend_ops && suspend_ops->begin)
 		return suspend_ops->begin(state);
 	else
 		return 0;
@@ -221,7 +229,7 @@ static void platform_resume_end(suspend_state_t state)
 {
 	if (state == PM_SUSPEND_FREEZE && freeze_ops && freeze_ops->end)
 		freeze_ops->end();
-	else if (suspend_ops->end)
+	else if (suspend_ops && suspend_ops->end)
 		suspend_ops->end();
 }