PM / Domains: Introduce dev_pm_domain_start()

For a subsystem/driver that either doesn't support runtime PM or makes use
of pm_runtime_set_active() during ->probe(), may try to access its device
when probing, even if it may not be fully powered on from the PM domain's
point of view. This may be the case when the used PM domain is a genpd
provider, that implements genpd's ->start|stop() device callbacks.

There are cases where the subsystem/driver managed to avoid the above
problem, simply by calling pm_runtime_enable() and pm_runtime_get_sync()
during ->probe(). However, this approach comes with a drawback, especially
if the subsystem/driver implements a ->runtime_resume() callback.

More precisely, the subsystem/driver then needs to use a device flag, which
is checked in its ->runtime_resume() callback, as to avoid powering on its
resources the first time the callback is invoked. This is needed because
the subsystem/driver has already powered on the resources for the device,
during ->probe() and before it called pm_runtime_get_sync().

In a way to avoid this boilerplate code and the inefficient check for "if
(first_time_suspend)" in the ->runtime_resume() callback for these
subsystems/drivers, let's introduce and export a dev_pm_domain_start()
function, that may be called during ->probe() instead.

Moreover, let the dev_pm_domain_start() invoke an optional ->start()
callback, added to the struct dev_pm_domain, as to allow a PM domain
specific implementation.

Signed-off-by: Ulf Hansson <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/base/power/common.c

···
 EXPORT_SYMBOL_GPL(dev_pm_domain_detach);
 
 /**
+ * dev_pm_domain_start - Start the device through its PM domain.
+ * @dev: Device to start.
+ *
+ * This function should typically be called during probe by a subsystem/driver,
+ * when it needs to start its device from the PM domain's perspective. Note
+ * that, it's assumed that the PM domain is already powered on when this
+ * function is called.
+ *
+ * Returns 0 on success and negative error values on failures.
+ */
+int dev_pm_domain_start(struct device *dev)
+{
+	if (dev->pm_domain && dev->pm_domain->start)
+		return dev->pm_domain->start(dev);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dev_pm_domain_start);
+
+/**
  * dev_pm_domain_set - Set PM domain of a device.
  * @dev: Device whose PM domain is to be set.
  * @pd: PM domain to be set, or NULL.

include/linux/pm.h

···
  * struct dev_pm_domain - power management domain representation.
  *
  * @ops: Power management operations associated with this domain.
+ * @start: Called when a user needs to start the device via the domain.
  * @detach: Called when removing a device from the domain.
  * @activate: Called before executing probe routines for bus types and drivers.
  * @sync: Called after successful driver probe.
···
  */
 struct dev_pm_domain {
 	struct dev_pm_ops	ops;
+	int (*start)(struct device *dev);
 	void (*detach)(struct device *dev, bool power_off);
 	int (*activate)(struct device *dev);
 	void (*sync)(struct device *dev);

include/linux/pm_domain.h

···
 struct device *dev_pm_domain_attach_by_name(struct device *dev,
 					    const char *name);
 void dev_pm_domain_detach(struct device *dev, bool power_off);
+int dev_pm_domain_start(struct device *dev);
 void dev_pm_domain_set(struct device *dev, struct dev_pm_domain *pd);
 #else
 static inline int dev_pm_domain_attach(struct device *dev, bool power_on)
···
 	return NULL;
 }
 static inline void dev_pm_domain_detach(struct device *dev, bool power_off) {}
+static inline int dev_pm_domain_start(struct device *dev)
+{
+	return 0;
+}
 static inline void dev_pm_domain_set(struct device *dev,
 				     struct dev_pm_domain *pd) {}
 #endif