PM / core: Add SMART_SUSPEND driver flag

Define and document a SMART_SUSPEND flag to instruct bus types and PM
domains that the system suspend callbacks provided by the driver can
cope with runtime-suspended devices, so from the driver's perspective
it should be safe to leave devices in runtime suspend during system
suspend.

Setting that flag may also cause middle-layer code (bus types,
PM domains etc.) to skip invocations of the ->suspend_late and
->suspend_noirq callbacks provided by the driver if the device
is in runtime suspend at the beginning of the "late" phase of
the system-wide suspend transition, in which case the driver's
system-wide resume callbacks may be invoked back-to-back with
its ->runtime_suspend callback, so the driver has to be able to
cope with that too.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/driver-api/pm/devices.rst

@@ -766,6 +766,26 @@ the state of devices (possibly except for resuming them from runtime suspend)
 from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
 invoking device drivers' ``->suspend`` callbacks (or equivalent).
 
+Some bus types and PM domains have a policy to resume all devices from runtime
+suspend upfront in their ``->suspend`` callbacks, but that may not be really
+necessary if the driver of the device can cope with runtime-suspended devices.
+The driver can indicate that by setting ``DPM_FLAG_SMART_SUSPEND`` in
+:c:member:`power.driver_flags` at the probe time, by passing it to the
+:c:func:`dev_pm_set_driver_flags` helper.  That also may cause middle-layer code
+(bus types, PM domains etc.) to skip the ``->suspend_late`` and
+``->suspend_noirq`` callbacks provided by the driver if the device remains in
+runtime suspend at the beginning of the ``suspend_late`` phase of system-wide
+suspend (or in the ``poweroff_late`` phase of hibernation), when runtime PM
+has been disabled for it, under the assumption that its state should not change
+after that point until the system-wide transition is over.  If that happens, the
+driver's system-wide resume callbacks, if present, may still be invoked during
+the subsequent system-wide resume transition and the device's runtime power
+management status may be set to "active" before enabling runtime PM for it,
+so the driver must be prepared to cope with the invocation of its system-wide
+resume callbacks back-to-back with its ``->runtime_suspend`` one (without the
+intervening ``->runtime_resume`` and so on) and the final state of the device
+must reflect the "active" status for runtime PM in that case.
+
 During system-wide resume from a sleep state it's easiest to put devices into
 the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
 Refer to that document for more information regarding this particular issue as

drivers/base/power/main.c

@@ -1652,6 +1652,9 @@ static int device_prepare(struct device *dev, pm_message_t state)
 	if (dev->power.syscore)
 		return 0;
 
+	WARN_ON(dev_pm_test_driver_flags(dev, DPM_FLAG_SMART_SUSPEND) &&
+		!pm_runtime_enabled(dev));
+
 	/*
 	 * If a device's parent goes into runtime suspend at the wrong time,
 	 * it won't be possible to resume the device.  To prevent this we

include/linux/pm.h

@@ -558,6 +558,7 @@ struct pm_subsys_data {
  *
  * NEVER_SKIP: Do not skip system suspend/resume callbacks for the device.
  * SMART_PREPARE: Check the return value of the driver's ->prepare callback.
+ * SMART_SUSPEND: No need to resume the device from runtime suspend.
  *
  * Setting SMART_PREPARE instructs bus types and PM domains which may want
  * system suspend/resume callbacks to be skipped for the device to return 0 from
@@ -565,9 +566,16 @@ struct pm_subsys_data {
  * other words, the system suspend/resume callbacks can only be skipped for the
  * device if its driver doesn't object against that).  This flag has no effect
  * if NEVER_SKIP is set.
+ *
+ * Setting SMART_SUSPEND instructs bus types and PM domains which may want to
+ * runtime resume the device upfront during system suspend that doing so is not
+ * necessary from the driver's perspective.  It also may cause them to skip
+ * invocations of the ->suspend_late and ->suspend_noirq callbacks provided by
+ * the driver if they decide to leave the device in runtime suspend.
  */
 #define DPM_FLAG_NEVER_SKIP	BIT(0)
 #define DPM_FLAG_SMART_PREPARE	BIT(1)
+#define DPM_FLAG_SMART_SUSPEND	BIT(2)
 
 struct dev_pm_info {
 	pm_message_t		power_state;