PM / core: Add NEVER_SKIP and SMART_PREPARE driver flags

The motivation for this change is to provide a way to work around
a problem with the direct-complete mechanism used for avoiding
system suspend/resume handling for devices in runtime suspend.

The problem is that some middle layer code (the PCI bus type and
the ACPI PM domain in particular) returns positive values from its
system suspend ->prepare callbacks regardless of whether the driver's
->prepare returns a positive value or 0, which effectively prevents
drivers from being able to control the direct-complete feature.
Some drivers need that control, however, and the PCI bus type has
grown its own flag to deal with this issue, but since it is not
limited to PCI, it is better to address it by adding driver flags at
the core level.

To that end, add a driver_flags field to struct dev_pm_info for flags
that can be set by device drivers at the probe time to inform the PM
core and/or bus types, PM domains and so on on the capabilities and/or
preferences of device drivers. Also add two static inline helpers
for setting that field and testing it against a given set of flags
and make the driver core clear it automatically on driver remove
and probe failures.

Define and document two PM driver flags related to the direct-
complete feature: NEVER_SKIP and SMART_PREPARE that can be used,
respectively, to indicate to the PM core that the direct-complete
mechanism should never be used for the device and to inform the
middle layer code (bus types, PM domains etc) that it can only
request the PM core to use the direct-complete mechanism for
the device (by returning a positive value from its ->prepare
callback) if it also has been requested by the driver.

While at it, make the core check pm_runtime_suspended() when
setting power.direct_complete so that it doesn't need to be
checked by ->prepare callbacks.

Signed-off-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Bjorn Helgaas <[e-mail]>
Reviewed-by: Ulf Hansson <[e-mail]>

Documentation/driver-api/pm/devices.rst

···
 	is because all such devices are initially set to runtime-suspended with
 	runtime PM disabled.
 
+	This feature also can be controlled by device drivers by using the
+	``DPM_FLAG_NEVER_SKIP`` and ``DPM_FLAG_SMART_PREPARE`` driver power
+	management flags.  [Typically, they are set at the time the driver is
+	probed against the device in question by passing them to the
+	:c:func:`dev_pm_set_driver_flags` helper function.]  If the first of
+	these flags is set, the PM core will not apply the direct-complete
+	procedure described above to the given device and, consequenty, to any
+	of its ancestors.  The second flag, when set, informs the middle layer
+	code (bus types, device types, PM domains, classes) that it should take
+	the return value of the ``->prepare`` callback provided by the driver
+	into account and it may only return a positive value from its own
+	``->prepare`` callback if the driver's one also has returned a positive
+	value.
+
     2.	The ``->suspend`` methods should quiesce the device to stop it from
 	performing I/O.  They also may save the device registers and put it into
 	the appropriate low-power state, depending on the bus type the device is

Documentation/power/pci.txt

···
 .suspend(), .freeze(), and .poweroff() members and one resume routine is to
 be pointed to by the .resume(), .thaw(), and .restore() members.
 
+3.1.19. Driver Flags for Power Management
+
+The PM core allows device drivers to set flags that influence the handling of
+power management for the devices by the core itself and by middle layer code
+including the PCI bus type.  The flags should be set once at the driver probe
+time with the help of the dev_pm_set_driver_flags() function and they should not
+be updated directly afterwards.
+
+The DPM_FLAG_NEVER_SKIP flag prevents the PM core from using the direct-complete
+mechanism allowing device suspend/resume callbacks to be skipped if the device
+is in runtime suspend when the system suspend starts.  That also affects all of
+the ancestors of the device, so this flag should only be used if absolutely
+necessary.
+
+The DPM_FLAG_SMART_PREPARE flag instructs the PCI bus type to only return a
+positive value from pci_pm_prepare() if the ->prepare callback provided by the
+driver of the device returns a positive value.  That allows the driver to opt
+out from using the direct-complete mechanism dynamically.
+
 3.2. Device Runtime Power Management
 ------------------------------------
 In addition to providing device power management callbacks PCI device drivers

drivers/acpi/device_pm.c

···
 int acpi_subsys_prepare(struct device *dev)
 {
 	struct acpi_device *adev = ACPI_COMPANION(dev);
-	int ret;
 
