PM: Run the driver callback directly if the subsystem one is not there

Make the PM core execute driver PM callbacks directly if the
corresponding subsystem callbacks are not present.

There are three reasons for doing that.  First, it reflects the
behavior of drivers/base/dd.c:really_probe() that runs the driver's
.probe() callback directly if the bus type's one is not defined, so
this change will remove one arbitrary difference between the PM core
and the remaining parts of the driver core.  Second, it will allow
some subsystems, whose PM callbacks don't do anything except for
executing driver callbacks, to be simplified quite a bit by removing
those "forward-only" callbacks.  Finally, it will allow us to remove
one level of indirection in the system suspend and resume code paths
where it is not necessary, which is going to lead to less debug noise
with initcall_debug passed in the kernel command line (messages won't
be printed for driverless devices whose subsystems don't provide
PM callbacks among other things).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

1 files changed, 21 insertions, 16 deletions

diff --git a/Documentation/power/devices.txt b/Documentation/power/devices.txt
index 3139fb505dc..20af7def23c 100644
--- a/Documentation/power/devices.txt
+++ b/Documentation/power/devices.txt
@@ -126,7 +126,9 @@ The core methods to suspend and resume devices reside in struct dev_pm_ops
 pointed to by the ops member of struct dev_pm_domain, or by the pm member of
 struct bus_type, struct device_type and struct class.  They are mostly of
 interest to the people writing infrastructure for platforms and buses, like PCI
-or USB, or device type and device class drivers.
+or USB, or device type and device class drivers.  They also are relevant to the
+writers of device drivers whose subsystems (PM domains, device types, device
+classes and bus types) don't provide all power management methods.
 
 Bus drivers implement these methods as appropriate for the hardware and the
 drivers using it; PCI works differently from USB, and so on.  Not many people
@@ -268,32 +270,35 @@ various phases always run after tasks have been frozen and before they are
 unfrozen.  Furthermore, the *_noirq phases run at a time when IRQ handlers have
 been disabled (except for those marked with the IRQF_NO_SUSPEND flag).
 
-All phases use PM domain, bus, type, or class callbacks (that is, methods
-defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, or dev->class->pm).
-These callbacks are regarded by the PM core as mutually exclusive.  Moreover,
-PM domain callbacks always take precedence over bus, type and class callbacks,
-while type callbacks take precedence over bus and class callbacks, and class
-callbacks take precedence over bus callbacks.  To be precise, the following
-rules are used to determine which callback to execute in the given phase:
+All phases use PM domain, bus, type, class or driver callbacks (that is, methods
+defined in dev->pm_domain->ops, dev->bus->pm, dev->type->pm, dev->class->pm or
+dev->driver->pm).  These callbacks are regarded by the PM core as mutually
+exclusive.  Moreover, PM domain callbacks always take precedence over all of the
+other callbacks and, for example, type callbacks take precedence over bus, class
+and driver callbacks.  To be precise, the following rules are used to determine
+which callback to execute in the given phase:
 
-    1.	If dev->pm_domain is present, the PM core will attempt to execute the
-	callback included in dev->pm_domain->ops.  If that callback is not
-	present, no action will be carried out for the given device.
+    1.	If dev->pm_domain is present, the PM core will choose the callback
+	included in dev->pm_domain->ops for execution
 
     2.	Otherwise, if both dev->type and dev->type->pm are present, the callback
-	included in dev->type->pm will be executed.
+	included in dev->type->pm will be chosen for execution.
 
     3.	Otherwise, if both dev->class and dev->class->pm are present, the
-	callback included in dev->class->pm will be executed.
+	callback included in dev->class->pm will be chosen for execution.
 
     4.	Otherwise, if both dev->bus and dev->bus->pm are present, the callback
-	included in dev->bus->pm will be executed.
+	included in dev->bus->pm will be chosen for execution.
 
 This allows PM domains and device types to override callbacks provided by bus
 types or device classes if necessary.
 
-These callbacks may in turn invoke device- or driver-specific methods stored in
-dev->driver->pm, but they don't have to.
+The PM domain, type, class and bus callbacks may in turn invoke device- or
+driver-specific methods stored in dev->driver->pm, but they don't have to do
+that.
+
+If the subsystem callback chosen for execution is not present, the PM core will
+execute the corresponding method from dev->driver->pm instead if there is one.
 
 
 Entering System Suspend