PM / runtime: Document autosuspend-helper side effects

Document the fact that the autosuspend delay and enable helpers may
change the power.usage_count and resume or suspend a device depending on
the values of power.autosuspend_delay and power.use_autosuspend.

Note that this means that a driver must disable autosuspend before
disabling runtime pm on probe errors and on driver unbind if the device
is to be suspended upon return (as a negative delay may otherwise keep
the device resumed).

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/runtime_pm.txt

@@ -478,15 +478,23 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
     - set the power.last_busy field to the current time
 
   void pm_runtime_use_autosuspend(struct device *dev);
-    - set the power.use_autosuspend flag, enabling autosuspend delays
+    - set the power.use_autosuspend flag, enabling autosuspend delays; call
+      pm_runtime_get_sync if the flag was previously cleared and
+      power.autosuspend_delay is negative
 
   void pm_runtime_dont_use_autosuspend(struct device *dev);
-    - clear the power.use_autosuspend flag, disabling autosuspend delays
+    - clear the power.use_autosuspend flag, disabling autosuspend delays;
+      decrement the device's usage counter if the flag was previously set and
+      power.autosuspend_delay is negative; call pm_runtime_idle
 
   void pm_runtime_set_autosuspend_delay(struct device *dev, int delay);
     - set the power.autosuspend_delay value to 'delay' (expressed in
       milliseconds); if 'delay' is negative then runtime suspends are
-      prevented
+      prevented; if power.use_autosuspend is set, pm_runtime_get_sync may be
+      called or the device's usage counter may be decremented and
+      pm_runtime_idle called depending on if power.autosuspend_delay is
+      changed to or from a negative value; if power.use_autosuspend is clear,
+      pm_runtime_idle is called
 
   unsigned long pm_runtime_autosuspend_expiration(struct device *dev);
     - calculate the time when the current autosuspend delay period will expire,