-	ret = pm_generic_prepare(dev);
-	if (ret < 0)
-		return ret;
+	if (dev->driver && dev->driver->pm && dev->driver->pm->prepare) {
+		int ret = dev->driver->pm->prepare(dev);
+
+		if (ret < 0)
+			return ret;
+
+		if (!ret && dev_pm_test_driver_flags(dev, DPM_FLAG_SMART_PREPARE))
+			return 0;
+	}
 
 	if (!adev || !pm_runtime_suspended(dev))
 		return 0;

drivers/base/dd.c

···
 	if (dev->pm_domain && dev->pm_domain->dismiss)
 		dev->pm_domain->dismiss(dev);
 	pm_runtime_reinit(dev);
+	dev_pm_set_driver_flags(dev, 0);
 
 	switch (ret) {
 	case -EPROBE_DEFER:
···
 		if (dev->pm_domain && dev->pm_domain->dismiss)
 			dev->pm_domain->dismiss(dev);
 		pm_runtime_reinit(dev);
+		dev_pm_set_driver_flags(dev, 0);
 
 		klist_remove(&dev->p->knode_driver);
 		device_pm_check_callbacks(dev);

drivers/base/power/main.c

···
 	 * applies to suspend transitions, however.
 	 */
 	spin_lock_irq(&dev->power.lock);
-	dev->power.direct_complete = ret > 0 && state.event == PM_EVENT_SUSPEND;
+	dev->power.direct_complete = state.event == PM_EVENT_SUSPEND &&
+		pm_runtime_suspended(dev) && ret > 0 &&
+		!dev_pm_test_driver_flags(dev, DPM_FLAG_NEVER_SKIP);
 	spin_unlock_irq(&dev->power.lock);
 	return 0;
 }

drivers/pci/pci-driver.c

···
 
 	if (drv && drv->pm && drv->pm->prepare) {
 		int error = drv->pm->prepare(dev);
-		if (error)
+		if (error < 0)
 			return error;
+
+		if (!error && dev_pm_test_driver_flags(dev, DPM_FLAG_SMART_PREPARE))
+			return 0;
 	}
 	return pci_dev_keep_suspended(to_pci_dev(dev));
 }

include/linux/device.h

···
 #endif
 }
 
+static inline void dev_pm_set_driver_flags(struct device *dev, u32 flags)
+{
+	dev->power.driver_flags = flags;
+}
+
+static inline bool dev_pm_test_driver_flags(struct device *dev, u32 flags)
+{
+	return !!(dev->power.driver_flags & flags);
+}
+
 static inline void device_lock(struct device *dev)
 {
 	mutex_lock(&dev->mutex);

include/linux/pm.h

···
 #endif
 };
 
+/*
+ * Driver flags to control system suspend/resume behavior.
+ *
+ * These flags can be set by device drivers at the probe time.  They need not be
+ * cleared by the drivers as the driver core will take care of that.
+ *
+ * NEVER_SKIP: Do not skip system suspend/resume callbacks for the device.
+ * SMART_PREPARE: Check the return value of the driver's ->prepare callback.
+ *
+ * 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
+ * their ->prepare callbacks if the driver's ->prepare callback returns 0 (in
+ * 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.
+ */
+#define DPM_FLAG_NEVER_SKIP	BIT(0)
+#define DPM_FLAG_SMART_PREPARE	BIT(1)
+
 struct dev_pm_info {
 	pm_message_t		power_state;
 	unsigned int		can_wakeup:1;
···
 	bool			is_late_suspended:1;
 	bool			early_init:1;	/* Owned by the PM core */
 	bool			direct_complete:1;	/* Owned by the PM core */
+	u32			driver_flags;
 	spinlock_t		lock;
 #ifdef CONFIG_PM_SLEEP
 	struct list_head	entry;