[PATCH] PM / Runtime: Update documentation of interactions with system sleep

[Rafael J. Wysocki]

From: Rafael J. Wysocki <rjw@xxxxxxx>

The documents describing the interactions between runtime PM and
system sleep generally refer to the model in which the system sleep
state is entered through a global firmware or hardware operation.
As a result, some recommendations given in there are not entirely
suitable for systems in which this is not the case.  Update the
documentation take the existence of those systems into accout.

Signed-off-by: Rafael J. Wysocki <rjw@xxxxxxx>
---
 Documentation/power/devices.txt    |    6 +++---
 Documentation/power/runtime_pm.txt |   25 +++++++++++++++++--------
 2 files changed, 20 insertions(+), 11 deletions(-)

Index: linux-2.6/Documentation/power/runtime_pm.txt
===================================================================
--- linux-2.6.orig/Documentation/power/runtime_pm.txt
+++ linux-2.6/Documentation/power/runtime_pm.txt
@@ -537,9 +537,9 @@ suspend routine).  It may be necessary t
 in order to do so.  The same is true if the driver uses different power levels
 or other settings for run-time suspend and system sleep.
 
-During system resume, devices generally should be brought back to full power,
-even if they were suspended before the system sleep began.  There are several
-reasons for this, including:
+During system resume, the simplest approach is to bring all devices back to full
+power, even if they were suspended before the system sleep began.  There are
+several reasons for this, including:
 
   * The device might need to switch power levels, wake-up settings, etc.
 
@@ -556,16 +556,25 @@ reasons for this, including:
   * Even though the device was suspended, if its usage counter was > 0 then most
     likely it would need a run-time resume in the near future anyway.
 
-  * Always going back to full power is simplest.
-
-If the device was suspended before the sleep began, then its run-time PM status
-will have to be updated to reflect the actual post-system sleep status.  The way
-to do this is:
+If the device had been suspended before the sleep began and it's brought back to
+full power during resume, then its run-time PM status will have to be updated to
+reflect the actual post-system sleep status.  The way to do this is:
 
 	pm_runtime_disable(dev);
 	pm_runtime_set_active(dev);
 	pm_runtime_enable(dev);
 
+On some systems, however, system sleep is not entered through a global firmware
+or hardware operation.  Instead, all hardware components are put into low-power
+states directly by the kernel in a coordinated way.  Then, the system sleep
+state effectively follows from the states the hardware components end up in
+and the system is woken up from that state by a hardware interrupt or a similar
+mechanism entirely under the kernel's control.  As a result, the kernel never
+gives control away and the states of all devices during resume are precisely
+known to it.  If that is the case and none of the situations listed above takes
+place, it may be more efficient to leave the devices that had been suspended
+before the system sleep began in the suspended state.
+
 7. Generic subsystem callbacks
 
 Subsystems may wish to conserve code space by using the set of generic power
Index: linux-2.6/Documentation/power/devices.txt
===================================================================
--- linux-2.6.orig/Documentation/power/devices.txt
+++ linux-2.6/Documentation/power/devices.txt
@@ -604,7 +604,7 @@ state temporarily, for example so that i
 disabled.  This all depends on the hardware and the design of the subsystem and
 device driver in question.
 
-During system-wide resume from a sleep state it's best to put devices into the
-full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer to
-that document for more information regarding this particular issue as well as
+During system-wide resume from a sleep state it's easiest to put devices into
+the full-power state, as explained in Documentation/power/runtime_pm.txt.  Refer
+to that document for more information regarding this particular issue as well as
 for information on the device runtime power management framework in general.
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